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Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Missing Reference: 'RFC-TBD' is mentioned on line 1102, but not defined ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Bittau 3 Internet-Draft Google 4 Intended status: Experimental D. Giffin 5 Expires: April 7, 2018 Stanford University 6 M. Handley 7 University College London 8 D. Mazieres 9 Stanford University 10 E. Smith 11 Kestrel Institute 12 October 4, 2017 14 TCP-ENO: Encryption Negotiation Option 15 draft-ietf-tcpinc-tcpeno-10 17 Abstract 19 Despite growing adoption of TLS, a significant fraction of TCP 20 traffic on the Internet remains unencrypted. The persistence of 21 unencrypted traffic can be attributed to at least two factors. 22 First, some legacy protocols lack a signaling mechanism (such as a 23 "STARTTLS" command) by which to convey support for encryption, making 24 incremental deployment impossible. Second, legacy applications 25 themselves cannot always be upgraded, requiring a way to implement 26 encryption transparently entirely within the transport layer. The 27 TCP Encryption Negotiation Option (TCP-ENO) addresses both of these 28 problems through a new TCP option kind providing out-of-band, fully 29 backward-compatible negotiation of encryption. 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 http://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 April 7, 2018. 48 Copyright Notice 50 Copyright (c) 2017 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 (http://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. Requirements language . . . . . . . . . . . . . . . . . . . . 3 66 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2.1. Design goals . . . . . . . . . . . . . . . . . . . . . . 3 68 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 4. TCP-ENO specification . . . . . . . . . . . . . . . . . . . . 5 70 4.1. ENO option . . . . . . . . . . . . . . . . . . . . . . . 6 71 4.2. The global suboption . . . . . . . . . . . . . . . . . . 8 72 4.3. TCP-ENO roles . . . . . . . . . . . . . . . . . . . . . . 9 73 4.4. Specifying suboption data length . . . . . . . . . . . . 9 74 4.5. The negotiated TEP . . . . . . . . . . . . . . . . . . . 11 75 4.6. TCP-ENO handshake . . . . . . . . . . . . . . . . . . . . 11 76 4.7. Data in SYN segments . . . . . . . . . . . . . . . . . . 12 77 4.8. Negotiation transcript . . . . . . . . . . . . . . . . . 14 78 5. Requirements for TEPs . . . . . . . . . . . . . . . . . . . . 15 79 5.1. Session IDs . . . . . . . . . . . . . . . . . . . . . . . 16 80 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 81 7. Future developments . . . . . . . . . . . . . . . . . . . . . 19 82 8. Design rationale . . . . . . . . . . . . . . . . . . . . . . 20 83 8.1. Handshake robustness . . . . . . . . . . . . . . . . . . 20 84 8.2. Suboption data . . . . . . . . . . . . . . . . . . . . . 20 85 8.3. Passive role bit . . . . . . . . . . . . . . . . . . . . 21 86 8.4. Use of ENO option kind by TEPs . . . . . . . . . . . . . 21 87 9. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 21 88 10. Security considerations . . . . . . . . . . . . . . . . . . . 22 89 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 90 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 91 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24 92 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 93 14.1. Normative References . . . . . . . . . . . . . . . . . . 24 94 14.2. Informative References . . . . . . . . . . . . . . . . . 25 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 97 1. Requirements language 99 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 100 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 101 document are to be interpreted as described in [RFC2119]. 103 2. Introduction 105 Many applications and protocols running on top of TCP today do not 106 encrypt traffic. This failure to encrypt lowers the bar for certain 107 attacks, harming both user privacy and system security. 108 Counteracting the problem demands a minimally intrusive, backward- 109 compatible mechanism for incrementally deploying encryption. The TCP 110 Encryption Negotiation Option (TCP-ENO) specified in this document 111 provides such a mechanism. 113 Introducing TCP options, extending operating system interfaces to 114 support TCP-level encryption, and extending applications to take 115 advantage of TCP-level encryption all require effort. To the 116 greatest extent possible, the effort invested in realizing TCP-level 117 encryption today needs to remain applicable in the future should the 118 need arise to change encryption strategies. To this end, it is 119 useful to consider two questions separately: 121 1. How to negotiate the use of encryption at the TCP layer, and 123 2. How to perform encryption at the TCP layer. 125 This document addresses question 1 with a new TCP option, ENO. TCP- 126 ENO provides a framework in which two endpoints can agree on one 127 among multiple possible TCP encryption protocols or _TEPs_. For 128 future compatibility, TEPs can vary widely in terms of wire format, 129 use of TCP option space, and integration with the TCP header and 130 segmentation. However, ENO abstracts these differences to ensure the 131 introduction of new TEPs can be transparent to applications taking 132 advantage of TCP-level encryption. 134 Question 2 is addressed by one or more companion TEP specification 135 documents. While current TEPs enable TCP-level traffic encryption 136 today, TCP-ENO ensures that the effort invested to deploy today's 137 TEPs will additionally benefit future ones. 139 2.1. Design goals 141 TCP-ENO was designed to achieve the following goals: 143 1. Enable endpoints to negotiate the use of a separately specified 144 TCP encryption protocol or _TEP_. 146 2. Transparently fall back to unencrypted TCP when not supported by 147 both endpoints. 149 3. Provide out-of-band signaling through which applications can 150 better take advantage of TCP-level encryption (for instance, by 151 improving authentication mechanisms in the presence of TCP-level 152 encryption). 154 4. Provide a standard negotiation transcript through which TEPs can 155 defend against tampering with TCP-ENO. 157 5. Make parsimonious use of TCP option space. 159 6. Define roles for the two ends of a TCP connection, so as to name 160 each end of a connection for encryption or authentication 161 purposes even following a symmetric simultaneous open. 163 3. Terminology 165 We define the following terms, which are used throughout this 166 document: 168 SYN segment 169 A TCP segment in which the SYN flag is set 171 ACK segment 172 A TCP segment in which the ACK flag is set (which includes most 173 segments other than an initial SYN segment) 175 non-SYN segment 176 A TCP segment in which the SYN flag is clear 178 SYN-only segment 179 A TCP segment in which the SYN flag is set but the ACK flag is 180 clear 182 SYN-ACK segment 183 A TCP segment in which the SYN and ACK flags are both set 185 Active opener 186 A host that initiates a connection by sending a SYN-only segment. 187 With the BSD socket API, an active opener calls "connect". In 188 client-server configurations, active openers are typically 189 clients. 191 Passive opener 192 A host that does not send a SYN-only segment, but responds to one 193 with a SYN-ACK segment. With the BSD socket API, passive openers 194 call "listen" and "accept", rather than "connect". In client- 195 server configurations, passive openers are typically servers. 197 Simultaneous open 198 The act of symmetrically establishing a TCP connection between two 199 active openers (both of which call "connect" with BSD sockets). 200 Each host of a simultaneous open sends both a SYN-only and a SYN- 201 ACK segment. Simultaneous open is less common than asymmetric 202 open with one active and one passive opener, but can be used for 203 NAT traversal by peer-to-peer applications [RFC5382]. 205 TEP 206 A TCP encryption protocol intended for use with TCP-ENO and 207 specified in a separate document. 209 TEP identifier 210 A unique 7-bit value in the range 0x20-0x7f that IANA has assigned 211 to a TEP. 213 Negotiated TEP 214 The single TEP governing a TCP connection, determined by use of 215 the TCP ENO option specified in this document. 217 4. TCP-ENO specification 219 TCP-ENO extends TCP connection establishment to enable encryption 220 opportunistically. It uses a new TCP option kind to negotiate one 221 among multiple possible TCP encryption protocols or TEPs. The 222 negotiation involves hosts exchanging sets of supported TEPs, where 223 each TEP is represented by a _suboption_ within a larger TCP ENO 224 option in the offering host's SYN segment. 226 If TCP-ENO succeeds, it yields the following information: 228 o A negotiated TEP, represented by a unique 7-bit TEP identifier, 230 o A few extra bytes of suboption data from each host, if needed by 231 the TEP, 233 o A negotiation transcript with which to mitigate attacks on the 234 negotiation itself, 236 o Role assignments designating one endpoint "host A" and the other 237 endpoint "host B", and 239 o A bit available to higher-layer protocols at each endpoint for 240 out-of-band negotiation of updated behavior in the presence of TCP 241 encryption. 243 If TCP-ENO fails, encryption is disabled and the connection falls 244 back to traditional unencrypted TCP. 246 The remainder of this section provides the normative description of 247 the TCP ENO option and handshake protocol. 249 4.1. ENO option 251 TCP-ENO employs an option in the TCP header [RFC0793]. Figure 1 252 illustrates the high-level format of this option. 254 byte 0 1 2 N+1 (N+2 bytes total) 255 +-----+-----+-----+--....--+-----+ 256 |Kind=|Len= | | 257 | TBD | N+2 | contents (N bytes) | 258 +-----+-----+-----+--....--+-----+ 260 Figure 1: The TCP-ENO option 262 The contents of an ENO option can take one of two forms. A SYN form, 263 illustrated in Figure 2, appears only in SYN segments. A non-SYN 264 form, illustrated in Figure 3, appears only in non-SYN segments. The 265 SYN form of ENO acts as a container for zero or more suboptions, 266 labeled "Opt_0", "Opt_1", ... in Figure 2. The non-SYN form, by its 267 presence, acts as a one-bit acknowledgment, with the actual contents 268 ignored by ENO. Particular TEPs MAY assign additional meaning to the 269 contents of non-SYN ENO options. When a negotiated TEP does not 270 assign such meaning, the contents of a non-SYN ENO option MUST be 271 zero bytes in sent segments and MUST be ignored in received segments. 273 byte 0 1 2 3 ... N+1 274 +-----+-----+-----+-----+--...--+-----+----...----+ 275 |Kind=|Len= |Opt_0|Opt_1| |Opt_i| Opt_i | 276 | TBD | N+2 | | | | | data | 277 +-----+-----+-----+-----+--...--+-----+----...----+ 279 Figure 2: SYN form of ENO 281 byte 0 1 2 N+1 282 +-----+-----+-----...----+ 283 |Kind=|Len= | ignored | 284 | TBD | N+2 | by TCP-ENO | 285 +-----+-----+-----...----+ 287 Figure 3: Non-SYN form of ENO, where N MAY be 0 289 Every suboption starts with a byte of the form illustrated in 290 Figure 4. The high bit "v", when set, introduces suboptions with 291 variable-length data. When "v = 0", the byte itself constitutes the 292 entirety of the suboption. The 7-bit value "glt" expresses one of: 294 o Global configuration data (discussed in Section 4.2), 296 o Suboption data length for the next suboption (discussed in 297 Section 4.4), or 299 o An offer to use a particular TEP defined in a separate TEP 300 specification document. 302 bit 7 6 5 4 3 2 1 0 303 +---+---+---+---+---+---+---+---+ 304 | v | glt | 305 +---+---+---+---+---+---+---+---+ 307 v - non-zero for use with variable-length suboption data 308 glt - Global suboption, Length, or TEP identifier 310 Figure 4: Format of initial suboption byte 312 Table 1 summarizes the meaning of initial suboption bytes. Values of 313 "glt" below 0x20 are used for global suboptions and length 314 information (the "gl" in "glt"), while those greater than or equal to 315 0x20 are TEP identifiers (the "t"). When "v = 0", the initial 316 suboption byte constitutes the entirety of the suboption and all 317 information is expressed by the 7-bit "glt" value, which can be 318 either a global suboption or a TEP identifier. When "v = 1", it 319 indicates a suboption with variable-length suboption data. Only TEP 320 identifiers may have suboption data, not global suboptions. Hence, 321 bytes with "v = 1" and "glt < 0x20" are not global suboptions but 322 rather length bytes governing the length of the next suboption (which 323 MUST be a TEP identifer). In the absence of a length byte, a TEP 324 identifier suboption with "v = 1" has suboption data extending to the 325 end of the TCP option. 327 +-----------+---+-------------------------------------------+ 328 | glt | v | Meaning | 329 +-----------+---+-------------------------------------------+ 330 | 0x00-0x1f | 0 | Global suboption (Section 4.2) | 331 | 0x00-0x1f | 1 | Length byte (Section 4.4) | 332 | 0x20-0x7f | 0 | TEP identifier without suboption data | 333 | 0x20-0x7f | 1 | TEP identifier followed by suboption data | 334 +-----------+---+-------------------------------------------+ 336 Table 1: Initial suboption byte values 338 A SYN segment MUST contain at most one TCP ENO option. If a SYN 339 segment contains more than one ENO option, the receiver MUST behave 340 as though the segment contained no ENO options and disable 341 encryption. A TEP MAY specify the use of multiple ENO options in a 342 non-SYN segment. For non-SYN segments, ENO itself only distinguishes 343 between the presence or absence of ENO options; multiple ENO options 344 are interpreted the same as one. 346 4.2. The global suboption 348 Suboptions 0x00-0x1f are used for global configuration that applies 349 regardless of the negotiated TEP. A TCP SYN segment MUST include at 350 most one ENO suboption in this range. A receiver MUST ignore all but 351 the first suboption in this range in any given TCP segment so as to 352 anticipate updates to ENO that assign new meaning to bits in 353 subsequent global suboptions. The value of a global suboption byte 354 is interpreted as a bitmask, illustrated in Figure 5. 356 bit 7 6 5 4 3 2 1 0 357 +---+---+---+---+---+---+---+---+ 358 | 0 | 0 | 0 |z1 |z2 |z3 | a | b | 359 +---+---+---+---+---+---+---+---+ 361 b - Passive role bit 362 a - Application-aware bit 363 z* - Zero bits (reserved for future use) 365 Figure 5: Format of the global suboption byte 367 The fields of the bitmask are interpreted as follows: 369 b 370 The passive role bit MUST be 1 for all passive openers. For 371 active openers, it MUST default to 0, but implementations MUST 372 provide an API through which an application can explicitly set "b 373 = 1" before initiating an active open. (Manual configuration of 374 "b" is necessary to enable encryption with a simultaneous open.) 376 a 377 Legacy applications can benefit from ENO-specific updates that 378 improve endpoint authentication or avoid double encryption. The 379 application-aware bit "a" is an out-of-band signal through which 380 higher-layer protocols can enable ENO-specific updates that would 381 otherwise not be backwards-compatible. Implementations MUST set 382 this bit to 0 by default, and MUST provide an API through which 383 applications can change the value of the bit as well as examine 384 the value of the bit sent by the remote host. Implementations 385 MUST furthermore support a _mandatory_ application-aware mode in 386 which TCP-ENO is automatically disabled if the remote host does 387 not set "a = 1". 389 z1, z2, z3 390 The "z" bits are reserved for future updates to TCP-ENO. They 391 MUST be set to zero in sent segments and MUST be ignored in 392 received segments. 394 A SYN segment without an explicit global suboption has an implicit 395 global suboption of 0x00. Because passive openers MUST always set "b 396 = 1", they cannot rely on this implicit 0x00 byte and MUST include an 397 explicit global suboption in their SYN-ACK segments. 399 4.3. TCP-ENO roles 401 TCP-ENO uses abstract roles to distinguish the two ends of a TCP 402 connection. These roles are determined by the "b" bit in the global 403 suboption. The host that sent an implicit or explicit suboption with 404 "b = 0" plays the "A" role. The host that sent "b = 1" plays the "B" 405 role. 407 If both sides of a connection set "b = 1" (which can happen if the 408 active opener misconfigures "b" before calling "connect"), or both 409 sides set "b = 0" (which can happen with simultaneous open), then 410 TCP-ENO MUST be disabled and the connection MUST fall back to 411 unencrypted TCP. 413 TEP specifications SHOULD refer to TCP-ENO's A and B roles to specify 414 asymmetric behavior by the two hosts. For the remainder of this 415 document, we will use the terms "host A" and "host B" to designate 416 the hosts with roles A and B, respectively, in a connection. 418 4.4. Specifying suboption data length 420 A TEP MAY optionally make use of one or more bytes of suboption data. 421 The presence of such data is indicated by setting "v = 1" in the 422 initial suboption byte (see Figure 4). By default, suboption data 423 extends to the end of the TCP option. Hence, if only one suboption 424 requires data, the most compact way to encode it is to place it last 425 in the ENO option, after all other suboptions. As an example, in 426 Figure 2, the last suboption, "Opt_i", has suboption data and thus 427 requires "v = 1"; however, the suboption data length is inferred from 428 the total length of the TCP option. 430 When a suboption with data is not last in an ENO option, the sender 431 MUST explicitly specify the suboption data length for the receiver to 432 know where the next suboption starts. The sender does so by 433 preceding the suboption with a length byte, depicted in Figure 6. 435 The length byte encodes a 5-bit value "nnnnn". Adding one to "nnnnn" 436 yields the length of the suboption data (not including the length 437 byte or the TEP identifier). Hence, a length byte can designate 438 anywhere from 1 to 32 bytes of suboption data (inclusive). 440 bit 7 6 5 4 3 2 1 0 441 +---+---+---+-------------------+ 442 | 1 0 0 nnnnn | 443 +---+---+---+-------------------+ 445 nnnnn - 5-bit value encoding (length - 1) 447 Figure 6: Format of a length byte 449 A suboption preceded by a length byte MUST be a TEP identifier ("glt 450 >= 0x20") and MUST have "v = 1". Figure 7 shows an example of such a 451 suboption. 453 byte 0 1 2 nnnnn+2 (nnnnn+3 bytes total) 454 +------+------+-------...-------+ 455 |length| TEP | suboption data | 456 | byte |ident.| (nnnnn+1 bytes) | 457 +------+------+-------...-------+ 459 length byte - specifies nnnnn 460 TEP identifier - MUST have v = 1 and glt >= 0x20 461 suboption data - length specified by nnnnn+1 463 Figure 7: Suboption with length byte 465 A host MUST ignore an ENO option in a SYN segment and MUST disable 466 encryption if either: 468 1. A length byte indicates that suboption data would extend beyond 469 the end of the TCP ENO option, or 471 2. A length byte is followed by an octet in the range 0x00-0x9f 472 (meaning the following byte has "v = 0" or "glt < 0x20"). 474 Because the last suboption in an ENO option is special-cased to have 475 its length inferred from the 8-bit TCP option length, it MAY contain 476 more than 32 bytes of suboption data. Other suboptions are limited 477 to 32 bytes by the length byte format. The TCP header itself can 478 only accommodate a maximum of 40 bytes of options, however. Hence, 479 regardless of the length byte format, a segment would not be able to 480 contain more than one suboption over 32 bytes in size. That said, 481 TEPs MAY define the use of multiple suboptions with the same TEP 482 identifier in the same SYN segment, providing another way to convey 483 over 32 bytes of suboption data even with length bytes. 485 4.5. The negotiated TEP 487 A TEP identifier "glt" (with "glt >= 0x20") is _valid_ for a 488 connection when: 490 1. Each side has sent a suboption for "glt" in its SYN-form ENO 491 option, 493 2. Any suboption data in these "glt" suboptions is valid according 494 to the TEP specification and satisfies any runtime constraints, 495 and 497 3. If an ENO option contains multiple suboptions with "glt", then 498 such repetition is well-defined by the TEP specification. 500 A passive opener (which is always host B) sees the remote host's SYN 501 segment before constructing its own SYN-ACK segment. Hence, a 502 passive opener SHOULD include only one TEP identifier in SYN-ACK 503 segments and SHOULD ensure this TEP identifier is valid. However, 504 simultaneous open or implementation considerations can prevent host B 505 from offering only one TEP. 507 To accommodate scenarios in which host B sends multiple TEP 508 identifiers in the SYN-ACK segment, the _negotiated TEP_ is defined 509 as the last valid TEP identifier in host B's SYN-form ENO option. 510 This definition means host B specifies TEP suboptions in order of 511 increasing priority, while host A does not influence TEP priority. 513 4.6. TCP-ENO handshake 515 A host employing TCP-ENO for a connection MUST include an ENO option 516 in every TCP segment sent until either encryption is disabled or the 517 host receives a non-SYN segment. In particular, this means an active 518 opener MUST include a non-SYN-form ENO option in the third segment of 519 a three-way handshake. 521 A host MUST disable encryption, refrain from sending any further ENO 522 options, and fall back to unencrypted TCP if any of the following 523 occurs: 525 1. Any segment it receives up to and including the first received 526 ACK segment does not contain a ENO option (or contains an ill- 527 formed SYN-form ENO option), 529 2. The SYN segment it receives does not contain a valid TEP 530 identifier, or 532 3. It receives a SYN segment with an incompatible global suboption. 533 (Specifically, incompatible means the two hosts set the same "b" 534 value or the connection is in mandatory application-aware mode 535 and the remote host set "a = 0".) 537 Hosts MUST NOT alter SYN-form ENO options in retransmitted segments, 538 or between the SYN and SYN-ACK segments of a simultaneous open, with 539 two exceptions for an active opener. First, an active opener MAY 540 unilaterally disable ENO (and thus remove the ENO option) between 541 retransmissions of a SYN-only segment. (Such removal could enable 542 recovery from middleboxes dropping segments with ENO options.) 543 Second, an active opener performing simultaneous open MAY include no 544 TCP-ENO option in its SYN-ACK if the received SYN caused it to 545 disable encryption according to the above rules (for instance because 546 role negotiation failed). 548 Once a host has both sent and received an ACK segment containing an 549 ENO option, encryption MUST be enabled. Once encryption is enabled, 550 hosts MUST follow the specification of the negotiated TEP and MUST 551 NOT present raw TCP payload data to the application. In particular, 552 data segments MUST NOT contain plaintext application data, but rather 553 ciphertext, key negotiation parameters, or other messages as 554 determined by the negotiated TEP. 556 A host MAY send a _vacuous_ SYN-form ENO option containing zero TEP 557 identifier suboptions. If either host sends a vacuous ENO option, it 558 follows that there are no valid TEP identifiers for the connection 559 and hence the connection must fall back to unencrypted TCP. Hosts 560 MAY send vacuous ENO options to indicate that ENO is supported but 561 unavailable by configuration, or to probe network paths for 562 robustness to ENO options. However, a passive opener MUST NOT send a 563 vacuous ENO option in a SYN-ACK segment unless there was an ENO 564 option in the SYN segment it received. Moreover, a passive opener's 565 SYN-form ENO option MUST still include a global suboption with "b = 566 1", as discussed in Section 4.3. 568 4.7. Data in SYN segments 570 TEPs MAY specify the use of data in SYN segments so as to reduce the 571 number of round trips required for connection setup. The meaning of 572 data in a SYN segment with an ENO option (a SYN+ENO segment) is 573 determined by the last TEP identifier in the ENO option, which we 574 term the segment's _SYN TEP_. 576 A host sending a SYN+ENO segment MUST NOT include data in the segment 577 unless the SYN TEP's specification defines the use of such data. 578 Furthermore, to avoid conflicting interpretations of SYN data, a 579 SYN+ENO segment MUST NOT include a non-empty TCP Fast Open (TFO) 580 option [RFC7413]. 582 Because a host can send SYN data before knowing which if any TEP will 583 govern a connection, hosts implementing ENO are REQUIRED to discard 584 data from SYN+ENO segments when the SYN TEP does not govern the 585 connection or when there is any ambiguity over the meaning of the SYN 586 data. This requirement applies to hosts that implement ENO even when 587 ENO has been disabled by configuration. However, note that 588 discarding SYN data is already common practice [RFC4987] and the new 589 requirement applies only to segments containing ENO options. 591 More specifically, a host that implements ENO MUST discard the data 592 in a received SYN+ENO segment if any of the following applies: 594 o ENO fails and TEP-indicated encryption is disabled for the 595 connection, 597 o The received segment's SYN TEP is not the negotiated TEP, 599 o The negotiated TEP does not define the use of SYN data, or 601 o The SYN segment contains a non-empty TFO option or any other TCP 602 option implying a conflicting definition of SYN data. 604 A host discarding SYN data in compliance with the above requirement 605 MUST NOT acknowledge the sequence number of the discarded data, but 606 rather MUST acknowledge the other host's initial sequence number as 607 if the received SYN segment contained no data. Furthermore, after 608 discarding SYN data, such a host MUST NOT assume the SYN data will be 609 identically retransmitted, and MUST process data only from non-SYN 610 segments. 612 If a host sends a SYN+ENO segment with data and receives 613 acknowledgment for the data, but the SYN TEP governing the data is 614 not the negotiated TEP (either because a different TEP was negotiated 615 or because ENO failed to negotiate encryption), then the host MUST 616 abort the TCP connection. Proceeding in any other fashion risks 617 misinterpreted SYN data. 619 If a host sends a SYN-only SYN+ENO segment bearing data and 620 subsequently receives a SYN-ACK segment without an ENO option, that 621 host MUST abort the connection even if the SYN-ACK segment does not 622 acknowledge the SYN data. The issue is that unacknowledged data may 623 nonetheless have been cached by the receiver; later retransmissions 624 intended to supersede this unacknowledged data could fail to do so if 625 the receiver gives precedence to the cached original data. 626 Implementations MAY provide an API call for a non-default mode in 627 which unacknowledged SYN data does not cause a connection abort, but 628 applications MUST use this mode only when a higher-layer integrity 629 check would anyway terminate a garbled connection. 631 To avoid unexpected connection aborts, ENO implementations MUST 632 disable the use of data in SYN-only segments by default. Such data 633 MAY be enabled by an API command. In particular, implementations MAY 634 provide a per-connection mandatory encryption mode that automatically 635 aborts a connection if ENO fails, and MAY enable SYN data in this 636 mode. 638 To satisfy the requirement of the previous paragraph, all TEPs SHOULD 639 support a normal mode of operation that avoids data in SYN-only 640 segments. An exception is TEPs intended to be disabled by default. 642 4.8. Negotiation transcript 644 To defend against attacks on encryption negotiation itself, a TEP 645 MUST with high probability fail to establish a working connection 646 between two ENO-compliant hosts when SYN-form ENO options have been 647 altered in transit. (Of course, in the absence of endpoint 648 authentication, two compliant hosts can each still be connected to a 649 man-in-the-middle attacker.) To detect SYN-form ENO option 650 tampering, TEPs must reference a transcript of TCP-ENO's negotiation. 652 TCP-ENO defines its negotiation transcript as a packed data structure 653 consisting of two TCP-ENO options exactly as they appeared in the TCP 654 header (including the TCP option kind and TCP option length byte as 655 illustrated in Figure 1). The transcript is constructed from the 656 following, in order: 658 1. The TCP-ENO option in host A's SYN segment, including the kind 659 and length bytes. 661 2. The TCP-ENO option in host B's SYN segment, including the kind 662 and length bytes. 664 Note that because the ENO options in the transcript contain length 665 bytes as specified by TCP, the transcript unambiguously delimits A's 666 and B's ENO options. 668 5. Requirements for TEPs 670 TCP-ENO affords TEP specifications a large amount of design 671 flexibility. However, to abstract TEP differences away from 672 applications requires fitting them all into a coherent framework. As 673 such, any TEP claiming an ENO TEP identifier MUST satisfy the 674 following normative list of properties. 676 o TEPs MUST protect TCP data streams with authenticated encryption. 677 (Note "authenticated encryption" designates the REQUIRED form 678 encryption algorithm [RFC5116]; it does not imply any actual 679 endpoint authentication.) 681 o TEPs MUST define a session ID whose value identifies the TCP 682 connection and, with overwhelming probability, is unique over all 683 time if either host correctly obeys the TEP. Section 5.1 684 describes the requirements of the session ID in more detail. 686 o TEPs MUST NOT permit the negotiation of any encryption algorithms 687 with significantly less than 128-bit security. 689 o TEPs MUST NOT allow the negotiation of null cipher suites, even 690 for debugging purposes. (Implementations MAY support debugging 691 modes that allow applications to extract their own session keys.) 693 o TEPs MUST NOT depend on long-lived secrets for data 694 confidentiality, as implementations SHOULD provide forward secrecy 695 some bounded, short time after the close of a TCP connection. 696 (Exceptions to forward secrecy are permissible only at the 697 implementation level, and only in response to hardware or 698 architectural constraints--e.g., storage that cannot be securely 699 erased.) 701 o TEPs MUST protect and authenticate the end-of-file marker conveyed 702 by TCP's FIN flag. In particular, a receiver MUST with high 703 probability detect a FIN flag that was set or cleared in transit 704 and does not match the sender's intent. A TEP MAY discard a 705 segment with such a corrupted FIN bit, or may abort the connection 706 in response to such a segment. However, any such abort MUST raise 707 an error condition distinct from an authentic end-of-file 708 condition. 710 o TEPs MUST prevent corrupted packets from causing urgent data to be 711 delivered when none has been sent. A TEP MAY do so by 712 cryptographically protecting the URG flag and urgent pointer 713 alongside ordinary payload data. Alternatively, a TEP MAY disable 714 urgent data functionality by clearing the URG flag on all received 715 segments and returning errors in response to sender-side urgent- 716 data API calls. Implementations SHOULD avoid negotiating TEPs 717 that disable urgent data by default. The exception is when 718 applications and protocols are known never to send urgent data. 720 5.1. Session IDs 722 Each TEP MUST define a session ID that is computable by both 723 endpoints and uniquely identifies each encrypted TCP connection. 724 Implementations MUST expose the session ID to applications via an API 725 extension. The API extension MUST return an error when no session ID 726 is available because ENO has failed to negotiate encryption or 727 because no connection is yet established. Applications that are 728 aware of TCP-ENO SHOULD, when practical, authenticate the TCP 729 endpoints by incorporating the values of the session ID and TCP-ENO 730 role (A or B) into higher-layer authentication mechanisms. 732 In order to avoid replay attacks and prevent authenticated session 733 IDs from being used out of context, session IDs MUST be unique over 734 all time with high probability. This uniqueness property MUST hold 735 even if one end of a connection maliciously manipulates the protocol 736 in an effort to create duplicate session IDs. In other words, it 737 MUST be infeasible for a host, even by violating the TEP 738 specification, to establish two TCP connections with the same session 739 ID to remote hosts properly implementing the TEP. 741 To prevent session IDs from being confused across TEPs, all session 742 IDs begin with the negotiated TEP identifier--that is, the last valid 743 TEP identifier in host B's SYN segment. Futhermore, this initial 744 byte has bit "v" set to the same value that accompanied the 745 negotiated TEP identifier in B's SYN segment. However, only this 746 single byte is included, not any suboption data. Figure 8 shows the 747 resulting format. This format is designed for TEPs to compute unique 748 identifiers; it is not intended for application authors to pick apart 749 session IDs. Applications SHOULD treat session IDs as monolithic 750 opaque values and SHOULD NOT discard the first byte to shorten 751 identifiers. (An exception is for non-security-relevant purposes, 752 such as gathering statistics about negotiated TEPs.) 754 byte 0 1 2 N-1 N 755 +-----+------------...------------+ 756 | sub-| collision-resistant hash | 757 | opt | of connection information | 758 +-----+------------...------------+ 760 Figure 8: Format of a session ID 762 Though TEP specifications retain considerable flexibility in their 763 definitions of the session ID, all session IDs MUST meet the 764 following normative list of requirements: 766 o The session ID MUST be at least 33 bytes (including the one-byte 767 suboption), though TEPs MAY choose longer session IDs. 769 o The session ID MUST depend in a collision-resistant way on all of 770 the following (meaning it is computationally infeasible to produce 771 collisions of the session ID derivation function unless all of the 772 following quantities are identical): 774 * Fresh data contributed by both sides of the connection, 776 * Any public keys, public Diffie-Hellman parameters, or other 777 public asymmetric cryptographic parameters that are employed by 778 the TEP and have corresponding private data that is known by 779 only one side of the connection, and 781 * The negotiation transcript specified in Section 4.8. 783 o Unless and until applications disclose information about the 784 session ID, all but the first byte MUST be computationally 785 indistinguishable from random bytes to a network eavesdropper. 787 o Applications MAY choose to make session IDs public. Therefore, 788 TEPs MUST NOT place any confidential data in the session ID (such 789 as data permitting the derivation of session keys). 791 6. Examples 793 This subsection illustrates the TCP-ENO handshake with a few non- 794 normative examples. 796 (1) A -> B: SYN ENO 797 (2) B -> A: SYN-ACK ENO 798 (3) A -> B: ACK ENO<> 799 [rest of connection encrypted according to TEP Y] 801 Figure 9: Three-way handshake with successful TCP-ENO negotiation 803 Figure 9 shows a three-way handshake with a successful TCP-ENO 804 negotiation. Host A includes two ENO suboptions with TEP identifiers 805 X and Y. The two sides agree to follow the TEP identified by 806 suboption Y. 808 (1) A -> B: SYN ENO 809 (2) B -> A: SYN-ACK 810 (3) A -> B: ACK 811 [rest of connection unencrypted legacy TCP] 813 Figure 10: Three-way handshake with failed TCP-ENO negotiation 815 Figure 10 shows a failed TCP-ENO negotiation. The active opener (A) 816 indicates support for TEPs corresponding to suboptions X and Y. 817 Unfortunately, at this point one of several things occurs: 819 1. The passive opener (B) does not support TCP-ENO, 821 2. B supports TCP-ENO, but supports neither of TEPs X and Y, and so 822 does not reply with an ENO option, 824 3. B supports TCP-ENO, but has the connection configured in 825 mandatory application-aware mode and thus disables ENO because 826 A's SYN segment does not set the application-aware bit, or 828 4. The network stripped the ENO option out of A's SYN segment, so B 829 did not receive it. 831 Whichever of the above applies, the connection transparently falls 832 back to unencrypted TCP. 834 (1) A -> B: SYN ENO 835 (2) B -> A: SYN-ACK ENO [ENO stripped by middlebox] 836 (3) A -> B: ACK 837 [rest of connection unencrypted legacy TCP] 839 Figure 11: Failed TCP-ENO negotiation because of option stripping 841 Figure 11 Shows another handshake with a failed encryption 842 negotiation. In this case, the passive opener B receives an ENO 843 option from A and replies. However, the reverse network path from B 844 to A strips ENO options. Hence, A does not receive an ENO option 845 from B, disables ENO, and does not include a non-SYN-form ENO option 846 in segment 3 when ACKing B's SYN. Had A not disabled encryption, 847 Section 4.6 would have required it to include a non-SYN ENO option in 848 segment 3. The omission of this option informs B that encryption 849 negotiation has failed, after which the two hosts proceed with 850 unencrypted TCP. 852 (1) A -> B: SYN ENO 853 (2) B -> A: SYN ENO 854 (3) A -> B: SYN-ACK ENO 855 (4) B -> A: SYN-ACK ENO 856 [rest of connection encrypted according to TEP Y] 858 Figure 12: Simultaneous open with successful TCP-ENO negotiation 860 Figure 12 shows a successful TCP-ENO negotiation with simultaneous 861 open. Here the first four segments contain a SYN-form ENO option, as 862 each side sends both a SYN-only and a SYN-ACK segment. The ENO 863 option in each host's SYN-ACK is identical to the ENO option in its 864 SYN-only segment, as otherwise connection establishment could not 865 recover from the loss of a SYN segment. The last valid TEP in host 866 B's ENO option is Y, so Y is the negotiated TEP. 868 7. Future developments 870 TCP-ENO is designed to capitalize on future developments that could 871 alter trade-offs and change the best approach to TCP-level encryption 872 (beyond introducing new cipher suites). By way of example, we 873 discuss a few such possible developments. 875 Various proposals exist to increase the maximum space for options in 876 the TCP header. These proposals are highly experimental-- 877 particularly those that apply to SYN segments. Hence, future TEPs 878 are unlikely to to benefit from extended SYN option space. In the 879 unlikely event that SYN option space is one day extended, however, 880 future TEPs could benefit by embedding key agreement messages 881 directly in SYN segments. Under such usage, the 32-byte limit on 882 length bytes could prove insufficient. This draft intentionally 883 aborts TCP-ENO if a length byte is followed by an octet in the range 884 0x00-0x9f. If necessary, a future update to this document can define 885 a format for larger suboptions by assigning meaning to such currently 886 undefined byte sequences. 888 New revisions to socket interfaces [RFC3493] could involve library 889 calls that simultaneously have access to hostname information and an 890 underlying TCP connection. Such an API enables the possibility of 891 authenticating servers transparently to the application, particularly 892 in conjunction with technologies such as DANE [RFC6394]. An update 893 to TCP-ENO can adopt one of the "z" bits in the global suboption to 894 negotiate the use of an endpoint authentication protocol before any 895 application use of the TCP connection. Over time, the consequences 896 of failed or missing endpoint authentication can gradually be 897 increased from issuing log messages to aborting the connection if 898 some as yet unspecified DNS record indicates authentication is 899 mandatory. Through shared library updates, such endpoint 900 authentication can potentially be added transparently to legacy 901 applications without recompilation. 903 TLS can currently only be added to legacy applications whose 904 protocols accommodate a STARTTLS command or equivalent. TCP-ENO, 905 because it provides out-of-band signaling, opens the possibility of 906 future TLS revisions being generically applicable to any TCP 907 application. 909 8. Design rationale 911 This section describes some of the design rationale behind TCP-ENO. 913 8.1. Handshake robustness 915 Incremental deployment of TCP-ENO depends critically on failure cases 916 devolving to unencrypted TCP rather than causing the entire TCP 917 connection to fail. 919 Because a network path may drop ENO options in one direction only, a 920 host must know not just that the peer supports encryption, but that 921 the peer has received an ENO option. To this end, ENO disables 922 encryption unless it receives an ACK segment bearing an ENO option. 923 To stay robust in the face of dropped segments, hosts continue to 924 include non-SYN form ENO options in segments until such point as they 925 have received a non-SYN segment from the other side. 927 One particularly pernicious middlebox behavior found in the wild is 928 load balancers that echo unknown TCP options found in SYN segments 929 back to an active opener. The passive role bit "b" in global 930 suboptions ensures encryption will always be disabled under such 931 circumstances, as sending back a verbatim copy of an active opener's 932 SYN-form ENO option always causes role negotiation to fail. 934 8.2. Suboption data 936 TEPs can employ suboption data for session caching, cipher suite 937 negotiation, or other purposes. However, TCP currently limits total 938 option space consumed by all options to only 40 bytes, making it 939 impractical to have many suboptions with data. For this reason, ENO 940 optimizes the case of a single suboption with data by inferring the 941 length of the last suboption from the TCP option length. Doing so 942 saves one byte. 944 8.3. Passive role bit 946 TCP-ENO, TEPs, and applications all have asymmetries that require an 947 unambiguous way to identify one of the two connection endpoints. As 948 an example, Section 4.8 specifies that host A's ENO option comes 949 before host B's in the negotiation transcript. As another example, 950 an application might need to authenticate one end of a TCP connection 951 with a digital signature. To ensure the signed message cannot not be 952 interpreted out of context to authenticate the other end, the signed 953 message would need to include both the session ID and the local role, 954 A or B. 956 A normal TCP three-way handshake involves one active and one passive 957 opener. This asymmetry is captured by the default configuration of 958 the "b" bit in the global suboption. With simultaneous open, both 959 hosts are active openers, so TCP-ENO requires that one host 960 explicitly configure "b = 1". An alternate design might 961 automatically break the symmetry to avoid this need for explicit 962 configuration. However, all such designs we considered either lacked 963 robustness or consumed precious bytes of SYN option space even in the 964 absence of simultaneous open. (One complicating factor is that TCP 965 does not know it is participating in a simultaneous open until after 966 it has sent a SYN segment. Moreover, with packet loss, one host 967 might never learn it has participated in a simultaneous open.) 969 8.4. Use of ENO option kind by TEPs 971 This draft does not specify the use of ENO options beyond the first 972 few segments of a connection. Moreover, it does not specify the 973 content of ENO options in non-SYN segments, only their presence. As 974 a result, any use of option kind TBD after the SYN exchange does not 975 conflict with this document. Because, in addition, ENO guarantees at 976 most one negotiated TEP per connection, TEPs will not conflict with 977 one another or ENO if they use ENO's option kind for out-of-band 978 signaling in non-SYN segments. 980 9. Experiments 982 This document has experimental status because TCP-ENO's viability 983 depends on middlebox behavior that can only be determined _a 984 posteriori_. Specifically, we must determine to what extent 985 middleboxes will permit the use of TCP-ENO. Once TCP-ENO is 986 deployed, we will be in a better position to gather data on two types 987 of failure: 989 1. Middleboxes downgrading TCP-ENO connections to unencrypted TCP. 990 This can happen if middleboxes strip unknown TCP options or if 991 they terminate TCP connections and relay data back and forth. 993 2. Middleboxes causing TCP-ENO connections to fail completely. This 994 can happen if middleboxes perform deep packet inspection and 995 start dropping segments that unexpectedly contain ciphertext, or 996 if middleboxes strip ENO options from non-SYN segments after 997 allowing them in SYN segments. 999 The first type of failure is tolerable since TCP-ENO is designed for 1000 incremental deployment anyway. The second type of failure is more 1001 problematic, and, if prevalent, will require the development of 1002 techniques to avoid and recover from such failures. 1004 10. Security considerations 1006 An obvious use case for TCP-ENO is opportunistic encryption--that is, 1007 encrypting some connections, but only where supported and without any 1008 kind of endpoint authentication. Opportunistic encryption protects 1009 against undetectable large-scale eavesdropping. However, it does not 1010 protect against detectable large-scale eavesdropping (for instance, 1011 if ISPs terminate TCP connections and proxy them, or simply downgrade 1012 connections to unencrypted). Moreover, opportunistic encryption 1013 emphatically does not protect against targeted attacks that employ 1014 trivial spoofing to redirect a specific high-value connection to a 1015 man-in-the-middle attacker. 1017 Achieving stronger security with TCP-ENO requires verifying session 1018 IDs. Any application relying on ENO for communications security MUST 1019 incorporate session IDs into its endpoint authentication. By way of 1020 example, an authentication mechanism based on keyed digests (such as 1021 Digest Access Authentication [RFC7616]) can be extended to include 1022 the role and session ID in the input of the keyed digest. Higher- 1023 layer protocols MAY use the application-aware "a" bit to negotiate 1024 the inclusion of session IDs in authentication even when there is no 1025 in-band way to carry out such a negotiation. Because there is only 1026 one "a" bit, however, a protocol extension that specifies use of the 1027 "a" bit will likely require a built-in versioning or negotiation 1028 mechanism to accommodate crypto agility and future updates. 1030 Because TCP-ENO enables multiple different TEPs to coexist, security 1031 could potentially be only as strong as the weakest available TEP. In 1032 particular, if session IDs do not depend on the TCP-ENO transcript in 1033 a strong way, an attacker can undetectably tamper with ENO options to 1034 force negotiation of a deprecated and vulnerable TEP. To avoid such 1035 problems, TEPs MUST compute session IDs using only well-studied and 1036 conservative hash functions. That way, even if other parts of a TEP 1037 are vulnerable, it is still intractable for an attacker to induce 1038 identical session IDs at both ends after tampering with ENO contents 1039 in SYN segments. 1041 Implementations MUST NOT send ENO options unless they have access to 1042 an adequate source of randomness [RFC4086]. Without secret 1043 unpredictable data at both ends of a connection, it is impossible for 1044 TEPs to achieve confidentiality and forward secrecy. Because systems 1045 typically have very little entropy on bootup, implementations might 1046 need to disable TCP-ENO until after system initialization. 1048 With a regular three-way handshake (meaning no simultaneous open), 1049 the non-SYN form ENO option in an active opener's first ACK segment 1050 MAY contain N > 0 bytes of TEP-specific data, as shown in Figure 3. 1051 Such data is not part of the TCP-ENO negotiation transcript, and 1052 hence MUST be separately authenticated by the TEP. 1054 11. IANA Considerations 1056 [RFC-editor: please replace TBD in this section, in Section 4.1, and 1057 in Section 8.4 with the assigned option kind number. Please also 1058 replace RFC-TBD with this document's final RFC number.] 1060 This document defines a new TCP option kind for TCP-ENO, assigned a 1061 value of TBD from the TCP option space. This value is defined as: 1063 +------+--------+----------------------------------+-----------+ 1064 | Kind | Length | Meaning | Reference | 1065 +------+--------+----------------------------------+-----------+ 1066 | TBD | N | Encryption Negotiation (TCP-ENO) | [RFC-TBD] | 1067 +------+--------+----------------------------------+-----------+ 1069 TCP Option Kind Numbers 1071 Early implementations of TCP-ENO and a predecessor TCP encryption 1072 protocol made unauthorized use of TCP option kind 69. 1074 [RFC-editor: please glue the following text to the previous paragraph 1075 iff TBD == 69, otherwise delete it.] These earlier uses of option 69 1076 are not compatible with TCP-ENO and could disable encryption or 1077 suffer complete connection failure when interoperating with TCP-ENO- 1078 compliant hosts. Hence, legacy use of option 69 MUST be disabled on 1079 hosts that cannot be upgraded to TCP-ENO. 1081 [RFC-editor: please glue this to the previous paragraph regardless of 1082 the value of TBD.] More recent implementations used experimental 1083 option 253 per [RFC6994] with 16-bit ExID 0x454E, and MUST migrate to 1084 option TBD. Section 4.1 requires at most one SYN-form ENO option per 1085 segment, which means hosts MUST NOT not include both option TBD and 1086 option 253 with ExID 0x454E in the same TCP segment. 1088 This document defines a 7-bit "glt" field in the range of 0x20-0x7f, 1089 for which IANA is to create and maintain a new registry entitled "TCP 1090 encryption protocol identifiers" under the "Transmission Control 1091 Protocol (TCP) Parameters" registry. The initial contents of the TCP 1092 encryption protocol identifier registry is shown in Table 2, 1093 reflecting that this document reserves one TEP identifier for 1094 experimental use. Subsequent assignments are to be made under the 1095 "RFC Required" policy detailed in [RFC8126], relying on early 1096 allocation [RFC7120] to facilitate testing before an RFC is 1097 finalized. 1099 +-------+------------------+-----------+ 1100 | Value | Meaning | Reference | 1101 +-------+------------------+-----------+ 1102 | 0x20 | Experimental Use | [RFC-TBD] | 1103 +-------+------------------+-----------+ 1105 Table 2: TCP encryption protocol identifiers 1107 12. Acknowledgments 1109 We are grateful for contributions, help, discussions, and feedback 1110 from the TCPINC working group, including Marcelo Bagnulo, David 1111 Black, Bob Briscoe, Jake Holland, Jana Iyengar, Tero Kivinen, Mirja 1112 Kuhlewind, Yoav Nir, Christoph Paasch, Eric Rescorla, Kyle Rose, 1113 Michael Scharf, and Joe Touch. This work was partially funded by 1114 DARPA CRASH and the Stanford Secure Internet of Things Project. 1116 13. Contributors 1118 Dan Boneh was a co-author of the draft that became this document. 1120 14. References 1122 14.1. Normative References 1124 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1125 RFC 793, DOI 10.17487/RFC0793, September 1981, 1126 . 1128 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1129 Requirement Levels", BCP 14, RFC 2119, 1130 DOI 10.17487/RFC2119, March 1997, . 1133 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 1134 "Randomness Requirements for Security", BCP 106, RFC 4086, 1135 DOI 10.17487/RFC4086, June 2005, . 1138 [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code 1139 Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 1140 2014, . 1142 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1143 Writing an IANA Considerations Section in RFCs", BCP 26, 1144 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1145 . 1147 14.2. Informative References 1149 [RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. 1150 Stevens, "Basic Socket Interface Extensions for IPv6", 1151 RFC 3493, DOI 10.17487/RFC3493, February 2003, 1152 . 1154 [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common 1155 Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, 1156 . 1158 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated 1159 Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, 1160 . 1162 [RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P. 1163 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 1164 RFC 5382, DOI 10.17487/RFC5382, October 2008, 1165 . 1167 [RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based 1168 Authentication of Named Entities (DANE)", RFC 6394, 1169 DOI 10.17487/RFC6394, October 2011, . 1172 [RFC6994] Touch, J., "Shared Use of Experimental TCP Options", 1173 RFC 6994, DOI 10.17487/RFC6994, August 2013, 1174 . 1176 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1177 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1178 . 1180 [RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP 1181 Digest Access Authentication", RFC 7616, 1182 DOI 10.17487/RFC7616, September 2015, . 1185 Authors' Addresses 1187 Andrea Bittau 1188 Google 1189 345 Spear Street 1190 San Francisco, CA 94105 1191 US 1193 Email: bittau@google.com 1195 Daniel B. Giffin 1196 Stanford University 1197 353 Serra Mall, Room 288 1198 Stanford, CA 94305 1199 US 1201 Email: dbg@scs.stanford.edu 1203 Mark Handley 1204 University College London 1205 Gower St. 1206 London WC1E 6BT 1207 UK 1209 Email: M.Handley@cs.ucl.ac.uk 1211 David Mazieres 1212 Stanford University 1213 353 Serra Mall, Room 290 1214 Stanford, CA 94305 1215 US 1217 Email: dm@uun.org 1218 Eric W. Smith 1219 Kestrel Institute 1220 3260 Hillview Avenue 1221 Palo Alto, CA 94304 1222 US 1224 Email: eric.smith@kestrel.edu