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Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Missing Reference: 'This-RFC' is mentioned on line 1443, but not defined ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) ** Obsolete normative reference: RFC 6824 (Obsoleted by RFC 8684) == Outdated reference: A later version (-03) exists of draft-boucadair-tcpm-dhc-converter-02 == Outdated reference: A later version (-11) exists of draft-olteanu-intarea-socks-6-06 -- Obsolete informational reference (is this intentional?): RFC 1323 (Obsoleted by RFC 7323) Summary: 3 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TCPM Working Group O. Bonaventure, Ed. 3 Internet-Draft Tessares 4 Intended status: Experimental M. Boucadair, Ed. 5 Expires: December 13, 2019 Orange 6 S. Gundavelli 7 Cisco 8 S. Seo 9 Korea Telecom 10 B. Hesmans 11 Tessares 12 June 11, 2019 14 0-RTT TCP Convert Protocol 15 draft-ietf-tcpm-converters-07 17 Abstract 19 This document specifies an application proxy, called Transport 20 Converter, to assist the deployment of TCP extensions such as 21 Multipath TCP. This proxy is designed to avoid inducing extra delay 22 when involved in a network-assisted connection (that is, 0-RTT). 24 This specification assumes an explicit model, where the proxy is 25 explicitly configured on hosts. 27 -- Editorial Note (To be removed by RFC Editor) 29 Please update these statements with the RFC number to be assigned to 30 this document: [This-RFC] 32 Please update TBA statements with the port number to be assigned to 33 the 0-RTT TCP Convert Protocol. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at https://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on December 13, 2019. 51 Copyright Notice 53 Copyright (c) 2019 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (https://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5 70 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 6 71 3.1. Functional Elements . . . . . . . . . . . . . . . . . . . 6 72 3.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 8 73 3.3. Sample Examples of Outgoing Converter-Assisted Multipath 74 TCP Connections . . . . . . . . . . . . . . . . . . . . . 11 75 3.4. Sample Example of Incoming Converter-Assisted Multipath 76 TCP Connection . . . . . . . . . . . . . . . . . . . . . 12 77 4. The Convert Protocol (Convert) . . . . . . . . . . . . . . . 13 78 4.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 13 79 4.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 14 80 4.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 14 81 4.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 15 82 4.2.3. The Info TLV . . . . . . . . . . . . . . . . . . . . 16 83 4.2.4. Supported TCP Extensions TLV . . . . . . . . . . . . 16 84 4.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 17 85 4.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 19 86 4.2.7. The Cookie TLV . . . . . . . . . . . . . . . . . . . 19 87 4.2.8. Error TLV . . . . . . . . . . . . . . . . . . . . . . 20 88 5. Compatibility of Specific TCP Options with the Conversion 89 Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 90 5.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 23 91 5.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 24 92 5.3. Selective Acknowledgements . . . . . . . . . . . . . . . 24 93 5.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 25 94 5.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 25 95 5.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 25 96 5.7. TCP User Timeout . . . . . . . . . . . . . . . . . . . . 26 97 5.8. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 26 98 5.9. TCP Experimental Options . . . . . . . . . . . . . . . . 26 99 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 26 100 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27 101 7.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 27 102 7.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 28 103 7.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 29 104 7.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 29 105 7.5. Multipath TCP-specific Considerations . . . . . . . . . . 29 106 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 107 8.1. Convert Service Port Number . . . . . . . . . . . . . . . 30 108 8.2. The Convert Protocol (Convert) Parameters . . . . . . . . 30 109 8.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 30 110 8.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 31 111 8.2.3. Convert Error Messages . . . . . . . . . . . . . . . 31 112 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 113 9.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 33 114 10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 34 115 11. Example Socket API Changes to Support the 0-RTT Convert 116 Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 35 117 11.1. Active Open (Client Side) . . . . . . . . . . . . . . . 35 118 11.2. Passive Open (Converter Side) . . . . . . . . . . . . . 36 119 12. Differences with SOCKSv5 . . . . . . . . . . . . . . . . . . 37 120 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 121 13.1. Normative References . . . . . . . . . . . . . . . . . . 39 122 13.2. Informative References . . . . . . . . . . . . . . . . . 41 123 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44 125 1. Introduction 127 Transport protocols like TCP evolve regularly [RFC7414]. TCP has 128 been improved in different ways. Some improvements such as changing 129 the initial window size [RFC6928] or modifying the congestion control 130 scheme can be applied independently on clients and servers. Other 131 improvements such as Selective Acknowledgements [RFC2018] or large 132 windows [RFC7323] require a new TCP option or to change the semantics 133 of some fields in the TCP header. These modifications must be 134 deployed on both clients and servers to be actually used on the 135 Internet. Experience with the latter TCP extensions reveals that 136 their deployment can require many years. Fukuda reports in 137 [Fukuda2011] results of a decade of measurements showing the 138 deployment of Selective Acknowledgements, Window Scale and TCP 139 Timestamps. [ANRW17] describes measurements showing that TCP Fast 140 Open (TFO) [RFC7413] is still not widely deployed. 142 There are some situations where the transport stack used on clients 143 (or servers) can be upgraded at a faster pace than the transport 144 stack running on servers (or clients). In those situations, clients 145 would typically want to benefit from the features of an improved 146 transport protocol even if the servers have not yet been upgraded and 147 conversely. Performance Enhancing Proxies [RFC3135], and other 148 service functions have been deployed as solutions to improve TCP 149 performance over links with specific characteristics. 151 Recent examples of TCP extensions include Multipath TCP [RFC6824] or 152 TCPINC [RFC8548]. Those extensions provide features that are 153 interesting for clients such as wireless devices. With Multipath 154 TCP, those devices could seamlessly use WLAN (Wireless Local Area 155 Network) and cellular networks, for bonding purposes, faster 156 handovers, or better resiliency. Unfortunately, deploying those 157 extensions on both a wide range of clients and servers remains 158 difficult. 160 More recently, experimentation of 5G bonding, which has very scarce 161 coverage, has been conducted into global range of the incumbent 4G 162 (LTE) connectivity in newly devised clients using Multipath TCP 163 proxy. Even if the 5G and the 4G bonding by using Multipath TCP 164 increases the bandwidth, it is as well crucial to minimize latency 165 for all the way between endhosts regardless of whether intermediate 166 nodes are inside or outside of the mobile core. In order to handle 167 uRLLC (Ultra-Reliable Low-Latency Communication) for the next 168 generation mobile network, Multipath TCP and its proxy mechanism such 169 as the one used to provide Access Taffic Steering, Switching, and 170 Splitting (ATSSS) must be optimised to reduce latency. 172 This document specifies an application proxy, called Transport 173 Converter. A Transport Converter is a function that is installed by 174 a network operator to aid the deployment of TCP extensions and to 175 provide the benefits of such extensions to clients. A Transport 176 Converter may provide conversion service for one or more TCP 177 extensions. Which TCP extensions are eligible to the conversion 178 service is deployment-specific. The conversion service is provided 179 by means of the 0-RTT TCP Convert Protocol (Convert), that is an 180 application-layer protocol which uses TCP port number TBA 181 (Section 8). 183 The Transport Converter adheres to the main principles drawn in 184 [RFC1919]. In particular, a Transport Converter achieves the 185 following: 187 o Listen for client sessions; 189 o Receive from a client the address of the final target server; 191 o Setup a session to the final server; 192 o Relay control messages and data between the client and the server; 194 o Perform access controls according to local policies. 196 The main advantage of network-assisted conversion services is that 197 they enable new TCP extensions to be used on a subset of the path 198 between endpoints, which encourages the deployment of these 199 extensions. Furthermore, the Transport Converter allows the client 200 and the server to directly negotiate TCP options for the sake of 201 native support along the full path. 203 The Convert Protocol is a generic mechanism to provide 0-RTT 204 conversion service. As a sample applicability use case, this 205 document specifies how the Convert Protocol applies for Multipath 206 TCP. It is out of scope of this document to provide a comprehensive 207 list of all potential conversion services. Applicability documents 208 may be defined in the future. 210 This document does not assume that all the traffic is eligible to the 211 network-assisted conversion service. Only a subset of the traffic 212 will be forwarded to a Transport Converter according to a set of 213 policies. These policies, and how they are communicated to 214 endpoints, are out of scope. Furthermore, it is possible to bypass 215 the Transport Converter to connect directly to the servers that 216 already support the required TCP extension(s). 218 This document assumes an explicit model in which a client is 219 configured with one or a list of Transport Converters (statically or 220 through protocols such as [I-D.boucadair-tcpm-dhc-converter]). 221 Configuration means are outside the scope of this document. 223 This document is organized as follows. We first provide a brief 224 explanation of the operation of Transport Converters in Section 3. 225 We describe the Convert Protocol in Section 4. We discuss in 226 Section 5 how Transport Converters can be used to support different 227 TCP extensions. We then discuss the interactions with middleboxes 228 (Section 6) and the security considerations (Section 7). 230 Appendix A discusses how a TCP stack would need to support the 231 protocol described in this document. Appendix B provides a 232 comparison with SOCKS proxies that are already used to deploy 233 Multipath TCP in some cellular networks (Section 2.2 of [RFC8041]). 235 2. Requirements 237 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 238 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 239 "OPTIONAL" in this document are to be interpreted as described in 241 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, 242 as shown here. 244 3. Architecture 246 3.1. Functional Elements 248 The Convert Protocol considers three functional elements: 250 o Clients; 252 o Transport Converters; 254 o Servers. 256 A Transport Converter is a network function that relays all data 257 exchanged over one upstream connection to one downstream connection 258 and vice versa (Figure 1). The Transport Converter, thus, maintains 259 state that associates one upstream connection to a corresponding 260 downstream connection. 262 A connection can be initiated from both sides of the Transport 263 Converter (Internet-facing interface, client-facing interface). 265 | 266 : 267 | 268 +------------+ 269 client <- upstream ->| Transport |<- downstream ->server 270 | Converter | 271 +------------+ 272 | 273 client-facing interface : Internet-facing interface 274 | 276 Figure 1: A Transport Converter Relays Data between Pairs of TCP 277 Connections 279 Transport Converters can be operated by network operators or third 280 parties. Nevertheless, this document focuses on the single 281 administrative deployment case where the entity offering the 282 connectivity service to a client is also the entity which owns and 283 operates the Transport Converter. 285 A Transport Converter can be embedded in a standalone device or be 286 activated as a service on a router. How such function is enabled is 287 deployment-specific. A sample deployment is depicted in Figure 2. 289 +-+ +-+ +-+ 290 Client - |R| -- |R| -- |R| - - - Server 291 +-+ +-+ +-+ 292 | 293 +-+ 294 |R| 295 +-+ 296 | 297 +---------+ 298 |Transport| 299 |Converter| 300 +---------+ 302 Figure 2: A Transport Converter Can Be Installed Anywhere in the 303 Network 305 The architecture assumes that new software will be installed on the 306 Client hosts to interact with one or more Transport Converters. 307 Further, the architecture allows for making use of new TCP extensions 308 even if those are not supported by a given server. 310 The Client is configured, through means that are outside the scope of 311 this document, with the names and/or the addresses of one or more 312 Transport Converters and the TCP extensions that they support. The 313 procedure for selecting a Transport Converter among a list of 314 configured Transport Converters is outside the scope of this 315 document. 317 One of the benefits of this design is that different transport 318 protocol extensions can be used on the upstream and the downstream 319 connections. This encourages the deployment of new TCP extensions 320 until they are widely supported by servers, in particular. 322 The architecture does not mandate anything on the server side. 324 Similar to address sharing mechanisms, the architecture does not 325 interfere with end-to-end TLS connections [RFC8446] between the 326 Client and the Server (Figure 3). In other words, end-to-end TLS is 327 supported in the presence of a Converter. 329 Client Transport Server 330 | Converter | 331 | | | 332 /==========================================\ 333 | End-to-end TLS | 334 \==========================================/ 336 * TLS messages exhanged between the Client 337 and the Server are not shown. 339 Figure 3: End-to-end TLS via a Transport Converter 341 It is out of scope of this document to elaborate on specific 342 considerations related to the use of TLS in the Client-Converter 343 connection leg to exchange Convert TLVs (in addition to the end-to- 344 end TLS connection). 346 3.2. Theory of Operation 348 At a high level, the objective of the Transport Converter is to allow 349 the use a specific extension, e.g., Multipath TCP, on a subset of the 350 path even if the peer does not support this extension. This is 351 illustrated in Figure 4 where the Client initiates a Multipath TCP 352 connection with the Transport Converter (packets belonging to the 353 Multipath TCP connection are shown with "===") while the Transport 354 Converter uses a regular TCP connection with the Server. 356 Client Transport Server 357 | Converter | 358 | | | 359 |==================>|--------------------->| 360 | | | 361 |<==================|<---------------------| 362 | | | 363 Multipath TCP packets Regular TCP packets 365 Figure 4: An Example of 0-RTT Network-Assisted MPTCP Connection 367 The packets belonging to the pair of connections between the Client 368 and Server passing through a Transport Converter may follow a 369 different path than the packets directly exchanged between the Client 370 and the Server. Deployments should minimize the possible additional 371 delay by carefully selecting the location of the Transport Converter 372 used to reach a given destination. 374 When establishing a connection, the Client can, depending on local 375 policies, either contact the Server directly (e.g., by sending a TCP 376 SYN towards the Server) or create the connection via a Transport 377 Converter. In the latter case (that is, the conversion service is 378 used), the Client initiates a connection towards the Transport 379 Converter and indicates the IP address and port number of the Server 380 within the connection establishment packet. Doing so enables the 381 Transport Converter to immediately initiate a connection towards that 382 Server, without experiencing an extra delay. The Transport Converter 383 waits until the receipt of the confirmation that the Server agrees to 384 establish the connection before confirming it to the Client. 386 The client places the destination address and port number of the 387 Server in the payload of the SYN sent to the Transport Converter to 388 minimize connection establishment delays. In accordance with 389 [RFC1919], the Transport Converter maintains two connections that are 390 combined together: 392 o the upstream connection is the one between the Client and the 393 Transport Converter. 395 o the downstream connection is between the Transport Converter and 396 the Server. 398 Any user data received by the Transport Converter over the upstream 399 (or downstream) connection is relayed over the downstream (or 400 upstream) connection. In particular, if the initial SYN message 401 contains data in its payload (e.g., [RFC7413]), that data MUST be 402 placed right after the Convert TLVs when generating the relayed SYN. 404 Figure 5 illustrates the establishment of an outbound TCP connection 405 by a Client through a Transport Converter. The information shown 406 between brackets denotes Convert Protocol messages described in 407 Section 4. 409 Transport 410 Client Converter Server 411 | | | 412 |SYN [->Server:port]| SYN | 413 |------------------>|--------------------->| 414 |<------------------|<---------------------| 415 | SYN+ACK [ ] | SYN+ACK | 416 | | | 418 Figure 5: Establishment of a TCP Connection Through a Transport 419 Converter (1) 421 The Client sends a SYN destined to the Transport Converter. The 422 payload of this SYN contains the address and port number of the 423 Server. The Transport Converter does not reply immediately to this 424 SYN. It first tries to create a TCP connection towards the target 425 Server. If this upstream connection succeeds, the Transport 426 Converter confirms the establishment of the connection to the Client 427 by returning a SYN+ACK and the first bytes of the bytestream contain 428 information about the TCP options that were negotiated with the 429 Server. This information is sent at the beginning of the bytestream, 430 either directly in the SYN+ACK or in a subsequent packet. For 431 graphical reasons, the figures in this section show that the 432 Transport Converter returns this information in the SYN+ACK packet. 433 An implementation could also place this information in a packet that 434 it sent shortly after the SYN+ACK. 436 The connection can also be established from the Internet towards a 437 Client via a Transport Converter. This is typically the case when an 438 application on the Client listens to a specific port (the Client 439 hosts a server, typically). 441 A Transport Converter MAY operate in address preservation or address 442 sharing modes as discussed in Section 5.4 of 443 [I-D.nam-mptcp-deployment-considerations]. Which behavior to use by 444 a Transport Converter is deployment-specific. If address sharing 445 mode is enabled, the Transport Converter MUST adhere to REQ-2 of 446 [RFC6888] which implies a default "IP address pooling" behavior of 447 "Paired" (as defined in Section 4.1 of [RFC4787]) must be supported. 448 This behavior is meant to avoid breaking applications that depend on 449 the external address remaining constant. 451 Standard TCP ([RFC0793], Section 3.4) allows a SYN packet to carry 452 data inside its payload but forbids the receiver from delivering it 453 to the application until completion of the three-way-handshake. To 454 enable applications to exchange data in a TCP handshake, this 455 specification follows an approach similar to TCP Fast Open [RFC7413] 456 and thus removes the constraint by allowing data in SYN packets to be 457 delivered to the Transport Converter application. 459 As discussed in [RFC7413], such change to TCP semantic raises two 460 issues. First, duplicate SYNs can cause problems for some 461 applications that rely on TCP. Second, TCP suffers from SYN flooding 462 attacks [RFC4987]. TFO solves these two problems for applications 463 that can tolerate replays by using the TCP Fast Open option that 464 includes a cookie. However, the utilization of this option consumes 465 space in the limited TCP extended header. Furthermore, there are 466 situations, as noted in Section 7.3 of [RFC7413] where it is possible 467 to accept the payload of SYN packets without creating additional 468 security risks such as a network where addresses cannot be spoofed 469 and the Transport Converter only serves a set of hosts that are 470 identified by these addresses. 472 For these reasons, this specification does not mandate the use of the 473 TCP Fast Open option when the Client sends a connection establishment 474 packet towards a Transport Converter. The Convert protocol includes 475 an optional Cookie TLV that provides similar protection as the TCP 476 Fast Open option without consuming space in the extended TCP header. 478 3.3. Sample Examples of Outgoing Converter-Assisted Multipath TCP 479 Connections 481 As an example, let us consider how the Convert protocol can help the 482 deployment of Multipath TCP. We assume that both the Client and the 483 Transport Converter support Multipath TCP, but consider two different 484 cases depending on whether the Server supports Multipath TCP or not. 486 As a reminder, a Multipath TCP connection is created by placing the 487 MP_CAPABLE (MPC) option in the SYN sent by the Client. 489 Figure 6 describes the operation of the Transport Converter if the 490 Server does not support Multipath TCP. 492 Transport 493 Client Converter Server 494 |SYN, | | 495 |MPC [->Server:port]| | 496 |------------------>| SYN, MPC | 497 | |--------------------->| 498 | |<---------------------| 499 |<------------------| SYN+ACK | 500 | SYN+ACK,MPC [.] | | 501 | | | 502 |------------------>| | 503 | ACK, MPC |--------------------->| 504 | | ACK | 506 Figure 6: Establishment of a Multipath TCP Connection Through a 507 Transport Converter towards a Server that Does Not Support Multipath 508 TCP 510 The Client tries to initiate a Multipath TCP connection by sending a 511 SYN with the MP_CAPABLE option (MPC in Figure 6). The SYN includes 512 the address and port number of the target Server, that are extracted 513 and used by the Transport Converter to initiate a Multipath TCP 514 connection towards this Server. Since the Server does not support 515 Multipath TCP, it replies with a SYN+ACK that does not contain the 516 MP_CAPABLE option. The Transport Converter notes that the connection 517 with the Server does not support Multipath TCP and returns the 518 extended TCP header received from the Server to the Client. 520 Figure 7 considers a Server that supports Multipath TCP. In this 521 case, it replies to the SYN sent by the Transport Converter with the 522 MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport 523 Converter confirms the establishment of the connection to the Client 524 and indicates to the Client that the Server supports Multipath TCP. 525 With this information, the Client has discovered that the Server 526 supports Multipath TCP natively. This will enable the Client to 527 bypass the Transport Converter for the subsequent Multipath TCP 528 connections that it will initiate towards this Server. 530 Transport 531 Client Converter Server 532 |SYN, | | 533 |MPC [->Server:port]| | 534 |------------------>| SYN, MPC | 535 | |--------------------->| 536 | |<---------------------| 537 |<------------------| SYN+ACK, MPC | 538 |SYN+ACK, | | 539 |MPC [MPC supported]| | 540 |------------------>| | 541 | ACK, MPC |--------------------->| 542 | | ACK, MPC | 544 Figure 7: Establishment of a Multipath TCP Connection Through a 545 Converter Towards an MPTCP-capable Server 547 3.4. Sample Example of Incoming Converter-Assisted Multipath TCP 548 Connection 550 An example of an incoming Converter-assisted Multipath TCP connection 551 is depicted in Figure 8. In order to support incoming connections 552 from remote hosts, the Client may use PCP [RFC6887] to instruct the 553 Transport Converter to create dynamic mappings. Those mappings will 554 be used by the Transport Converter to intercept an incoming TCP 555 connection destined to the Client and convert it into a Multipath TCP 556 connection. 558 Typically, the Client sends a PCP request to the Converter asking to 559 create an explicit TCP mapping for (internal IP address, internal 560 port number). The Converter accepts the request by creating a TCP 561 mapping (internal IP address, internal port number, external IP 562 address, external port number). The external IP address and external 563 port number will be then advertised using an out-of-band mechanism so 564 that remote hosts can initiate TCP connections to the Client via the 565 Converter. Note that the external and internal information may be 566 the same. 568 Then, when the Converter receives an incoming SYN, it checks its 569 mapping table to verify if there is an active mapping matching the 570 destination IP address and destination port of that SYN. If an entry 571 is found, the Converter inserts an MP_CAPABLE option and Connect TLV 572 in the SYN packet, rewrites the source IP address to one of its IP 573 addresses and, eventually, the destination IP address and port number 574 in accordance with the information stored in the mapping. SYN-ACK 575 and ACK will be then exchanged between the Client and the Converter 576 to confirm the establishment of the initial subflow. The Client can 577 add new subflows following normal Multipath TCP procedures. 579 Transport 580 Client Converter Server 581 | | | 582 | |<-------------------| 583 |<--------------------| SYN | 584 |SYN, | | 585 |MPC[Remote Host:port]| | 586 |-------------------->| | 587 | SYN+ACK, MPC |------------------->| 588 | | SYN+ACK | 589 | |<-------------------| 590 |<--------------------| ACK | 591 | ACK, MPC | | 592 | | | 594 Figure 8: Establishment of an Incoming TCP Connection through a 595 Transport Converter 597 It is out of scope of this document to define specific Convert TLVs 598 to manage incoming connections. These TLVs can be defined in a 599 separate document. 601 4. The Convert Protocol (Convert) 603 This section describes the messages that are exchanged between a 604 Client and a Transport Converter. The Convert Protocol (Convert, for 605 short) uses a 32 bits long fixed header that is sent by both the 606 Client and the Transport Converter over each established connection. 607 This header indicates both the version of the protocol used and the 608 length of the Convert message. 610 4.1. The Convert Fixed Header 612 The Fixed Header is used to convey information about the version and 613 length of the messages exchanged between the Client and the Transport 614 Converter. 616 The Client and the Transport Converter MUST send the fixed-sized 617 header, shown in Figure 9, as the first four bytes of the bytestream. 619 1 2 3 620 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 621 +---------------+---------------+-------------------------------+ 622 | Version | Total Length | Unassigned | 623 +---------------+---------------+-------------------------------+ 625 Figure 9: The Fixed-Sized Header of the Convert Protocol 627 The Version is encoded as an 8 bits unsigned integer value. This 628 document specifies version 1. Version 0 is reserved by this document 629 and MUST NOT be used. 631 The Total Length is the number of 32 bits word, including the header, 632 of the bytestream that are consumed by the Convert messages. Since 633 Total Length is also an 8 bits unsigned integer, those messages 634 cannot consume more than 1020 bytes of data. This limits the number 635 of bytes that a Transport Converter needs to process. A Total Length 636 of zero is invalid and the connection MUST be reset upon reception of 637 a header with such total length. 639 The Unassigned field MUST be set to zero in this version of the 640 protocol. These bits are available for future use [RFC8126]. 642 Data added by the Convert protocol to the TCP bytestream in the 643 upstream connection is unambiguously distinguished from payload data 644 in the downstream connection by the Total Length field in the Convert 645 messages. 647 4.2. Convert TLVs 649 4.2.1. Generic Convert TLV Format 651 The Convert protocol uses variable length messages that are encoded 652 using the generic TLV (Type, Length, Value) format depicted in 653 Figure 10. 655 The length of all TLVs used by the Convert protocol is always a 656 multiple of four bytes. All TLVs are aligned on 32 bits boundaries. 657 All TLV fields are encoded using the network byte order. 659 1 2 3 660 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 661 +---------------+---------------+-------------------------------+ 662 | Type | Length | (optional) Value ... | 663 +---------------+---------------+-------------------------------+ 664 | ... (optional) Value | 665 +---------------------------------------------------------------+ 667 Figure 10: Convert Generic TLV Format 669 The Length field is expressed in units of 32 bits words. In general 670 zero padding MUST be added if the value's length in bytes can not be 671 expressed as 2+(4 * n). 673 A given TLV MUST only appear once on a connection. If two or more 674 instances of the same TLV are exchanged over a Convert connection, 675 the associated TCP connections MUST be closed. 677 4.2.2. Summary of Supported Convert TLVs 679 This document specifies the following Convert TLVs: 681 +------+-----+----------+------------------------------------------+ 682 | Type | Hex | Length | Description | 683 +------+-----+----------+------------------------------------------+ 684 | 1 | 0x1 | 1 | Info TLV | 685 | 10 | 0xA | Variable | Connect TLV | 686 | 20 | 0x14| Variable | Extended TCP Header TLV | 687 | 21 | 0x15| Variable | Supported TCP Extensions TLV | 688 | 22 | 0x16| Variable | Cookie TLV | 689 | 30 | 0x1E| Variable | Error TLV | 690 +------+-----+----------+------------------------------------------+ 692 Figure 11: The TLVs used by the Convert Protocol 694 Type 0x0 is a reserved valued. Implementations MUST discard messages 695 with such TLV. 697 The Client can request the establishment of connections to servers by 698 using the Connect TLV (Section 4.2.5). If the connection can be 699 established with the final server, the Transport Converter replies 700 with the Extended TCP Header TLV (Section 4.2.4). If not, the 701 Transport Converter returns an Error TLV (Section 4.2.8) and then 702 closes the connection. 704 As a general rule, when an error is encountered an Error TLV with the 705 appropriate error code MUST be returned by the Transport Converter. 707 4.2.3. The Info TLV 709 The Info TLV (Figure 12) is an optional TLV which can be sent by a 710 Client to request the TCP extensions that are supported by a 711 Transport Converter. It is typically sent on the first connection 712 that a Client establishes with a Transport Converter to learn its 713 capabilities. Assuming a Client is entitled to invoke the Transport 714 Converter, the latter replies with the Supported TCP Extensions TLV 715 described in Section 4.2.4. 717 1 2 3 718 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 719 +---------------+---------------+-------------------------------+ 720 | Type=0x1 | Length | Zero | 721 +---------------+---------------+-------------------------------+ 723 Figure 12: The Info TLV 725 4.2.4. Supported TCP Extensions TLV 727 The Supported TCP Extensions TLV (Figure 13) is used by a Transport 728 Converter to announce the TCP options for which it provides a 729 conversion service. A Transport Converter SHOULD include in this 730 list the TCP options that it accepts from Clients and that it 731 includes the SYN packets that it sends to initiate connections. 733 Each supported TCP option is encoded with its TCP option Kind listed 734 in the "TCP Parameters" registry maintained by IANA. 736 1 2 3 737 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 738 +---------------+---------------+-------------------------------+ 739 | Type=0x15 | Length | Unassigned | 740 +---------------+---------------+-------------------------------+ 741 | Kind #1 | Kind #2 | ... | 742 +---------------+---------------+-------------------------------+ 743 / ... / 744 / / 745 +---------------------------------------------------------------+ 747 Figure 13: The Supported TCP Extensions TLV 749 TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by 750 all TCP implementations and thus MUST NOT appear in this list. 752 The list of Supported TCP Extension is padded with 0 to end on a 32 753 bits boundary. 755 For example, if the Transport Converter supports Multipath TCP, 756 Kind=30 will be present in the Supported TCP Extensions TLV that it 757 returns in response to Info TLV. 759 4.2.5. Connect TLV 761 The Connect TLV (Figure 14) is used to request the establishment of a 762 connection via a Transport Converter. This connection can be from or 763 to a client. 765 The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain 766 the destination port number and IP address of the Server, for 767 outgoing connections. For incoming connections destined to a Client 768 serviced via a Transport Converter, these fields convey the source 769 port number and IP address. 771 The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4 772 addresses MUST be encoded using the IPv4-Mapped IPv6 Address format 773 defined in [RFC4291]. Further, Remote Peer IP address field MUST NOT 774 include multicast, broadcast, and host loopback addresses [RFC6890]. 775 Connect TLVs witch such messages MUST be discarded by the Transport 776 Converter. 778 We distinguish two types of Connect TLV based on their length: (1) 779 the base Connect TLV has a length of 20 bytes and contains a remote 780 address and a remote port, (2) the extended Connect TLV spans more 781 than 20 bytes and also includes the optional 'TCP Options' field. 782 This field is used to specify how specific TCP options should be 783 advertised by the Transport Converter to the server. 785 1 2 3 786 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 787 +---------------+---------------+-------------------------------+ 788 | Type=0xA | Length | Remote Peer Port | 789 +---------------+---------------+-------------------------------+ 790 | | 791 | Remote Peer IP Address (128 bits) | 792 | | 793 | | 794 +---------------------------------------------------------------+ 795 | TCP Options (Variable) | 796 | ... | 797 +---------------------------------------------------------------+ 799 Figure 14: The Connect TLV 801 The 'TCP Options' field is a variable length field that carries a 802 list of TCP option fields (Figure 15). Each TCP option field is 803 encoded as a block of 2+n bytes where the first byte is the TCP 804 option Kind and the second byte is the length of the TCP option as 805 specified in [RFC0793]. The minimum value for the TCP option Length 806 is 2. The TCP options that do not include a length subfield, i.e., 807 option types 0 (EOL) and 1 (NOP) defined in [RFC0793] MUST NOT be 808 placed inside the TCP options field of the Connect TLV. The optional 809 Value field contains the variable-length part of the TCP option. A 810 length of two indicates the absence of the Value field. The TCP 811 options field always ends on a 32 bits boundary after being padded 812 with zeros. 814 1 2 3 815 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 816 +---------------+---------------+---------------+---------------+ 817 | TCPOpt kind | TCPOpt Length | Value (opt) | .... | 818 +---------------+---------------+---------------+---------------+ 819 | .... | 820 +---------------------------------------------------------------+ 821 | ... | 822 +---------------------------------------------------------------+ 824 Figure 15: The TCP Options Field 826 Upon reception of a Connect TLV, and absent any policy (e.g., rate- 827 limit) or resource exhaustion conditions, a Transport Converter 828 attempts to establish a connection to the address and port that it 829 contains. The Transport Converter MUST use by default the TCP 830 options that correspond to its local policy to establish this 831 connection. These are the options that it advertises in the 832 Supported TCP Extensions TLV. 834 Upon reception of an extended Connect TLV, and absent any rate limit 835 policy or resource exhaustion conditions, a Transport Converter MUST 836 attempt to establish a connection to the address and port that it 837 contains. It MUST include the options of the 'TCP Options' subfield 838 in the SYN sent to the Server in addition to the TCP options that it 839 would have used according to its local policies. For the TCP options 840 that are listed without an optional value, the Transport Converter 841 MUST generate its own value. For the TCP options that are included 842 in the 'TCP Options' field with an optional value, it MUST copy the 843 entire option for use in the connection with the destination peer. 844 This feature is required to support TCP Fast Open. 846 The Transport Converter may discard a Connect TLV request for various 847 reasons (e.g., authorization failed, out of resources, invalid 848 address type). An error message indicating the encountered error is 849 returned to the requesting Client (Section 4.2.8). In order to 850 prevent denial-of-service attacks, error messages sent to a Client 851 SHOULD be rate-limited. 853 4.2.6. Extended TCP Header TLV 855 The Extended TCP Header TLV (Figure 16) is used by the Transport 856 Converter to send to the Client the extended TCP header that was 857 returned by the Server in the SYN+ACK packet. This TLV is only sent 858 if the Client sent a Connect TLV to request the establishment of a 859 connection. 861 1 2 3 862 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 863 +---------------+---------------+-------------------------------+ 864 | Type=0x14 | Length | Unassigned | 865 +---------------+---------------+-------------------------------+ 866 | Returned Extended TCP header | 867 | ... | 868 +---------------------------------------------------------------+ 870 Figure 16: The Extended TCP Header TLV 872 The Returned Extended TCP header field is a copy of the extended 873 header that was received in the SYN+ACK by the Transport Converter. 875 The Unassigned field MUST be set to zero by the transmitter and 876 ignored by the receiver. These bits are available for future use 877 [RFC8126]. 879 4.2.7. The Cookie TLV 881 The Cookie TLV (Figure 17 is an optional TLV which use is similar to 882 the TCP Fast Open Cookie [RFC7413]. A Transport Converter may want 883 to verify that its Clients can receive the packets that it sends to 884 prevent attacks from spoofed addresses. This verification can be 885 done by using a Cookie that is bound to, for example, the IP 886 address(es) of the Client. This Cookie can be configured on the 887 Client by means that are outside of this document or provided by the 888 Transport Converter as follows. 890 A Transport Converter that has been configured to use the optional 891 Cookie TLV MUST verify the presence of this TLV in the payload of the 892 received SYN. If this TLV is present, the Transport Converter MUST 893 validate the Cookie by means similar to those in Section 4.1.2 of 894 [RFC7413] (i.e., IsCookieValid). If the Cookie is valid, the 895 connection establishment procedure can continue. Otherwise, the 896 Transport Converter MUST return an Error TLV set to "Not Authorized" 897 and close the connection. 899 If the received SYN did not contain a Cookie TLV, and cookie 900 validation is required, the Transport Converter should compute a 901 Cookie bound to this Client address and return a Convert message 902 containing the fixed header, an Error TLV set to "Missing Cookie" and 903 the computed Cookie and close the connection. The Client will react 904 to this error by storing the received Cookie in its cache and attempt 905 to reestablish a new connection to the Transport Converter that 906 includes the Cookie. 908 The format of the Cookie TLV is shown in the below figure. 910 1 2 3 911 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 912 +---------------+---------------+-------------------------------+ 913 | Type=0x16 | Length | Zero | 914 +---------------+---------------+-------------------------------+ 915 | Opaque Cookie | 916 | ... | 917 +---------------------------------------------------------------+ 919 Figure 17: The Cookie TLV 921 4.2.8. Error TLV 923 The Error TLV (Figure 18) is used by the Transport Converter to 924 provide information about some errors that occurred during the 925 processing of Convert message. This TLV has a variable length. It 926 appears after the Convert fixed-header in the bytestream returned by 927 the Transport Converter. Upon reception of an Error TLV, a Client 928 MUST close the associated connection. 930 1 2 3 931 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 932 +---------------+---------------+----------------+--------------+ 933 | Type=0x1E | Length | Error code | Value | 934 +---------------+---------------+----------------+--------------+ 936 Figure 18: The Error TLV 938 Different types of errors can occur while processing Convert 939 messages. Each error is identified by an Error code represented as 940 an unsigned integer. Four classes of Error codes are defined: 942 o Message validation and processing errors (0-31 range): returned 943 upon reception of an invalid message (including valid messages but 944 with invalid or unknown TLVs). 946 o Client-side errors (32-63 range): the Client sent a request that 947 could not be accepted by the Transport Converter (e.g., 948 unsupported operation). 950 o Converter-side errors (64-95 range): problems encountered on the 951 Transport Converter (e.g., lack of resources) which prevent it 952 from fulfilling the Client's request. 954 o Errors caused by the destination server (96-127 range): the final 955 destination could not be reached or it replied with a reset. 957 The following error codes are defined in this document: 959 o Unsupported Version (0): The version number indicated in the fixed 960 header of a message received from a peer is not supported. 962 This error code MUST be generated by a Transport Converter when it 963 receives a request having a version number that it does not 964 support. 966 The value field MUST be set to the version supported by the 967 Transport Converter. When multiple versions are supported by the 968 Transport Converter, it includes the list of supported version in 969 the value field; each version is encoded in 8 bits. The list of 970 supported versions should be padded with zeros to end on a 32 bits 971 boundary. 973 Upon receipt of this error code, the client checks whether it 974 supports one of the versions returned by the Transport Converter. 975 The highest common supported version MUST be used by the client in 976 subsequent exchanges with the Transport Converter. 978 o Malformed Message (1): This error code is sent to indicate that a 979 message can not be successfully parsed and validated. 981 Typically, this error code is sent by the Transport Converter if 982 it receives a Connect TLV enclosing a multicast, broadcast, or 983 loopback IP address. 985 To ease troubleshooting, the value field MUST echo the received 986 message shifted by one byte to keep to original alignment of the 987 message. 989 o Unsupported Message (2): This error code is sent to indicate that 990 a message type is not supported by the Transport Converter. 992 To ease troubleshooting, the value field MUST echo the received 993 message shifted by one byte to keep to original alignment of the 994 message. 996 o Missing Cookie (3): If a Transport Converter requires the 997 utilization of Cookies to prevent spoofing attacks and a Cookie 998 TLV was not included in the Convert message, the Transport 999 Converter MUST return this error to the requesting client. The 1000 first byte of the value field MUST be set to zero and the 1001 remaining bytes of the Error TLV contain the Cookie computed by 1002 the Transport Converter for this Client. 1004 A Client which receives this error code MUST cache the received 1005 Cookie and include it in subsequent Convert messages sent to that 1006 Transport Converter. 1008 o Not Authorized (32): This error code indicates that the Transport 1009 Converter refused to create a connection because of a lack of 1010 authorization (e.g., administratively prohibited, authorization 1011 failure, invalid Cookie TLV, etc.). The Value field MUST be set 1012 to zero. 1014 This error code MUST be sent by the Transport Converter when a 1015 request cannot be successfully processed because the authorization 1016 failed. 1018 o Unsupported TCP Option (33): A TCP option that the Client 1019 requested to advertise to the final Server cannot be safely used. 1021 The Value field is set to the type of the unsupported TCP option. 1022 If several unsupported TCP options were specified in the Connect 1023 TLV, then the list of unsupported TCP options is returned. The 1024 list of unsupported TCP options MUST be padded with zeros to end 1025 on a 32 bits boundary. 1027 o Resource Exceeded (64): This error indicates that the Transport 1028 Converter does not have enough resources to perform the request. 1030 This error MUST be sent by the Transport Converter when it does 1031 not have sufficient resources to handle a new connection. The 1032 Transport Converter may indicate in the Value field the suggested 1033 delay (in seconds) that the Client SHOULD wait before soliciting 1034 the Transport Converter for a new proxied connection. A Value of 1035 zero corresponds to a default delay of at least 30 seconds. 1037 o Network Failure (65): This error indicates that the Transport 1038 Converter is experiencing a network failure to relay the request. 1040 The Transport Converter MUST send this error code when it 1041 experiences forwarding issues to relay a connection. The 1042 Transport Converter may indicate in the Value field the suggested 1043 delay (in seconds) that the Client SHOULD wait before soliciting 1044 the Transport Converter for a new proxied connection. A Value of 1045 zero corresponds to a default delay of at least 30 seconds. 1047 o Connection Reset (96): This error indicates that the final 1048 destination responded with a RST packet. The Value field MUST be 1049 set to zero. 1051 o Destination Unreachable (97): This error indicates that an ICMP 1052 destination unreachable, port unreachable, or network unreachable 1053 was received by the Transport Converter. The Value field MUST 1054 echo the Code field of the received ICMP message. 1056 Figure 19 summarizes the different error codes. 1058 +-------+------+-----------------------------------------------+ 1059 | Error | Hex | Description | 1060 +-------+------+-----------------------------------------------+ 1061 | 0 | 0x00 | Unsupported Version | 1062 | 1 | 0x01 | Malformed Message | 1063 | 2 | 0x02 | Unsupported Message | 1064 | 3 | 0x03 | Missing Cookie | 1065 | 32 | 0x20 | Not Authorized | 1066 | 33 | 0x21 | Unsupported TCP Option | 1067 | 64 | 0x40 | Resource Exceeded | 1068 | 65 | 0x41 | Network Failure | 1069 | 96 | 0x60 | Connection Reset | 1070 | 97 | 0x61 | Destination Unreachable | 1071 +-------+------+-----------------------------------------------+ 1073 Figure 19: Convert Error Values 1075 5. Compatibility of Specific TCP Options with the Conversion Service 1077 In this section, we discuss how several standard track TCP options 1078 can be supported through the Convert protocol. The non-standard 1079 track options and the experimental options will be discussed in other 1080 documents. 1082 5.1. Base TCP Options 1084 Three TCP options were initially defined in [RFC0793]: End-of-Option 1085 List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size 1086 (Kind=2). The first two options are mainly used to pad the TCP 1087 extended header. There is no reason for a client to request a 1088 Transport Converter to specifically send these options towards the 1089 final destination. 1091 The Maximum Segment Size option (Kind=2) is used by a host to 1092 indicate the largest segment that it can receive over each 1093 connection. This value is function of the stack that terminates the 1094 TCP connection. There is no reason for a Client to request a 1095 Transport Converter to advertise a specific MSS value to a remote 1096 server. 1098 A Transport Converter MUST ignore options with Kind=0, 1 or 2 if they 1099 appear in a Connect TLV. It MUST NOT announce them in a Supported 1100 TCP Extensions TLV. 1102 5.2. Window Scale (WS) 1104 The Window Scale option (Kind=3) is defined in [RFC7323]. As for the 1105 MSS option, the window scale factor that is used for a connection 1106 strongly depends on the TCP stack that handles the connection. When 1107 a Transport Converter opens a TCP connection towards a remote server 1108 on behalf of a Client, it SHOULD use a WS option with a scaling 1109 factor that corresponds to the configuration of its stack. A local 1110 configuration MAY allow for WS option in the proxied message to be 1111 function of the scaling factor of the incoming connection. 1113 There is no benefit from a deployment viewpoint in enabling a Client 1114 of a Transport Converter to specifically request the utilisation of 1115 the WS option (Kind=3) with a specific scaling factor towards a 1116 remote Server. For this reason, a Transport Converter MUST ignore 1117 option Kind=3 if it appears in a Connect TLV. It MUST NOT announce 1118 it in a Supported TCP Extensions TLV. 1120 5.3. Selective Acknowledgements 1122 Two distinct TCP options were defined to support selective 1123 acknowledgements in [RFC2018]. This first one, SACK Permitted 1124 (Kind=4), is used to negotiate the utilisation of selective 1125 acknowledgements during the three-way handshake. The second one, 1126 SACK (Kind=5), carries the selective acknowledgements inside regular 1127 segments. 1129 The SACK Permitted option (Kind=4) MAY be advertised by a Transport 1130 Converter in the Supported TCP Extensions TLV. Clients connected to 1131 this Transport Converter MAY include the SACK Permitted option in the 1132 Connect TLV. 1134 The SACK option (Kind=5) cannot be used during the three-way 1135 handshake. For this reason, a Transport Converter MUST ignore option 1136 Kind=5 if it appears in a Connect TLV. It MUST NOT announce it in a 1137 TCP Supported Extensions TLV. 1139 5.4. Timestamp 1141 The Timestamp option was initially defined in [RFC1323] and later 1142 refined in [RFC7323]. It can be used during the three-way handshake 1143 to negotiate the utilization of timestamps during the TCP connection. 1144 It is notably used to improve round-trip-time estimations and to 1145 provide protection against wrapped sequence numbers (PAWS). As for 1146 the WS option, the timestamps are a property of a connection and 1147 there is limited benefit in enabling a client to request a Transport 1148 Converter to use the timestamp option when establishing a connection 1149 to a remote server. Furthermore, the timestamps that are used by TCP 1150 stacks are specific to each stack and there is no benefit in enabling 1151 a client to specify the timestamp value that a Transport Converter 1152 could use to establish a connection to a remote server. 1154 A Transport Converter MAY advertise the Timestamp option (Kind=8) in 1155 the TCP Supported Extensions TLV. The clients connected to this 1156 Transport Converter MAY include the Timestamp option in the Connect 1157 TLV but without any timestamp. 1159 5.5. Multipath TCP 1161 The Multipath TCP options are defined in [RFC6824]. [RFC6824] 1162 defines one variable length TCP option (Kind=30) that includes a 1163 subtype field to support several Multipath TCP options. There are 1164 several operational use cases where clients would like to use 1165 Multipath TCP through a Transport Converter [IETFJ16]. However, none 1166 of these use cases require the Client to specify the content of the 1167 Multipath TCP option that the Transport Converter should send to a 1168 remote server. 1170 A Transport Converter which supports Multipath TCP conversion service 1171 MUST advertise the Multipath TCP option (Kind=30) in the Supported 1172 TCP Extensions TLV. Clients serviced by this Transport Converter may 1173 include the Multipath TCP option in the Connect TLV but without any 1174 content. 1176 5.6. TCP Fast Open 1178 The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413]. 1179 There are two different usages of this option that need to be 1180 supported by Transport Converters. The first utilization of the TCP 1181 Fast Open cookie option is to request a cookie from the server. In 1182 this case, the option is sent with an empty cookie by the client and 1183 the server returns the cookie. The second utilization of the TCP 1184 Fast Open cookie option is to send a cookie to the server. In this 1185 case, the option contains a cookie. 1187 A Transport Converter MAY advertise the TCP Fast Open cookie option 1188 (Kind=34) in the Supported TCP Extensions TLV. If a Transport 1189 Converter has advertised the support for TCP Fast Open in its 1190 Supported TCP Extensions TLV, it needs to be able to process two 1191 types of Connect TLV. If such a Transport Converter receives a 1192 Connect TLV with the TCP Fast Open cookie option that does not 1193 contain a cookie, it MUST add an empty TCP Fast Open cookie option in 1194 the SYN sent to the remote server. If such a Transport Converter 1195 receives a Connect TLV with the TCP Fast Open cookie option that 1196 contains a cookie, it MUST copy the TCP Fast Open cookie option in 1197 the SYN sent to the remote server. 1199 5.7. TCP User Timeout 1201 The TCP User Timeout option is defined in [RFC5482]. The associated 1202 TCP option (Kind=28) does not appear to be widely deployed. 1204 5.8. TCP-AO 1206 TCP-AO [RFC5925] provides a technique to authenticate all the packets 1207 exchanged over a TCP connection. Given the nature of this extension, 1208 it is unlikely that the applications that require their packets to be 1209 authenticated end-to-end would want their connections to pass through 1210 a converter. For this reason, we do not recommend the support of the 1211 TCP-AO option by Transport Converters. The only use cases where it 1212 could make sense to combine TCP-AO and the solution in this document 1213 are those where the TCP-AO-NAT extension [RFC6978] is in use. 1215 A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29) 1216 in the Supported TCP Extensions TLV. If a Transport Converter 1217 receives a Connect TLV that contains the TCP-AO option, it MUST 1218 reject the establishment of the connection with error code set to 1219 "Unsupported TCP Option", except if the TCP-AO-NAT option is used. 1221 5.9. TCP Experimental Options 1223 The TCP Experimental options are defined in [RFC4727]. Given the 1224 variety of semantics for these options and their experimental nature, 1225 it is impossible to discuss them in details in this document. 1227 6. Interactions with Middleboxes 1229 The Convert Protocol is designed to be used in networks that do not 1230 contain middleboxes that interfere with TCP. Under such conditions, 1231 it is assumed that the network provider ensures that all involved on- 1232 path nodes are not breaking TCP signals (e.g., strip TCP options, 1233 discard some SYNs, etc.). 1235 Nevertheless, and in order to allow for a robust service, this 1236 section describes how a Client can detect middlebox interference and 1237 stop using the Transport Converter affected by this interference. 1239 Internet measurements [IMC11] have shown that middleboxes can affect 1240 the deployment of TCP extensions. In this section, we only discuss 1241 the middleboxes that modify SYN and SYN+ACK packets since the Convert 1242 Protocol places its messages in such packets. 1244 Consider a middlebox that removes the SYN payload. The Client can 1245 detect this problem by looking at the acknowledgement number field of 1246 the SYN+ACK returned by the Transport Converter. The Client MUST 1247 stop to use this Transport Converter given the middlebox 1248 interference. 1250 As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect 1251 the operation of TFO if they assign different IP addresses to the 1252 same end host. Such CGNs could affect the operation of the TFO 1253 Option used by the Convert Protocol. As a reminder CGNs, enabled on 1254 the path between a Client and a Transport Converter, must adhere to 1255 the address preservation defined in [RFC6888]. See also the 1256 discussion in Section 7.1 of [RFC7413]. 1258 7. Security Considerations 1260 7.1. Privacy & Ingress Filtering 1262 The Transport Converter may have access to privacy-related 1263 information (e.g., subscriber credentials). The Transport Converter 1264 is designed to not leak such sensitive information outside a local 1265 domain. 1267 Given its function and its location in the network, a Transport 1268 Converter has access to the payload of all the packets that it 1269 processes. As such, it MUST be protected as a core IP router (e.g., 1270 [RFC1812]). 1272 Furthermore, ingress filtering policies MUST be enforced at the 1273 network boundaries [RFC2827]. 1275 This document assumes that all network attachments are managed by the 1276 same administrative entity. Therefore, enforcing anti-spoofing 1277 filters at these network ensures that hosts are not sending traffic 1278 with spoofed source IP addresses. 1280 7.2. Authorization 1282 The Convert Protocol is intended to be used in managed networks where 1283 end hosts can be identified by their IP address. 1285 Stronger mutual authentication schemes MUST be defined to use the 1286 Convert Protocol in more open network environments. One possibility 1287 is to use TLS to perform mutual authentication between the client and 1288 the Converter. That is, use TLS when a Client retrieves a Cookie 1289 from the Converter and rely on certificate-based client 1290 authentication, pre-shared key based [RFC4279] or raw public key 1291 based client authentication [RFC7250] to secure this connection. 1293 If the authentication succeeds, the Converter returns a cookie to the 1294 Client. Subsequent Connect messages will be authorized as a function 1295 of the content of the Cookie TLV. 1297 In deployments where network-assisted connections are not allowed 1298 between hosts of a domain (i.e., hairpinning), the Converter may be 1299 instructed to discard such connections. Hairpinned connections are 1300 thus rejected by the Transport Converter by returning an Error TLV 1301 set to "Not Authorized". Absent explicit configuration otherwise, 1302 hairpinning is enabled by the Converter (see Figure 20. 1304 <===Network Provider===> 1306 +----+ from X1:x1 to X2':x2' +-----+ X1':x1' 1307 | C1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+--- 1308 +----+ | v | 1309 | v | 1310 | v | 1311 | v | 1312 +----+ from X1':x1' to X2:x2 | v | X2':x2' 1313 | C2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+--- 1314 +----+ +-----+ 1315 Converter 1317 Note: X2':x2' may be equal to 1318 X2:x2 1320 Figure 20: Hairpinning Example 1322 See below for authorization considerations that are specific for 1323 Multipath TCP. 1325 7.3. Denial of Service 1327 Another possible risk is the amplification attacks since a Transport 1328 Converter sends a SYN towards a remote Server upon reception of a SYN 1329 from a Client. This could lead to amplification attacks if the SYN 1330 sent by the Transport Converter were larger than the SYN received 1331 from the Client or if the Transport Converter retransmits the SYN. 1332 To mitigate such attacks, the Transport Converter SHOULD rate limit 1333 the number of pending requests for a given Client. It SHOULD also 1334 avoid sending to remote Servers SYNs that are significantly longer 1335 than the SYN received from the Client. Finally, the Transport 1336 Converter SHOULD only retransmit a SYN to a Server after having 1337 received a retransmitted SYN from the corresponding Client. Means to 1338 protect against SYN flooding attacks MUST also be enabled [RFC4987]. 1340 7.4. Traffic Theft 1342 Traffic theft is a risk if an illegitimate Converter is inserted in 1343 the path. Indeed, inserting an illegitimate Converter in the 1344 forwarding path allows traffic interception and can therefore provide 1345 access to sensitive data issued by or destined to a host. Converter 1346 discovery and configuration are out of scope of this document. 1348 7.5. Multipath TCP-specific Considerations 1350 Multipath TCP-related security threats are discussed in [RFC6181] and 1351 [RFC6824]. 1353 The operator that manages the various network attachments (including 1354 the Transport Converters) can enforce authentication and 1355 authorization policies using appropriate mechanisms. For example, a 1356 non-exhaustive list of methods to achieve authorization is provided 1357 hereafter: 1359 o The network provider may enforce a policy based on the 1360 International Mobile Subscriber Identity (IMSI) to verify that a 1361 user is allowed to benefit from the aggregation service. If that 1362 authorization fails, the Packet Data Protocol (PDP) context/bearer 1363 will not be mounted. This method does not require any interaction 1364 with the Transport Converter. 1366 o The network provider may enforce a policy based upon Access 1367 Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG) 1368 to control the hosts that are authorized to communicate with a 1369 Transport Converter. These ACLs may be installed as a result of 1370 RADIUS exchanges, e.g., [I-D.boucadair-radext-tcpm-converter]. 1371 This method does not require any interaction with the Transport 1372 Converter. 1374 o A device that embeds a Transport Converter may also host a RADIUS 1375 client that will solicit an AAA server to check whether 1376 connections received from a given source IP address are authorized 1377 or not [I-D.boucadair-radext-tcpm-converter]. 1379 A first safeguard against the misuse of Transport Converter resources 1380 by illegitimate users (e.g., users with access networks that are not 1381 managed by the same provider that operates the Transport Converter) 1382 is the Transport Converter to reject Multipath TCP connections 1383 received on its Internet-facing interfaces. Only Multipath TCP 1384 connections received on the customer-facing interfaces of a Transport 1385 Converter will be accepted. 1387 8. IANA Considerations 1389 8.1. Convert Service Port Number 1391 IANA is requested to assign a TCP port number (TBA) for the Convert 1392 Protocol from the "Service Name and Transport Protocol Port Number 1393 Registry" available at https://www.iana.org/assignments/service- 1394 names-port-numbers/service-names-port-numbers.xhtml. 1396 8.2. The Convert Protocol (Convert) Parameters 1398 IANA is requested to create a new "The Convert Protocol (Convert) 1399 Parameters" registry. 1401 The following subsections detail new registries within "The Convert 1402 Protocol (Convert) Parameters" registry. 1404 8.2.1. Convert Versions 1406 IANA is requested to create the "Convert versions" sub-registry. New 1407 values are assigned via IETF Review (Section 4.8 of [RFC8126]). 1409 The initial values to be assigned at the creation of the registry are 1410 as follows: 1412 +---------+--------------------------------------+-------------+ 1413 | Version | Description | Reference | 1414 +---------+--------------------------------------+-------------+ 1415 | 0 | Reserved by this document | [This-RFC] | 1416 | 1 | Assigned by this document | [This-RFC] | 1417 +---------+--------------------------------------+-------------+ 1419 8.2.2. Convert TLVs 1421 IANA is requested to create the "Convert TLVs" sub-registry. The 1422 procedure for assigning values from this registry is as follows: 1424 o The values in the range 1-127 can be assigned via IETF Review. 1426 o The values in the range 128-191 can be assigned via Specification 1427 Required. 1429 o The values in the range 192-255 can be assigned for Private Use. 1431 The initial values to be assigned at the creation of the registry are 1432 as follows: 1434 +---------+--------------------------------------+-------------+ 1435 | Code | Name | Reference | 1436 +---------+--------------------------------------+-------------+ 1437 | 0 | Reserved | [This-RFC] | 1438 | 1 | Info TLV | [This-RFC] | 1439 | 10 | Connect TLV | [This-RFC] | 1440 | 20 | Extended TCP Header TLV | [This-RFC] | 1441 | 21 | Supported TCP Extension TLV | [This-RFC] | 1442 | 22 | Cookie TLV | [This-RFC] | 1443 | 30 | Error TLV | [This-RFC] | 1444 +---------+--------------------------------------+-------------+ 1446 8.2.3. Convert Error Messages 1448 IANA is requested to create the "Convert Errors" sub-registry. Codes 1449 in this registry are assigned as a function of the error type. Four 1450 types are defined; the following ranges are reserved for each of 1451 these types: 1453 o Message validation and processing errors: 0-31 1455 o Client-side errors: 32-63 1457 o Transport Converter-side errors: 64-95 1459 o Errors caused by destination server: 96-127 1461 The procedure for assigning values from this sub-registry is as 1462 follows: 1464 o 0-191: Values in this range are assigned via IETF Review. 1466 o 192-255: Values in this range are assigned via Specification 1467 Required. 1469 The initial values to be assigned at the creation of the registry are 1470 as follows: 1472 +-------+------+-----------------------------------+-----------+ 1473 | Error | Hex | Description | Reference | 1474 +-------+------+-----------------------------------+-----------+ 1475 | 0 | 0x00 | Unsupported Version | [This-RFC]| 1476 | 1 | 0x01 | Malformed Message | [This-RFC]| 1477 | 2 | 0x02 | Unsupported Message | [This-RFC]| 1478 | 3 | 0x03 | Missing Cookie | [This-RFC]| 1479 | 32 | 0x20 | Not Authorized | [This-RFC]| 1480 | 33 | 0x21 | Unsupported TCP Option | [This-RFC]| 1481 | 64 | 0x40 | Resource Exceeded | [This-RFC]| 1482 | 65 | 0x41 | Network Failure | [This-RFC]| 1483 | 96 | 0x60 | Connection Reset | [This-RFC]| 1484 | 97 | 0x61 | Destination Unreachable | [This-RFC]| 1485 +-------+------+-----------------------------------+-----------+ 1487 Figure 21: The Convert Error Codes 1489 9. Acknowledgements 1491 Although they could disagree with the contents of the document, we 1492 would like to thank Joe Touch and Juliusz Chroboczek whose comments 1493 on the MPTCP mailing list have forced us to reconsider the design of 1494 the solution several times. 1496 We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha 1497 Nandugudi and Gregory Vander Schueren for their help in preparing 1498 this document. Nandini Ganesh provided valuable feedback about the 1499 handling of TFO and the error codes. Thanks to them. 1501 Thanks to Yuchung Cheng and Praveen Balasubramanian for the 1502 discussion on supplying data in SYNs. 1504 This document builds upon earlier documents that proposed various 1505 forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode], 1506 [I-D.peirens-mptcp-transparent] and [HotMiddlebox13b]. 1508 From [I-D.boucadair-mptcp-plain-mode]: 1510 Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi 1511 Nishida, and Christoph Paasch for their valuable comments. 1513 Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and 1514 Sri Gundavelli for the fruitful discussions in IETF#95 (Buenos 1515 Aires). 1517 Special thanks to Pierrick Seite, Yannick Le Goff, Fred Klamm, and 1518 Xavier Grall for their inputs. 1520 Thanks also to Olaf Schleusing, Martin Gysi, Thomas Zasowski, Andreas 1521 Burkhard, Silka Simmen, Sandro Berger, Michael Melloul, Jean-Yves 1522 Flahaut, Adrien Desportes, Gregory Detal, Benjamin David, Arun 1523 Srinivasan, and Raghavendra Mallya for the discussion. 1525 9.1. Contributors 1527 Bart Peirens contributed to an early version of the document. 1529 As noted above, this document builds on two previous documents. 1531 The authors of [I-D.boucadair-mptcp-plain-mode] were: 1533 o Mohamed Boucadair 1535 o Christian Jacquenet 1537 o Olivier Bonaventure 1539 o Denis Behaghel 1541 o Stefano Secci 1543 o Wim Henderickx 1545 o Robert Skog 1547 o Suresh Vinapamula 1549 o SungHoon Seo 1551 o Wouter Cloetens 1553 o Ullrich Meyer 1555 o Luis M. Contreras 1557 o Bart Peirens 1559 The authors of [I-D.peirens-mptcp-transparent] were: 1561 o Bart Peirens 1563 o Gregory Detal 1565 o Sebastien Barre 1567 o Olivier Bonaventure 1569 10. Change Log 1571 This section to be removed before publication. 1573 o 00 : initial version, designed to support Multipath TCP and TFO 1574 only 1576 o 00 to -01 : added section Section 5 describing the support of 1577 different standard tracks TCP options by Transport Converters, 1578 clarification of the IANA section, moved the SOCKS comparison to 1579 the appendix and various minor modifications 1581 o 01 to -02: Minor modifications 1583 o 02 to -03: Minor modifications 1585 o 03 to -04: Minor modifications 1587 o 04 to -05: Integrate a lot of feedback from implementors who have 1588 worked on client and server side implementations. The main 1589 modifications are the following : 1591 * TCP Fast Open is not strictly required anymore. Several 1592 implementors expressed concerns about this requirement. The 1593 TFO Cookie protects from some attack scenarios that affect open 1594 servers like web servers. The Convert protocol is different 1595 and as discussed in RFC7413, there are different ways to 1596 protect from such attacks. Instead of using a TFO cookie 1597 inside the TCP options, which consumes precious space in the 1598 extended TCP header, this version supports the utilisation of a 1599 Cookie that is placed in the SYN payload. This provides the 1600 same level of protection as a TFO Cookie in environments were 1601 such protection is required. 1603 * the Boostrap procedure has been simplified based on feedback 1604 from implementers 1606 * Error messages are not included in RST segments anymore but 1607 sent in the bytestream. Implementors have indicated that 1608 processing such segments on clients was difficult on some 1609 platforms. This change simplifies client implementations. 1611 * Many minor editorial changes to clarify the text based on 1612 implementors feedback. 1614 o 05 to -06: Many clarifications to integrate the comments from the 1615 chairs in preparation to the WGLC: 1617 * Updated IANA policy to require "IETF Review" instead of 1618 "Standard Action" 1620 * Call out explicilty that data in SYNs are relayed by the 1621 Converter 1623 * Reiterate the scope 1625 * Hairpinning behavior can be disabled (policy-based) 1627 * Fix nits 1629 o 07: 1631 * Update the text about supplying data in SYNs to make it clear 1632 that a constraint defined in RFC793 is relaxed folloiwng the 1633 same rationale as in RFC7413. 1635 * Nits 1637 * Added Appendix A on example Socket API changes 1639 11. Example Socket API Changes to Support the 0-RTT Convert Protocol 1641 11.1. Active Open (Client Side) 1643 On the client side, the support of the 0-RTT Converter protocol does 1644 not require any other changes than those identified in Appendix A of 1645 [RFC7413]. Those modifications are already supported by multiple TCP 1646 stacks. 1648 As an example, on Linux, a client can send the 0-RTT Convert message 1649 inside a SYN by using sendto with the MSG_FASTOPEN flag as shown in 1650 the example below: 1652 s = socket(AF_INET, SOCK_STREAM, 0); 1654 sendto(s, buffer, buffer_len, MSG_FASTOPEN, 1655 (struct sockaddr *) &server_addr, addr_len); 1657 The client side of the Linux TCP TFO can be used in two different 1658 modes depending on the host configuration (sysctl tcp_fastopen 1659 variable): 1661 o 0x1: (client) enables sending data in the opening SYN on the 1662 client. 1664 o 0x4: (client) send data in the opening SYN regardless of cookie 1665 availability and without a cookie option. 1667 By setting this configuration variable to 0x5, a Linux client using 1668 the above code would send data inside the SYN without using a TFO 1669 option. 1671 11.2. Passive Open (Converter Side) 1673 The Converter needs to enable the reception of data inside the SYN 1674 independently of the utilisation of the TFO option. This implies 1675 that the Transport Converter application cannot rely on the TFO 1676 cookies to validate the reachability of the IP address that sent the 1677 SYN. It must rely on other techniques, such as the Cookie TLV 1678 described in this document, to verify this reachability. 1680 [RFC7413] suggested the utilisation of a TCP_FASTOPEN socket option 1681 the enable the reception of SYNs containing data. Later, Appendix A 1682 of [RFC7413], mentionned: 1684 Traditionally, accept() returns only after a socket is connected. 1685 But, for a Fast Open connection, accept() returns upon receiving 1686 SYN with a valid Fast Open cookie and data, and the data is available 1687 to be read through, e.g., recvmsg(), read(). 1689 To support the 0-RTT Convert protocol, this behaviour should be 1690 modified as follows: 1692 Traditionally, accept() returns only after a socket is connected. 1693 But, for a Fast Open connection, accept() returns upon receiving a 1694 SYN with data, and the data is available to be read through, e.g., 1695 recvmsg(), read(). The application that receives such SYNs with data 1696 must be able to validate the reachability of the source of the SYN 1697 and also deal with replayed SYNs. 1699 The Linux server side can be configured with the following sysctls: 1701 o 0x2: (server) enables the server support, i.e., allowing data in a 1702 SYN packet to be accepted and passed to the application before 1703 3-way handshake finishes. 1705 o 0x200: (server) accept data-in-SYN w/o any cookie option present. 1707 However, this configuration is system-wide. This is convenient for 1708 typical Transport Converter deployments where no other applications 1709 relying on TFO are collocated on the same device. 1711 Recently, the TCP_FASTOPEN_NO_COOKIE socket option has been added to 1712 provide the same behaviour on a per socket basis. This enables a 1713 single host to support both servers that require the TFO cookie and 1714 servers that do not use it. 1716 12. Differences with SOCKSv5 1718 At a first glance, the solution proposed in this document could seem 1719 similar to the SOCKS v5 protocol [RFC1928] which is used to proxy TCP 1720 connections. The Client creates a connection to a SOCKS proxy, 1721 exchanges authentication information and indicates the destination 1722 address and port of the final server. At this point, the SOCKS proxy 1723 creates a connection towards the final server and relays all data 1724 between the two proxied connections. The operation of an 1725 implementation based on SOCKSv5 is illustrated in Figure 22. 1727 Client SOCKS Proxy Server 1728 --------------------> 1729 SYN 1730 <-------------------- 1731 SYN+ACK 1732 --------------------> 1733 ACK 1735 --------------------> 1736 Version=5, Auth Methods 1737 <-------------------- 1738 Method 1739 --------------------> 1740 Auth Request (unless "No auth" method negotiated) 1741 <-------------------- 1742 Auth Response 1743 --------------------> 1744 Connect Server:Port --------------------> 1745 SYN 1747 <-------------------- 1748 SYN+ACK 1749 <-------------------- 1750 Succeeded 1752 --------------------> 1753 Data1 1754 --------------------> 1755 Data1 1757 <-------------------- 1758 Data2 1759 <-------------------- 1760 Data2 1762 Figure 22: Establishment of a TCP connection through a SOCKS proxy 1763 without authentication 1765 The Convert protocol also relays data between an upstream and a 1766 downstream connection, but there are important differences with 1767 SOCKSv5. 1769 A first difference is that the Convert protocol exchanges all control 1770 information during the three-way handshake. This reduces the 1771 connection establishment delay compared to SOCKS that requires two or 1772 more round-trip-times before the establishment of the downstream 1773 connection towards the final destination. In today's Internet, 1774 latency is a important metric and various protocols have been tuned 1775 to reduce their latency [I-D.arkko-arch-low-latency]. A recently 1776 proposed extension to SOCKS also leverages the TFO option 1777 [I-D.olteanu-intarea-socks-6]. 1779 A second difference is that the Convert protocol explicitly takes the 1780 TCP extensions into account. By using the Convert protocol, the 1781 Client can learn whether a given TCP extension is supported by the 1782 destination Server. This enables the Client to bypass the Transport 1783 Converter when the destination supports the required TCP extension. 1784 Neither SOCKS v5 [RFC1928] nor the proposed SOCKS v6 1785 [I-D.olteanu-intarea-socks-6] provide such a feature. 1787 A third difference is that a Transport Converter will only accept the 1788 connection initiated by the Client provided that the downstream 1789 connection is accepted by the Server. If the Server refuses the 1790 connection establishment attempt from the Transport Converter, then 1791 the upstream connection from the Client is rejected as well. This 1792 feature is important for applications that check the availability of 1793 a Server or use the time to connect as a hint on the selection of a 1794 Server [RFC8305]. 1796 A fourth difference is that the Convert protocol only allows the 1797 client to specify the address/port of the destination server and not 1798 a DNS name. We evaluated an alternate design for the Connect TLV 1799 that included the DNS name of the remote peer instead of its IP 1800 address as in SOCKS [RFC1928]. However, that design was not adopted 1801 because it induces both an extra load and increased delays on the 1802 Transport Converter to handle and manage DNS resolution requests. 1804 13. References 1806 13.1. Normative References 1808 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1809 RFC 793, DOI 10.17487/RFC0793, September 1981, 1810 . 1812 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1813 Requirement Levels", BCP 14, RFC 2119, 1814 DOI 10.17487/RFC2119, March 1997, 1815 . 1817 [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key 1818 Ciphersuites for Transport Layer Security (TLS)", 1819 RFC 4279, DOI 10.17487/RFC4279, December 2005, 1820 . 1822 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1823 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 1824 2006, . 1826 [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, 1827 ICMPv6, UDP, and TCP Headers", RFC 4727, 1828 DOI 10.17487/RFC4727, November 2006, 1829 . 1831 [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address 1832 Translation (NAT) Behavioral Requirements for Unicast 1833 UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 1834 2007, . 1836 [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common 1837 Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, 1838 . 1840 [RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option", 1841 RFC 5482, DOI 10.17487/RFC5482, March 2009, 1842 . 1844 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP 1845 Authentication Option", RFC 5925, DOI 10.17487/RFC5925, 1846 June 2010, . 1848 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1849 "TCP Extensions for Multipath Operation with Multiple 1850 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1851 . 1853 [RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa, 1854 A., and H. Ashida, "Common Requirements for Carrier-Grade 1855 NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888, 1856 April 2013, . 1858 [RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, 1859 "Special-Purpose IP Address Registries", BCP 153, 1860 RFC 6890, DOI 10.17487/RFC6890, April 2013, 1861 . 1863 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 1864 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 1865 Transport Layer Security (TLS) and Datagram Transport 1866 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 1867 June 2014, . 1869 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1870 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1871 . 1873 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1874 Writing an IANA Considerations Section in RFCs", BCP 26, 1875 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1876 . 1878 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1879 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1880 May 2017, . 1882 13.2. Informative References 1884 [ANRW17] Trammell, B., Kuhlewind, M., De Vaere, P., Learmonth, I., 1885 and G. Fairhurst, "Tracking transport-layer evolution with 1886 PATHspider", Applied Networking Research Workshop 2017 1887 (ANRW17) , July 2017. 1889 [Fukuda2011] 1890 Fukuda, K., "An Analysis of Longitudinal TCP Passive 1891 Measurements (Short Paper)", Traffic Monitoring and 1892 Analysis. TMA 2011. Lecture Notes in Computer Science, vol 1893 6613. , 2011. 1895 [HotMiddlebox13b] 1896 Detal, G., Paasch, C., and O. Bonaventure, "Multipath in 1897 the Middle(Box)", HotMiddlebox'13 , December 2013, 1898 . 1901 [I-D.arkko-arch-low-latency] 1902 Arkko, J. and J. Tantsura, "Low Latency Applications and 1903 the Internet Architecture", draft-arkko-arch-low- 1904 latency-02 (work in progress), October 2017. 1906 [I-D.boucadair-mptcp-plain-mode] 1907 Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel, 1908 D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R., 1909 Vinapamula, S., Seo, S., Cloetens, W., Meyer, U., 1910 Contreras, L., and B. Peirens, "Extensions for Network- 1911 Assisted MPTCP Deployment Models", draft-boucadair-mptcp- 1912 plain-mode-10 (work in progress), March 2017. 1914 [I-D.boucadair-radext-tcpm-converter] 1915 Boucadair, M. and C. Jacquenet, "RADIUS Extensions for 1916 0-RTT TCP Converters", draft-boucadair-radext-tcpm- 1917 converter-02 (work in progress), April 2019. 1919 [I-D.boucadair-tcpm-dhc-converter] 1920 Boucadair, M., Jacquenet, C., and R. K, "DHCP Options for 1921 0-RTT TCP Converters", draft-boucadair-tcpm-dhc- 1922 converter-02 (work in progress), April 2019. 1924 [I-D.nam-mptcp-deployment-considerations] 1925 Boucadair, M., Jacquenet, C., Bonaventure, O., Henderickx, 1926 W., and R. Skog, "Network-Assisted MPTCP: Use Cases, 1927 Deployment Scenarios and Operational Considerations", 1928 draft-nam-mptcp-deployment-considerations-01 (work in 1929 progress), December 2016. 1931 [I-D.olteanu-intarea-socks-6] 1932 Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6", 1933 draft-olteanu-intarea-socks-6-06 (work in progress), March 1934 2019. 1936 [I-D.peirens-mptcp-transparent] 1937 Peirens, B., Detal, G., Barre, S., and O. Bonaventure, 1938 "Link bonding with transparent Multipath TCP", draft- 1939 peirens-mptcp-transparent-00 (work in progress), July 1940 2016. 1942 [IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment", 1943 IETF Journal, Fall 2016 , n.d.. 1945 [IMC11] Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A., 1946 Handley, M., and T. Hideyuki, "Is it still possible to 1947 extend TCP?", Proceedings of the 2011 ACM SIGCOMM 1948 conference on Internet measurement conference , 2011. 1950 [RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions 1951 for High Performance", RFC 1323, DOI 10.17487/RFC1323, May 1952 1992, . 1954 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 1955 RFC 1812, DOI 10.17487/RFC1812, June 1995, 1956 . 1958 [RFC1919] Chatel, M., "Classical versus Transparent IP Proxies", 1959 RFC 1919, DOI 10.17487/RFC1919, March 1996, 1960 . 1962 [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and 1963 L. Jones, "SOCKS Protocol Version 5", RFC 1928, 1964 DOI 10.17487/RFC1928, March 1996, 1965 . 1967 [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP 1968 Selective Acknowledgment Options", RFC 2018, 1969 DOI 10.17487/RFC2018, October 1996, 1970 . 1972 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 1973 Defeating Denial of Service Attacks which employ IP Source 1974 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 1975 May 2000, . 1977 [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. 1978 Shelby, "Performance Enhancing Proxies Intended to 1979 Mitigate Link-Related Degradations", RFC 3135, 1980 DOI 10.17487/RFC3135, June 2001, 1981 . 1983 [RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for 1984 Multipath Operation with Multiple Addresses", RFC 6181, 1985 DOI 10.17487/RFC6181, March 2011, 1986 . 1988 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 1989 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 1990 DOI 10.17487/RFC6887, April 2013, 1991 . 1993 [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis, 1994 "Increasing TCP's Initial Window", RFC 6928, 1995 DOI 10.17487/RFC6928, April 2013, 1996 . 1998 [RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT 1999 Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013, 2000 . 2002 [RFC7323] Borman, D., Braden, B., Jacobson, V., and R. 2003 Scheffenegger, Ed., "TCP Extensions for High Performance", 2004 RFC 7323, DOI 10.17487/RFC7323, September 2014, 2005 . 2007 [RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. 2008 Zimmermann, "A Roadmap for Transmission Control Protocol 2009 (TCP) Specification Documents", RFC 7414, 2010 DOI 10.17487/RFC7414, February 2015, 2011 . 2013 [RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and 2014 Operational Experience with Multipath TCP", RFC 8041, 2015 DOI 10.17487/RFC8041, January 2017, 2016 . 2018 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 2019 Better Connectivity Using Concurrency", RFC 8305, 2020 DOI 10.17487/RFC8305, December 2017, 2021 . 2023 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2024 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2025 . 2027 [RFC8548] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, 2028 Q., and E. Smith, "Cryptographic Protection of TCP Streams 2029 (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019, 2030 . 2032 Authors' Addresses 2034 Olivier Bonaventure (editor) 2035 Tessares 2037 Email: Olivier.Bonaventure@tessares.net 2039 Mohamed Boucadair (editor) 2040 Orange 2042 Email: mohamed.boucadair@orange.com 2044 Sri Gundavelli 2045 Cisco 2047 Email: sgundave@cisco.com 2048 SungHoon Seo 2049 Korea Telecom 2051 Email: sh.seo@kt.com 2053 Benjamin Hesmans 2054 Tessares 2056 Email: Benjamin.Hesmans@tessares.net