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Checking references for intended status: Experimental ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) ** Obsolete normative reference: RFC 6824 (Obsoleted by RFC 8684) == Outdated reference: A later version (-11) exists of draft-olteanu-intarea-socks-6-08 Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). 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: August 16, 2020 Orange 6 S. Gundavelli 7 Cisco 8 S. Seo 9 Korea Telecom 10 B. Hesmans 11 Tessares 12 February 13, 2020 14 0-RTT TCP Convert Protocol 15 draft-ietf-tcpm-converters-16 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. A Transport Converter may provide conversion service 22 for one or more TCP extensions. The conversion service is provided 23 by means of the TCP Convert Protocol (Convert). 25 This protocol provides 0-RTT (Zero Round-Trip Time) conversion 26 service since no extra delay is induced by the protocol compared to 27 connections that are not proxied. Also, the Convert Protocol does 28 not require any encapsulation (no tunnels, whatsoever). 30 This specification assumes an explicit model, where the Transport 31 Converter is explicitly configured on hosts. 33 Status of This Memo 35 This Internet-Draft is submitted in full conformance with the 36 provisions of BCP 78 and BCP 79. 38 Internet-Drafts are working documents of the Internet Engineering 39 Task Force (IETF). Note that other groups may also distribute 40 working documents as Internet-Drafts. The list of current Internet- 41 Drafts is at https://datatracker.ietf.org/drafts/current/. 43 Internet-Drafts are draft documents valid for a maximum of six months 44 and may be updated, replaced, or obsoleted by other documents at any 45 time. It is inappropriate to use Internet-Drafts as reference 46 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on August 16, 2020. 50 Copyright Notice 52 Copyright (c) 2020 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (https://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 68 1.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 3 69 1.2. Network-Assisted Connections: The Rationale . . . . . . . 4 70 2. Differences with SOCKSv5 . . . . . . . . . . . . . . . . . . 6 71 3. Conventions and Definitions . . . . . . . . . . . . . . . . . 8 72 4. Architecture & Behaviors . . . . . . . . . . . . . . . . . . 9 73 4.1. Functional Elements . . . . . . . . . . . . . . . . . . . 9 74 4.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 11 75 4.3. Data Processing at the Transport Converter . . . . . . . 14 76 4.4. Address Preservation vs. Address Sharing . . . . . . . . 16 77 4.4.1. Address Preservation . . . . . . . . . . . . . . . . 16 78 4.4.2. Address/Prefix Sharing . . . . . . . . . . . . . . . 17 79 5. Sample Examples . . . . . . . . . . . . . . . . . . . . . . . 18 80 5.1. Outgoing Converter-Assisted Multipath TCP Connections . . 18 81 5.2. Incoming Converter-Assisted Multipath TCP Connection . . 20 82 6. The Convert Protocol (Convert) . . . . . . . . . . . . . . . 21 83 6.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 22 84 6.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 22 85 6.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 22 86 6.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 23 87 6.2.3. The Info TLV . . . . . . . . . . . . . . . . . . . . 24 88 6.2.4. Supported TCP Extensions TLV . . . . . . . . . . . . 24 89 6.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 25 90 6.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 28 91 6.2.7. The Cookie TLV . . . . . . . . . . . . . . . . . . . 28 92 6.2.8. Error TLV . . . . . . . . . . . . . . . . . . . . . . 29 93 7. Compatibility of Specific TCP Options with the Conversion 94 Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 95 7.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 32 96 7.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 33 97 7.3. Selective Acknowledgments . . . . . . . . . . . . . . . . 33 98 7.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 34 99 7.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 34 100 7.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 34 101 7.7. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 35 102 8. Interactions with Middleboxes . . . . . . . . . . . . . . . . 35 103 9. Security Considerations . . . . . . . . . . . . . . . . . . . 36 104 9.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 36 105 9.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 37 106 9.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 38 107 9.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 38 108 9.5. Authentication Considerations . . . . . . . . . . . . . . 38 109 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 110 10.1. Convert Service Name . . . . . . . . . . . . . . . . . . 39 111 10.2. The Convert Protocol (Convert) Parameters . . . . . . . 39 112 10.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 40 113 10.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 40 114 10.2.3. Convert Error Messages . . . . . . . . . . . . . . . 41 115 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 42 116 11.1. Normative References . . . . . . . . . . . . . . . . . . 42 117 11.2. Informative References . . . . . . . . . . . . . . . . . 44 118 Appendix A. Example Socket API Changes to Support the 0-RTT 119 Convert Protocol . . . . . . . . . . . . . . . . . . 47 120 A.1. Active Open (Client Side) . . . . . . . . . . . . . . . . 47 121 A.2. Passive Open (Converter Side) . . . . . . . . . . . . . . 47 122 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 48 123 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 49 124 Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 125 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52 127 1. Introduction 129 1.1. The Problem 131 Transport protocols like TCP evolve regularly [RFC7414]. TCP has 132 been improved in different ways. Some improvements such as changing 133 the initial window size [RFC6928] or modifying the congestion control 134 scheme can be applied independently on clients and servers. Other 135 improvements such as Selective Acknowledgments [RFC2018] or large 136 windows [RFC7323] require a new TCP option or to change the semantics 137 of some fields in the TCP header. These modifications must be 138 deployed on both clients and servers to be actually used on the 139 Internet. Experience with the latter TCP extensions reveals that 140 their deployment can require many years. Fukuda reports in 141 [Fukuda2011] results of a decade of measurements showing the 142 deployment of Selective Acknowledgments, Window Scale and TCP 143 Timestamps. [ANRW17] describes measurements showing that TCP Fast 144 Open (TFO) [RFC7413] is still not widely deployed. 146 There are some situations where the transport stack used on clients 147 (or servers) can be upgraded at a faster pace than the transport 148 stack running on servers (or clients). In those situations, clients 149 would typically want to benefit from the features of an improved 150 transport protocol even if the servers have not yet been upgraded and 151 conversely. Some assistance from the network to make use of these 152 features is valuable. For example, Performance Enhancing Proxies 153 [RFC3135], and other service functions have been deployed as 154 solutions to improve TCP performance over links with specific 155 characteristics. 157 Recent examples of TCP extensions include Multipath TCP (MPTCP) 158 [RFC6824] or TCPINC [RFC8548]. Those extensions provide features 159 that are interesting for clients such as wireless devices. With 160 Multipath TCP, those devices could seamlessly use WLAN (Wireless 161 Local Area Network) and cellular networks, for bonding purposes, 162 faster hand-overs, or better resiliency. Unfortunately, deploying 163 those extensions on both a wide range of clients and servers remains 164 difficult. 166 More recently, 5G bonding experimentation has been conducted into 167 global range of the incumbent 4G (LTE) connectivity using newly 168 devised clients and a Multipath TCP proxy. Even if the 5G and the 4G 169 bonding relying upon Multipath TCP increases the bandwidth, it is as 170 well crucial to minimize latency for all the way between endhosts 171 regardless of whether intermediate nodes are inside or outside of the 172 mobile core. In order to handle URLLC (Ultra Reliable Low Latency 173 Communication) for the next generation mobile network, Multipath TCP 174 and its proxy mechanism such as the one used to provide Access 175 Traffic Steering, Switching, and Splitting (ATSSS) must be optimized 176 to reduce latency [TS23501]. 178 1.2. Network-Assisted Connections: The Rationale 180 This document specifies an application proxy, called Transport 181 Converter. A Transport Converter is a function that is installed by 182 a network operator to aid the deployment of TCP extensions and to 183 provide the benefits of such extensions to clients. A Transport 184 Converter may provide conversion service for one or more TCP 185 extensions. Which TCP extensions are eligible to the conversion 186 service is deployment-specific. The conversion service is provided 187 by means of the 0-RTT TCP Convert Protocol (Convert), that is an 188 application-layer protocol which uses a dedicated TCP port number. 190 The Convert Protocol provides 0-RTT (Zero Round-Trip Time) conversion 191 service since no extra delay is induced by the protocol compared to 192 connections that are not proxied. Particularly, the Convert Protocol 193 does not require extra signaling setup delays before making use of 194 the conversion service. The Convert Protocol does not require any 195 encapsulation (no tunnels, whatsoever). 197 The Transport Converter adheres to the main principles drawn in 198 [RFC1919]. In particular, a Transport Converter achieves the 199 following: 201 o Listen for client sessions; 203 o Receive from a client the address of the final target server; 205 o Setup a session to the final server; 207 o Relay control messages and data between the client and the server; 209 o Perform access controls according to local policies. 211 The main advantage of network-assisted conversion services is that 212 they enable new TCP extensions to be used on a subset of the path 213 between endpoints, which encourages the deployment of these 214 extensions. Furthermore, the Transport Converter allows the client 215 and the server to directly negotiate TCP extensions for the sake of 216 native support along the full path. 218 The Convert Protocol is a generic mechanism to provide 0-RTT 219 conversion service. As a sample applicability use case, this 220 document specifies how the Convert Protocol applies for Multipath 221 TCP. It is out of scope of this document to provide a comprehensive 222 list of all potential conversion services. Applicability documents 223 may be defined in the future. 225 This document does not assume that all the traffic is eligible to the 226 network-assisted conversion service. Only a subset of the traffic 227 will be forwarded to a Transport Converter according to a set of 228 policies. These policies, and how they are communicated to 229 endpoints, are out of scope. Furthermore, it is possible to bypass 230 the Transport Converter to connect directly to the servers that 231 already support the required TCP extension(s). 233 This document assumes an explicit model in which a client is 234 configured with one or a list of Transport Converters (statically or 235 through protocols such as [I-D.boucadair-tcpm-dhc-converter]). 236 Configuration means are outside the scope of this document. 238 The use of a Transport Converter means that there is no end-to-end 239 transport connection between the client and server. This could 240 potentially create problems in some scenarios such as those discussed 241 in Section 4 of [RFC3135]. Some of these problems may not be 242 applicable, for example, a Transport Converter can inform a client by 243 means of Network Failure (65) or Destination Unreachable (97) error 244 messages (Section 6.2.8) that it encounters a failure problem; the 245 client can react accordingly. An endpoint, or its network 246 administrator, can assess the benefit provided by the Transport 247 Converter service versus the risk. This is one reason why the 248 Transport Converter functionality has to be explicitly requested by 249 an endpoint. 251 This document is organized as follows. First, Section 2 provides a 252 brief overview of the differences between the well-known SOCKS 253 protocol and the 0-RTT Convert protocol. Section 4 provides a brief 254 explanation of the operation of Transport Converters. Then, 255 Section 6 describes the Convert Protocol. Section 7 discusses how 256 Transport Converters can be used to support different TCP extensions. 257 Section 8 then discusses the interactions with middleboxes, while 258 Section 9 focuses on the security considerations. Appendix A 259 describes how a TCP stack would need to support the protocol 260 described in this document. 262 2. Differences with SOCKSv5 264 Several IETF protocols provide proxy services; the closest to the 265 0-RTT Convert protocol being the SOCKSv5 protocol [RFC1928]. This 266 protocol is already used to deploy Multipath TCP in some cellular 267 networks (Section 2.2 of [RFC8041]). 269 A SOCKS Client creates a connection to a SOCKS Proxy, exchanges 270 authentication information, and indicates the IP address and port 271 number of the target Server. At this point, the SOCKS Proxy creates 272 a connection towards the target Server and relays all data between 273 the two proxied connections. The operation of an implementation 274 based on SOCKSv5 (without authentication) is illustrated in Figure 1. 276 Client SOCKS Proxy Server 277 | | | 278 | --------------------> | | 279 | SYN | | 280 | <-------------------- | | 281 | SYN+ACK | | 282 | --------------------> | | 283 | ACK | | 284 | | | 285 | --------------------> | | 286 |Version=5, Auth Methods| | 287 | <-------------------- | | 288 | Method | | 289 | --------------------> | | 290 |Auth Request (unless "No auth" method negotiated) 291 | <-------------------- | | 292 | Auth Response | | 293 | --------------------> | | 294 | Connect Server:Port | --------------------> | 295 | | SYN | 296 | | <-------------------- | 297 | | SYN+ACK | 298 | <-------------------- | | 299 | Succeeded | | 300 | --------------------> | | 301 | Data1 | | 302 | | --------------------> | 303 | | Data1 | 304 | | <-------------------- | 305 | | Data2 | 306 | <-------------------- | | 307 | Data2 | | 308 ... 310 Figure 1: Establishment of a TCP Connection through a SOCKS Proxy 311 Without Authentication 313 When SOCKS is used, an "end-to-end" connection between a Client and a 314 Server becomes a sequence of two TCP connections that are glued 315 together on the SOCKS Proxy. The SOCKS Client and Server exchange 316 control information at the beginning of the bytestream on the Client- 317 Proxy connection. The SOCKS Proxy then creates the connection with 318 the target Server and then glues the two connections together so that 319 all bytes sent by the application (Client) to the SOCKS Proxy are 320 relayed to the Server and vice versa. 322 The Convert Protocol is also used on TCP proxies that relay data 323 between an upstream and a downstream connection, but there are 324 important differences with SOCKSv5. A first difference is that the 325 0-RTT Convert protocol exchanges all the control information during 326 the initial RTT. This reduces the connection establishment delay 327 compared to SOCKS which requires two or more round-trip-times before 328 the establishment of the downstream connection towards the final 329 destination. In today's Internet, latency is a important metric and 330 various protocols have ben tuned to reduce their latency 331 [I-D.arkko-arch-low-latency]. A recently proposed extension to SOCKS 332 leverages the TFO (TCP Fast Open) option 333 [I-D.olteanu-intarea-socks-6] to reduce this delay. 335 A second difference is that the Convert Protocol explicitly takes the 336 TCP extensions into account. By using the Convert Protocol, the 337 Client can learn whether a given TCP extension is supported by the 338 destination Server. This enables the Client to bypass the Transport 339 Converter when the Server supports the required TCP extension(s). 340 Neither SOCKSv5 [RFC1928] nor the proposed SOCKSv6 341 [I-D.olteanu-intarea-socks-6] provide such a feature. 343 A third difference is that a Transport Converter will only confirm 344 the establishment of the connection initiated by the Client provided 345 that the downstream connection has already been accepted by the 346 Server. If the Server refuses the connection establishment attempt 347 from the Transport Converter, then the upstream connection from the 348 Client is rejected as well. This feature is important for 349 applications that check the availability of a Server or use the time 350 to connect as a hint on the selection of a Server [RFC8305]. 352 A fourth difference is that the 0-RTT Convert protocol only allows 353 the Client to specify the IP address/port number of the destination 354 server and not a DNS name. We evaluated an alternate design that 355 included the DNS name of the remote peer instead of its IP address as 356 in SOCKS [RFC1928]. However, that design was not adopted because it 357 induces both an extra load and increased delays on the Transport 358 Converter to handle and manage DNS resolution requests. 360 3. Conventions and Definitions 362 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 363 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 364 "OPTIONAL" in this document are to be interpreted as described in BCP 365 14 [RFC2119][RFC8174] when, and only when, they appear in all 366 capitals, as shown here. 368 4. Architecture & Behaviors 370 4.1. Functional Elements 372 The Convert Protocol considers three functional elements: 374 o Clients; 376 o Transport Converters; 378 o Servers. 380 A Transport Converter is a network function that proxies all data 381 exchanged over one upstream connection to one downstream connection 382 and vice versa (Figure 2). The Transport Converter, thus, maintains 383 state that associates one upstream connection to a corresponding 384 downstream connection. 386 A connection can be initiated from both sides of the Transport 387 Converter (Internet-facing interface, customer-facing interface). 389 | 390 : 391 | 392 +------------+ 393 Client <- upstream ->| Transport |<- downstream -> Server 394 connection | Converter | connection 395 +------------+ 396 | 397 customer-facing interface : Internet-facing interface 398 | 400 Figure 2: A Transport Converter Proxies Data between Pairs of TCP 401 Connections 403 "Client" refers to a software instance embedded on a host that can 404 reach a Transport Converter via its customer-facing interface. The 405 "Client" can initiate connections via a Transport Converter (referred 406 to as outgoing connections). Also, the "Client" can accept incoming 407 connections via a Transport Converter (referred to as incoming 408 connections). 410 Transport Converters can be operated by network operators or third 411 parties. Nevertheless, this document focuses on the single 412 administrative deployment case where the entity offering the 413 connectivity service to a client is also the entity which owns and 414 operates the Transport Converter. 416 A Transport Converter can be embedded in a standalone device or be 417 activated as a service on a router. How such function is enabled is 418 deployment-specific. 420 The architecture assumes that new software will be installed on the 421 Client hosts to interact with one or more Transport Converters. 422 Furthermore, the architecture allows for making use of new TCP 423 extensions even if those are not supported by a given server. 425 A Client is configured, through means that are outside the scope of 426 this document, with the names and/or the addresses of one or more 427 Transport Converters and the TCP extensions that they support. The 428 procedure for selecting a Transport Converter among a list of 429 configured Transport Converters is outside the scope of this 430 document. 432 One of the benefits of this design is that different transport 433 protocol extensions can be used on the upstream and the downstream 434 connections. This encourages the deployment of new TCP extensions 435 until they are widely supported by servers, in particular. 437 The architecture does not mandate anything on the Server side. 439 Similar to SOCKS, the architecture does not interfere with end-to-end 440 TLS connections [RFC8446] between the Client and the Server 441 (Figure 3). In other words, end-to-end TLS is supported in the 442 presence of a Converter. 444 Client Transport Server 445 | Converter | 446 | | | 447 /==========================================\ 448 | End-to-end TLS | 449 \==========================================/ 451 * TLS messages exchanged between the Client 452 and the Server are not shown. 454 Figure 3: End-to-end TLS via a Transport Converter 456 It is out of scope of this document to elaborate on specific 457 considerations related to the use of TLS in the Client-Converter 458 connection leg to exchange Convert messages (in addition to the end- 459 to-end TLS connection). 461 4.2. Theory of Operation 463 At a high level, the objective of the Transport Converter is to allow 464 the use a specific extension, e.g., Multipath TCP, on a subset of the 465 path even if the peer does not support this extension. This is 466 illustrated in Figure 4 where the Client initiates a Multipath TCP 467 connection with the Transport Converter (packets belonging to the 468 Multipath TCP connection are shown with "===") while the Transport 469 Converter uses a regular TCP connection with the Server. 471 Client Transport Server 472 | Converter | 473 | | | 474 |==================>|--------------------->| 475 | | | 476 |<==================|<---------------------| 477 | | | 478 Multipath TCP packets Regular TCP packets 480 Figure 4: An Example of 0-RTT Network-Assisted Outgoing MPTCP 481 Connection 483 The packets belonging to a connection established through a Transport 484 Converter may follow a different path than the packets directly 485 exchanged between the Client and the Server. Deployments should 486 minimize the possible additional delay by carefully selecting the 487 location of the Transport Converter used to reach a given 488 destination. 490 When establishing a connection, the Client can, depending on local 491 policies, either contact the Server directly (e.g., by sending a TCP 492 SYN towards the Server) or create the connection via a Transport 493 Converter. In the latter case (that is, the conversion service is 494 used), the Client initiates a connection towards the Transport 495 Converter and indicates the IP address and port number of the Server 496 within the connection establishment packet. Doing so enables the 497 Transport Converter to immediately initiate a connection towards that 498 Server, without experiencing an extra delay. The Transport Converter 499 waits until the receipt of the confirmation that the Server agrees to 500 establish the connection before confirming it to the Client. 502 The Client places the destination address and port number of the 503 Server in the payload of the SYN sent to the Transport Converter to 504 minimize connection establishment delays. The Transport Converter 505 maintains two connections that are combined together: 507 o the upstream connection is the one between the Client and the 508 Transport Converter. 510 o the downstream connection is the one between the Transport 511 Converter and the Server. 513 Any user data received by the Transport Converter over the upstream 514 (or downstream) connection is proxied over the downstream (or 515 upstream) connection. In particular, if the initial SYN message 516 contains user data in its payload (e.g., [RFC7413]), that data MUST 517 be placed right after the Convert TLVs when generating the SYN. 519 Figure 5 illustrates the establishment of an outgoing TCP connection 520 by a Client through a Transport Converter. 522 o Note: The information shown between brackets in Figure 5 (and 523 other figures in the document) refers to Convert Protocol messages 524 described in Section 6. 526 Transport 527 Client Converter Server 528 | | | 529 |SYN [->Server:port]| SYN | 530 |------------------>|--------------------->| 531 |<------------------|<---------------------| 532 | SYN+ACK [ ] | SYN+ACK | 533 | ... | ... | 535 Figure 5: Establishment of an Outgoing TCP Connection Through a 536 Transport Converter 538 The Client sends a SYN destined to the Transport Converter. The 539 payload of this SYN contains the address and port number of the 540 Server. The Transport Converter does not reply immediately to this 541 SYN. It first tries to create a TCP connection towards the target 542 Server. If this upstream connection succeeds, the Transport 543 Converter confirms the establishment of the connection to the Client 544 by returning a SYN+ACK and the first bytes of the bytestream contain 545 information about the TCP options that were negotiated with the 546 Server. Also, a state entry is instantiated for this connection. 547 This state entry is used by the Converter to handle subsequent 548 messages belonging to the connection. 550 The connection can also be established from the Internet towards a 551 Client via a Transport Converter (Figure 6). This is typically the 552 case when the Client hosts an application server that listens to a 553 specific port number. When the Converter receives an incoming SYN 554 from a remote host, it checks if it can provide the conversion 555 service for the destination IP address and destination port number of 556 that SYN. The Transport Converter receives this SYN because it is, 557 for example, on the path between the remote host and the Client or it 558 provides address sharing service for the Client. If the check fails, 559 the packet is silently ignored by the Converter. If the check is 560 successful, the Converter tries to initiate a TCP connection towards 561 the Client from its own address and using its configured TCP options. 562 In the SYN that corresponds to this connection attempt, the Transport 563 Convert inserts a TLV message that indicates the source address and 564 port number of the remote host. A transport session entry is created 565 by the Converter for this connection. SYN+ACK and ACK will be then 566 exchanged between the Client, the Converter, and remote host to 567 confirm the establishment of the connection. The Converter uses the 568 transport session entry to proxy packets belonging to the connection. 570 Transport Remote 571 Client Converter Host (RH) 572 | | | 573 |SYN [<-RH IP@:port]| SYN | 574 |<------------------|<---------------------| 575 |------------------>|--------------------->| 576 | SYN+ACK [ ] | SYN+ACK | 577 | ... | ... | 579 Figure 6: Establishment of an Incoming TCP Connection Through a 580 Transport Converter 582 Standard TCP ([RFC0793], Section 3.4) allows a SYN packet to carry 583 data inside its payload but forbids the receiver from delivering it 584 to the application until completion of the three-way-handshake. To 585 enable applications to exchange data in a TCP handshake, this 586 specification follows an approach similar to TCP Fast Open [RFC7413] 587 and thus removes the constraint by allowing data in SYN packets to be 588 delivered to the Transport Converter application. 590 As discussed in [RFC7413], such change to TCP semantic raises two 591 issues. First, duplicate SYNs can cause problems for some 592 applications that rely on TCP. Second, TCP suffers from SYN flooding 593 attacks [RFC4987]. TFO solves these two problems for applications 594 that can tolerate replays by using the TCP Fast Open option that 595 includes a cookie. However, the utilization of this option consumes 596 space in the limited TCP header. Furthermore, there are situations, 597 as noted in Section 7.3 of [RFC7413] where it is possible to accept 598 the payload of SYN packets without creating additional security risks 599 such as a network where addresses cannot be spoofed and the Transport 600 Converter only serves a set of hosts that are identified by these 601 addresses. 603 For these reasons, this specification does not mandate the use of the 604 TCP Fast Open option when the Client sends a connection establishment 605 packet towards a Transport Converter. The Convert Protocol includes 606 an optional Cookie TLV that provides similar protection as the TCP 607 Fast Open option without consuming space in the TCP header. 608 Furthermore, this design allows for the use of longer cookies than 609 [RFC7413]. 611 If the downstream (or upstream) connection fails for some reason 612 (excessive retransmissions, reception of an RST segment, etc.), then 613 the Converter reacts by forcing the tear-down of the upstream (or 614 downstream) connection. 616 The same reasoning applies when the upstream connection ends with an 617 exchange of FIN packets. In this case, the Converter should also 618 terminate the downstream connection by using FIN packets. If the 619 downstream connection terminates with the exchange of FIN packets, 620 the Converter should initiate a graceful termination of the upstream 621 connection. 623 4.3. Data Processing at the Transport Converter 625 As mentioned in Section 4.2, the Transport Converter acts as a TCP 626 proxy between the upstream connection (i.e., between the Client and 627 the Transport Converter) and the downstream connection (i.e., between 628 the Transport Converter and the Server). 630 The control messages, discussed in Section 6, establish state 631 (called, transport session entry) in the Transport Converter that 632 will enable it to proxy between the two TCP connections. 634 The Transport Converter uses the transport session entry to proxy 635 packets belonging to the connection. An implementation example of a 636 transport session entry for TCP connections is shown in Figure 7. 638 (C,c) <--> (T,t), (S,s), Lifetime 640 Where: 641 * C and c are the source IP address and source port number 642 used by the Client for the upstream connection. 643 * S and s are the Server's IP address and port number. 644 * T and t are the source IP address and source port number 645 used by the Transport Converter to proxy the connection. 646 * Lifetime is the validity lifetime of the entry as assigned 647 by the Converter. 649 Figure 7: An Example of Transport Session Entry (TCP) 651 Clients send packets bound to connections eligible to the conversion 652 service to the provisioned Transport Converter and destination port 653 number. This applies for both control messages and data. Additional 654 information is supplied by Clients to the Transport Converter by 655 means of Convert messages as detailed in Section 6. User data can be 656 included in SYN or non-SYN messages. User data is unambiguously 657 distinguished from Convert TLVs by a Transport Converter owing to the 658 Convert Fixed Header in the Convert messages (Section 6.1). These 659 Convert TLVs are destined to the Transport Convert and are, thus, 660 removed by the Transport Converter when proxying between the two 661 connections. 663 Upon receipt of a packet that belongs to an existing connection 664 between a Client and the Transport Converter the Converter proxies 665 the user data to the Server using the information stored in the 666 corresponding transport session entry. For example, in reference to 667 Figure 7, the Transport Converter proxies the data received from (C, 668 c) downstream using (T,t) as source transport address and (S,s) as 669 destination transport address. 671 A similar process happens for data sent from the Server. The 672 Converter acts as a TCP proxy and sends the data to the Client 673 relying upon the information stored in a transport session entry. 674 The Converter associates a lifetime with state entries used to bind 675 an upstream connection with its downstream connection. 677 When Multipath TCP is used between the Client and the Transport 678 Converter, the Converter maintains more state (e.g. information about 679 the subflows) for each Multipath TCP connection. The procedure 680 described above continues to apply except that the Converter needs to 681 manage the establishment/termination of subflows and schedule packets 682 among the established ones. These operations are part of the 683 Multipath TCP implementation. They are independent of the Convert 684 protocol that only processes the Convert messages in the beginning of 685 the bytestream. 687 A Transport Converter may operate in address preservation mode (that 688 is, the Converter does not rewrite the source IP address (i.e., 689 C==T)) or address sharing mode (that is, an address pool is shared 690 among all Clients serviced by the Converter (i.e., C!=T)); refer to 691 Section 4.4 for more details. Which behavior to use by a Transport 692 Converter is deployment-specific. If address sharing mode is 693 enabled, the Transport Converter MUST adhere to REQ-2 of [RFC6888] 694 which implies a default "IP address pooling" behavior of "Paired" (as 695 defined in Section 4.1 of [RFC4787]) MUST be supported. This 696 behavior is meant to avoid breaking applications that depend on the 697 source address remaining constant. 699 4.4. Address Preservation vs. Address Sharing 701 The Transport Converter is provided with instructions about the 702 behavior to adopt with regards to the processing of source addresses 703 of outgoing packets. The following sub-sections discusses two 704 deployment models for illustration purposes. It is out of the scope 705 of this document to make a recommendation. 707 4.4.1. Address Preservation 709 In this model, the visible source IP address of a packet proxied by a 710 Transport Converter to a Server is an IP address of the end host 711 (Client). No dedicated IP address pool is provisioned to the 712 Transport Converter, but the the Transport Converter is located on 713 the path between the Client and the Server. 715 For Multipath TCP, the Transport Converter preserves the source IP 716 address used by the Client when establishing the initial subflow. 717 Data conveyed in secondary subflows will be proxied by the Transport 718 Converter using the source IP address of the initial subflow. An 719 example of a proxied Multipath TCP connection with address 720 preservation is shown in Figure 8. 722 Transport 723 Client Converter Server 725 @:C1,C2 @:Tc @:S 726 || | | 727 |src:C1 SYN dst:Tc|src:C1 dst:S| 728 |-------MPC [->S:port]------->|-------SYN------->| 729 || | | 730 ||dst:C1 src:Tc|dst:C1 src:S| 731 |<---------SYN/ACK------------|<-----SYN/ACK-----| 732 || | | 733 |src:C1 dst:Tc|src:C1 dst:S| 734 |------------ACK------------->|-------ACK------->| 735 | | | 736 |src:C2 ... dst:Tc| ... | 737 ||<-----Secondary Subflow---->|src:C1 dst:S| 738 || |-------data------>| 739 | .. | ... | 741 Legend: 742 Tc: IP address used by the Transport Converter on its customer-facing 743 interface. 745 Figure 8: Example of Address Preservation 747 The Transport Converter must be on the forwarding path of incoming 748 traffic. Because the same (destination) IP address is used for both 749 proxied and non-proxied connections, the Transport Converter should 750 not drop incoming packets it intercepts if no matching entry is found 751 for the packets. Unless explicitly configured otherwise, such 752 packets are forwarded according to the instructions of a local 753 forwarding table. 755 4.4.2. Address/Prefix Sharing 757 A pool of global IPv4 addresses is provisioned to the Transport 758 Converter along with possible instructions about the address sharing 759 ratio to apply (see Appendix B of [RFC6269]). An address is thus 760 shared among multiple clients. 762 Likewise, rewriting the source IPv6 prefix [RFC6296] may be used to 763 ease redirection of incoming IPv6 traffic towards the appropriate 764 Transport Converter. A pool of IPv6 prefixes is then provisioned to 765 the Transport Converter for this purpose. 767 Adequate forwarding policies are enforced so that traffic destined to 768 an address of such pool is intercepted by the appropriate Transport 769 Converter. Unlike Section 4.4.1, the Transport Converter drops 770 incoming packets which do not match an active transport session 771 entry. 773 An example is shown in Figure 9. 775 Transport 776 Client Converter Server 778 @:C @:Tc|Te @:S 779 | | | 780 |src:C dst:Tc|src:Te dst:S| 781 |-------SYN [->S:port]------->|-------SYN------->| 782 | | | 783 |dst:C src:Tc|dst:Te src:S| 784 |<---------SYN/ACK------------|<-----SYN/ACK-----| 785 | | | 786 |src:C dst:Tc|src:Te dst:S| 787 |------------ACK------------->|-------ACK------->| 788 | | | 789 | ... | ... | 791 Legend: 792 Tc: IP address used by the Transport Converter for its customer-facing 793 interface. 794 Te: IP address used by the Transport Converter for its Internet-facing 795 interface. 797 Figure 9: Address Sharing 799 5. Sample Examples 801 5.1. Outgoing Converter-Assisted Multipath TCP Connections 803 As an example, let us consider how the Convert Protocol can help the 804 deployment of Multipath TCP. We assume that both the Client and the 805 Transport Converter support Multipath TCP, but consider two different 806 cases depending on whether the Server supports Multipath TCP or not. 808 As a reminder, a Multipath TCP connection is created by placing the 809 MP_CAPABLE (MPC) option in the SYN sent by the Client. 811 Figure 10 describes the operation of the Transport Converter if the 812 Server does not support Multipath TCP. 814 Transport 815 Client Converter Server 816 |SYN, MPC | | 817 |[->Server:port] | SYN, MPC | 818 |------------------>|--------------------->| 819 |<------------------|<---------------------| 820 | SYN+ACK,MPC [.] | SYN+ACK | 821 |------------------>|--------------------->| 822 | ACK, MPC | ACK | 823 | ... | ... | 825 Figure 10: Establishment of a Multipath TCP Connection through a 826 Transport Converter towards a Server that does not support Multipath 827 TCP 829 The Client tries to initiate a Multipath TCP connection by sending a 830 SYN with the MP_CAPABLE option (MPC in Figure 10). The SYN includes 831 the address and port number of the target Server, that are extracted 832 and used by the Transport Converter to initiate a Multipath TCP 833 connection towards this Server. Since the Server does not support 834 Multipath TCP, it replies with a SYN+ACK that does not contain the 835 MP_CAPABLE option. The Transport Converter notes that the connection 836 with the Server does not support Multipath TCP and returns the 837 extended TCP header received from the Server to the Client. 839 Note that, if the TCP connection is reset for some reason, the 840 Converter tears down the Multipath TCP connection by transmitting a 841 MP_FASTCLOSE. Likewise, if the Multipath TCP connection ends with 842 the transmission of DATA_FINs, the Converter terminates the TCP 843 connection by using FIN segments. As a side note, given that with 844 Multipath TCP, RST only has the scope of the subflow and will only 845 close the concerned subflow but not affect the remaining subflows, 846 the Converter does not terminate the downstream TCP connection upon 847 receipt of an RST over a Multipath subflow. 849 Figure 11 considers a Server that supports Multipath TCP. In this 850 case, it replies to the SYN sent by the Transport Converter with the 851 MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport 852 Converter confirms the establishment of the connection to the Client 853 and indicates to the Client that the Server supports Multipath TCP. 854 With this information, the Client has discovered that the Server 855 supports Multipath TCP. This will enable the Client to bypass the 856 Transport Converter for the subsequent Multipath TCP connections that 857 it will initiate towards this Server. 859 Transport 860 Client Converter Server 861 |SYN, MPC | | 862 |[->Server:port] | SYN, MPC | 863 |------------------>|--------------------->| 864 |<------------------|<---------------------| 865 |SYN+ACK, MPC | SYN+ACK, MPC | 866 |[MPC supported] | | 867 |------------------>|--------------------->| 868 | ACK, MPC | ACK, MPC | 869 | ... | ... | 871 Figure 11: Establishment of a Multipath TCP Connection through a 872 Converter towards an MPTCP-capable Server 874 5.2. Incoming Converter-Assisted Multipath TCP Connection 876 An example of an incoming Converter-assisted Multipath TCP connection 877 is depicted in Figure 12. In order to support incoming connections 878 from remote hosts, the Client may use PCP [RFC6887] to instruct the 879 Transport Converter to create dynamic mappings. Those mappings will 880 be used by the Transport Converter to intercept an incoming TCP 881 connection destined to the Client and convert it into a Multipath TCP 882 connection. 884 Typically, the Client sends a PCP request to the Converter asking to 885 create an explicit TCP mapping for (internal IP address, internal 886 port number). The Converter accepts the request by creating a TCP 887 mapping (internal IP address, internal port number, external IP 888 address, external port number). The external IP address and external 889 port number will be then advertised using an out-of-band mechanism so 890 that remote hosts can initiate TCP connections to the Client via the 891 Converter. Note that the external and internal information may be 892 the same. 894 Then, when the Converter receives an incoming SYN, it checks its 895 mapping table to verify if there is an active mapping matching the 896 destination IP address and destination port of that SYN. If no entry 897 is found, the Converter silently ignores the message. If an entry is 898 found, the Converter inserts an MP_CAPABLE option and Connect TLV in 899 the SYN packet, rewrites the source IP address to one of its IP 900 addresses and, eventually, the destination IP address and port number 901 in accordance with the information stored in the mapping. SYN+ACK 902 and ACK will be then exchanged between the Client and the Converter 903 to confirm the establishment of the initial subflow. The Client can 904 add new subflows following normal Multipath TCP procedures. 906 Transport Remote 907 Client Converter Host 908 | | | 909 |<--------------------|<-------------------| 910 |SYN, MPC | SYN | 911 |[Remote Host:port] | | 912 |-------------------->|------------------->| 913 | SYN+ACK, MPC | SYN+ACK | 914 |<--------------------|<-------------------| 915 | ACK, MPC | ACK | 916 | ... | ... | 918 Figure 12: Establishment of an Incoming Multipath TCP Connection 919 through a Transport Converter 921 It is out of scope of this document to define specific Convert TLVs 922 to manage incoming connections. These TLVs can be defined in a 923 separate document. 925 6. The Convert Protocol (Convert) 927 This section defines the Convert Protocol (Convert, for short) 928 messages that are exchanged between a Client and a Transport 929 Converter. 931 The Transport Converter listens on a dedicated TCP port number for 932 Convert messages from Clients. That port number is configured by an 933 administrator. 935 Convert messages MUST be included as the first bytes of the 936 bytestream. All Convert messages starts with a 32 bits long fixed 937 header (Section 6.1) followed by one or more Convert TLVs (Type, 938 Length, Value) (Section 6.2). 940 o Implementation note 1: Several implementers expressed concerns 941 about the use of TFO. As a reminder, the TFO Cookie protects from 942 some attack scenarios that affect open servers like web servers. 943 The Convert Protocol is different and, as discussed in RFC7413, 944 there are different ways to protect from such attacks. Instead of 945 using a TFO cookie inside the TCP options, which consumes precious 946 space in the extended TCP header, the Convert Protocol supports 947 the utilization of a Cookie that is placed in the SYN payload. 948 This provides the same level of protection as a TFO Cookie in 949 environments were such protection is required. 951 o Implementation note 2: Error messages are not included in RST but 952 sent in the bytestream. Implementers have indicated that 953 processing RST on clients was difficult on some platforms. This 954 design simplifies client implementations. 956 6.1. The Convert Fixed Header 958 The Convert Protocol uses a 32 bits long fixed header that is sent by 959 both the Client and the Transport Converter over each established 960 connection. This header indicates both the version of the protocol 961 used and the length of the Convert message. 963 The Client and the Transport Converter MUST send the fixed-sized 964 header, shown in Figure 13, as the first four bytes of the 965 bytestream. 967 1 2 3 968 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 1 969 +---------------+---------------+-------------------------------+ 970 | Version | Total Length | Unassigned | 971 +---------------+---------------+-------------------------------+ 973 Figure 13: The Convert Fixed Header 975 The Version is encoded as an 8 bits unsigned integer value. This 976 document specifies version 1. Version 0 is reserved by this document 977 and MUST NOT be used. 979 The Total Length is the number of 32 bits word, including the header, 980 of the bytestream that are consumed by the Convert messages. Since 981 Total Length is also an 8 bits unsigned integer, those messages 982 cannot consume more than 1020 bytes of data. This limits the number 983 of bytes that a Transport Converter needs to process. A Total Length 984 of zero is invalid and the connection MUST be reset upon reception of 985 a header with such total length. 987 The Unassigned field MUST be set to zero in this version of the 988 protocol. These bits are available for future use. 990 The Total Length field unambiguously marks the number of 32 bits 991 words that carry Convert TLVs in the beginning of the bytestream. 993 6.2. Convert TLVs 995 6.2.1. Generic Convert TLV Format 997 The Convert Protocol uses variable length messages that are encoded 998 using the generic TLV format depicted in Figure 14. 1000 The length of all TLVs used by the Convert Protocol is always a 1001 multiple of four bytes. All TLVs are aligned on 32 bits boundaries. 1002 All TLV fields are encoded using the network byte order. 1004 1 2 3 1005 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 1006 +---------------+---------------+-------------------------------+ 1007 | Type | Length | Value ... | 1008 +---------------+---------------+-------------------------------+ 1009 // ... (optional) Value // 1010 +---------------------------------------------------------------+ 1012 Figure 14: Convert Generic TLV Format 1014 The Length field covers Type, Length, and Value fields. It is 1015 expressed in units of 32 bits words. If necessary, Value MUST be 1016 padded with zeroes so that the length of the TLV is a multiple of 32 1017 bits. 1019 A given TLV MUST only appear once on a connection. If a Client 1020 receives two or more instances of the same TLV over a Convert 1021 connection, it MUST reset the associated TCP connection. If a 1022 Converter receives two or more instances of the same TLV over a 1023 Convert connection, it MUST return a Malformed Message Error TLV and 1024 close the associated TCP connection. 1026 6.2.2. Summary of Supported Convert TLVs 1028 This document specifies the following Convert TLVs: 1030 +------+-----+----------+------------------------------------------+ 1031 | Type | Hex | Length | Description | 1032 +------+-----+----------+------------------------------------------+ 1033 | 1 | 0x1 | 1 | Info TLV | 1034 | 10 | 0xA | Variable | Connect TLV | 1035 | 20 | 0x14| Variable | Extended TCP Header TLV | 1036 | 21 | 0x15| Variable | Supported TCP Extensions TLV | 1037 | 22 | 0x16| Variable | Cookie TLV | 1038 | 30 | 0x1E| Variable | Error TLV | 1039 +------+-----+----------+------------------------------------------+ 1041 Figure 15: The TLVs used by the Convert Protocol 1043 Type 0x0 is a reserved value. If a Client receives a TLV of type 1044 0x0, it MUST reset the associated TCP connection. If a Converter 1045 receives a TLV of type 0x0, it MUST return an Unsupported Message 1046 Error TLV and close the associated TCP connection. 1048 Implementations MUST reset the connection upon reception of messages 1049 with such TLV. 1051 The Client typically sends in the first connection it established 1052 with a Transport Converter the Info TLV (Section 6.2.3) to learn its 1053 capabilities. Assuming the Client is authorized to invoke the 1054 Transport Converter, the latter replies with the Supported TCP 1055 Extensions TLV (Section 6.2.4). 1057 The Client can request the establishment of connections to servers by 1058 using the Connect TLV (Section 6.2.5). If the connection can be 1059 established with the final server, the Transport Converter replies 1060 with the Extended TCP Header TLV (Section 6.2.6). If not, the 1061 Transport Converter returns an Error TLV (Section 6.2.8) and then 1062 closes the connection. The Transport Converter MUST NOT send a RST 1063 immediately after the detection of an error to let the Error TLV 1064 reach the Client. As explained later, the Client will anyway send a 1065 RST upon reception of the Error TLV. 1067 When an error is encountered an Error TLV with the appropriate error 1068 code MUST be returned by the Transport Converter. 1070 6.2.3. The Info TLV 1072 The Info TLV (Figure 16) is an optional TLV which can be sent by a 1073 Client to request the TCP extensions that are supported by a 1074 Transport Converter. It is typically sent on the first connection 1075 that a Client establishes with a Transport Converter to learn its 1076 capabilities. Assuming a Client is entitled to invoke the Transport 1077 Converter, the latter replies with the Supported TCP Extensions TLV 1078 described in Section 6.2.4. 1080 1 2 3 1081 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 1082 +---------------+---------------+-------------------------------+ 1083 | Type=0x1 | Length | Zero | 1084 +---------------+---------------+-------------------------------+ 1086 Figure 16: The Info TLV 1088 6.2.4. Supported TCP Extensions TLV 1090 The Supported TCP Extensions TLV (Figure 17) is used by a Transport 1091 Converter to announce the TCP options for which it provides a 1092 conversion service. A Transport Converter SHOULD include in this 1093 list the TCP options that it supports in outgoing SYNs. 1095 Each supported TCP option is encoded with its TCP option Kind listed 1096 in the "TCP Parameters" registry maintained by IANA. 1098 1 2 3 1099 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 1100 +---------------+---------------+-------------------------------+ 1101 | Type=0x15 | Length | Unassigned | 1102 +---------------+---------------+-------------------------------+ 1103 | Kind #1 | Kind #2 | ... | 1104 +---------------+---------------+-------------------------------+ 1105 / ... / 1106 / / 1107 +---------------------------------------------------------------+ 1109 Figure 17: The Supported TCP Extensions TLV 1111 TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by 1112 all TCP implementations and thus MUST NOT appear in this list. 1114 The list of Supported TCP Extensions is padded with 0 to end on a 32 1115 bits boundary. 1117 For example, if the Transport Converter supports Multipath TCP, 1118 Kind=30 will be present in the Supported TCP Extensions TLV that it 1119 returns in response to Info TLV. 1121 6.2.5. Connect TLV 1123 The Connect TLV (Figure 18) is used to request the establishment of a 1124 connection via a Transport Converter. This connection can be from or 1125 to a Client. 1127 The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain 1128 the destination port number and IP address of the Server, for 1129 outgoing connections. For incoming connections destined to a Client 1130 serviced via a Transport Converter, these fields convey the source 1131 port number and IP address of the SYN packet received by the 1132 Transport Converter from the server. 1134 The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4 1135 addresses MUST be encoded using the IPv4-Mapped IPv6 Address format 1136 defined in [RFC4291]. Further, Remote Peer IP address field MUST NOT 1137 include multicast, broadcast, and host loopback addresses [RFC6890]. 1138 If a Converter receives a Connect TLVs with such invalid addresses, 1139 it MUST reply with a Malformed Message Error TLV and close the 1140 associated TCP connection. 1142 We distinguish two types of Connect TLV based on their length: (1) 1143 the Base Connect TLV has a length of 20 bytes and contains a remote 1144 address and a remote port (Figure 18), (2) the Extended Connect TLV 1145 spans more than 20 bytes and also includes the optional 'TCP Options' 1146 field (Figure 19). This field is used to request the advertisement 1147 of specific TCP options to the server. 1149 1 2 3 1150 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 1151 +---------------+---------------+-------------------------------+ 1152 | Type=0xA | Length | Remote Peer Port | 1153 +---------------+---------------+-------------------------------+ 1154 | | 1155 | Remote Peer IP Address (128 bits) | 1156 | | 1157 | | 1158 +---------------------------------------------------------------+ 1160 Figure 18: The Base Connect TLV 1162 1 2 3 1163 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 1164 +---------------+---------------+-------------------------------+ 1165 | Type=0xA | Length | Remote Peer Port | 1166 +---------------+---------------+-------------------------------+ 1167 | | 1168 | Remote Peer IP Address (128 bits) | 1169 | | 1170 | | 1171 +---------------------------------------------------------------+ 1172 / TCP Options (Variable) / 1173 / ... / 1174 +---------------------------------------------------------------+ 1176 Figure 19: The Extended Connect TLV 1178 The 'TCP Options' field is a variable length field that carries a 1179 list of TCP option fields (Figure 20). Each TCP option field is 1180 encoded as a block of 2+n bytes where the first byte is the TCP 1181 option Kind and the second byte is the length of the TCP option as 1182 specified in [RFC0793]. The minimum value for the TCP option Length 1183 is 2. The TCP options that do not include a length sub-field, i.e., 1184 option types 0 (EOL) and 1 (NOP) defined in [RFC0793] MUST NOT be 1185 placed inside the TCP options field of the Connect TLV. The optional 1186 Value field contains the variable-length part of the TCP option. A 1187 length of two indicates the absence of the Value field. The TCP 1188 options field always ends on a 32 bits boundary after being padded 1189 with zeros. 1191 1 2 3 1192 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 1193 +---------------+---------------+---------------+---------------+ 1194 | TCPOpt kind | TCPOpt Length | Value (opt) | .... | 1195 +---------------+---------------+---------------+---------------+ 1196 | .... | 1197 +---------------------------------------------------------------+ 1198 | ... | 1199 +---------------------------------------------------------------+ 1201 Figure 20: The TCP Options Field 1203 Upon reception of a Base Connect TLV, and absent any policy (e.g., 1204 rate-limit) or resource exhaustion conditions, a Transport Converter 1205 attempts to establish a connection to the address and port that it 1206 contains. The Transport Converter MUST use by default the TCP 1207 options that correspond to its local policy to establish this 1208 connection. These are the options that it advertises in the 1209 Supported TCP Extensions TLV. 1211 Upon reception of an Extended Connect TLV, a Transport Converter 1212 first checks whether it supports the TCP Options listed in the 'TCP 1213 Options' field. If not, it returns an error message (Section 6.2.8). 1214 If the above check succeeded and absent any rate limit policy or 1215 resource exhaustion conditions, a Transport Converter MUST attempt to 1216 establish a connection to the address and port that it contains. It 1217 MUST include in the SYN that it sends to the Server the options 1218 listed in the 'TCP Options' sub-field and the TCP options that it 1219 would have used according to its local policies. For the TCP options 1220 that are included in the TCP Options field without an optional value, 1221 the Transport Converter MUST generate its own value. For the TCP 1222 options that are included in the 'TCP Options' field with an optional 1223 value, it MUST copy the entire option in the SYN sent to the remote 1224 server. This feature is required to support TCP Fast Open. See 1225 Section 7 for a detailed discussion of the different types of TCP 1226 options. 1228 The Transport Converter may refuse a Connect TLV request for various 1229 reasons (e.g., authorization failed, out of resources, invalid 1230 address type, unsupported TCP option). An error message indicating 1231 the encountered error is returned to the requesting Client 1232 (Section 6.2.8). In order to prevent denial-of-service attacks, 1233 error messages sent to a Client SHOULD be rate-limited. 1235 6.2.6. Extended TCP Header TLV 1237 The Extended TCP Header TLV (Figure 21) is used by the Transport 1238 Converter to return to the Client the TCP options that were returned 1239 by the Server in the SYN+ACK packet. A Transport Converter MUST 1240 return this TLV if the Client sent an Extended Connect TLV and the 1241 connection was accepted by the server. 1243 1 2 3 1244 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 1245 +---------------+---------------+-------------------------------+ 1246 | Type=0x14 | Length | Unassigned | 1247 +---------------+---------------+-------------------------------+ 1248 / Returned Extended TCP header / 1249 / ... / 1250 +---------------------------------------------------------------+ 1252 Figure 21: The Extended TCP Header TLV 1254 The Returned Extended TCP header field is a copy of the TCP Options 1255 that were included in the SYN+ACK received by the Transport 1256 Converter. 1258 The Unassigned field MUST be set to zero by the sender and ignored by 1259 the receiver. 1261 6.2.7. The Cookie TLV 1263 The Cookie TLV (Figure 22) is an optional TLV which is similar to the 1264 TCP Fast Open Cookie [RFC7413]. A Transport Converter may want to 1265 verify that a Client can receive the packets that it sends to prevent 1266 attacks from spoofed addresses. This verification can be done by 1267 using a Cookie that is bound to, for example, the IP address(es) of 1268 the Client. This Cookie can be configured on the Client by means 1269 that are outside of this document or provided by the Transport 1270 Converter as follows. 1272 A Transport Converter that has been configured to use the optional 1273 Cookie TLV MUST verify the presence of this TLV in the payload of the 1274 received SYN. If this TLV is present, the Transport Converter MUST 1275 validate the Cookie by means similar to those in Section 4.1.2 of 1276 [RFC7413] (i.e., IsCookieValid). If the Cookie is valid, the 1277 connection establishment procedure can continue. Otherwise, the 1278 Transport Converter MUST return an Error TLV set to "Not Authorized" 1279 and close the connection. 1281 If the received SYN did not contain a Cookie TLV, and cookie 1282 validation is required, the Transport Converter MAY compute a Cookie 1283 bound to this Client address and return a Convert message containing 1284 the fixed header, an Error TLV set to "Missing Cookie" and the 1285 computed Cookie and close the connection. The Client will react to 1286 this error by first issuing a reset to terminate the connection. It 1287 also stores the received Cookie in its cache and attempts to 1288 reestablish a new connection to the Transport Converter that includes 1289 the Cookie TLV. 1291 The format of the Cookie TLV is shown in Figure 22. 1293 1 2 3 1294 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 1295 +---------------+---------------+-------------------------------+ 1296 | Type=0x16 | Length | Zero | 1297 +---------------+---------------+-------------------------------+ 1298 / Opaque Cookie / 1299 / ... / 1300 +---------------------------------------------------------------+ 1302 Figure 22: The Cookie TLV 1304 6.2.8. Error TLV 1306 The Error TLV (Figure 23) is meant to provide information about some 1307 errors that occurred during the processing of a Convert message. 1308 This TLV has a variable length. Upon reception of an Error TLV, a 1309 Client MUST reset the associated connection. 1311 1 2 3 1312 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 1313 +---------------+---------------+----------------+--------------+ 1314 | Type=0x1E | Length | Error Code | Value | 1315 +---------------+---------------+----------------+--------------+ 1316 // ... (optional) Value // 1317 +---------------------------------------------------------------+ 1319 Figure 23: The Error TLV 1321 Different types of errors can occur while processing Convert 1322 messages. Each error is identified by an Error Code represented as 1323 an unsigned integer. Four classes of error codes are defined: 1325 o Message validation and processing errors (0-31 range): returned 1326 upon reception of an invalid message (including valid messages but 1327 with invalid or unknown TLVs). 1329 o Client-side errors (32-63 range): the Client sent a request that 1330 could not be accepted by the Transport Converter (e.g., 1331 unsupported operation). 1333 o Converter-side errors (64-95 range): problems encountered on the 1334 Transport Converter (e.g., lack of resources) which prevent it 1335 from fulfilling the Client's request. 1337 o Errors caused by the destination server (96-127 range): the final 1338 destination could not be reached or it replied with a reset. 1340 The following error codes are defined in this document: 1342 o Unsupported Version (0): The version number indicated in the fixed 1343 header of a message received from a peer is not supported. 1345 This error code MUST be generated by a peer (e.g. Transport 1346 Converter) when it receives a request having a version number that 1347 it does not support. 1349 The value field MUST be set to the version supported by the peer. 1350 When multiple versions are supported by the peer, it includes the 1351 list of supported version in the value field; each version is 1352 encoded in 8 bits. The list of supported versions should be 1353 padded with zeros to end on a 32 bits boundary. 1355 Upon receipt of this error code, the remote peer (e.g., Client) 1356 checks whether it supports one of the versions returned by the 1357 peer. The highest common supported version MUST be used by the 1358 remote peer in subsequent exchanges with the peer. 1360 o Malformed Message (1): This error code is sent to indicate that a 1361 message received from a peer cannot be successfully parsed and 1362 validated. 1364 Typically, this error code is sent by the Transport Converter if 1365 it receives a Connect TLV enclosing a multicast, broadcast, or 1366 loopback IP address. 1368 To ease troubleshooting, the value field MUST echo the received 1369 message shifted by one byte to keep to original alignment of the 1370 message. 1372 o Unsupported Message (2): This error code is sent to indicate that 1373 a message type received from a Client is not supported. 1375 To ease troubleshooting, the value field MUST echo the received 1376 message shifted by one byte to keep to original alignment of the 1377 message. 1379 o Missing Cookie (3): If a Transport Converter requires the 1380 utilization of Cookies to prevent spoofing attacks and a Cookie 1381 TLV was not included in the Convert message, the Transport 1382 Converter MUST return this error to the requesting client. The 1383 first byte of the value field MUST be set to zero and the 1384 remaining bytes of the Error TLV contain the Cookie computed by 1385 the Transport Converter for this Client. 1387 A Client which receives this error code SHOULD cache the received 1388 Cookie and include it in subsequent Convert messages sent to that 1389 Transport Converter. 1391 o Not Authorized (32): This error code indicates that the Transport 1392 Converter refused to create a connection because of a lack of 1393 authorization (e.g., administratively prohibited, authorization 1394 failure, invalid Cookie TLV, etc.). The Value field MUST be set 1395 to zero. 1397 This error code MUST be sent by the Transport Converter when a 1398 request cannot be successfully processed because the authorization 1399 failed. 1401 o Unsupported TCP Option (33): A TCP option that the Client 1402 requested to advertise to the final Server cannot be safely used. 1404 The Value field is set to the type of the unsupported TCP option. 1405 If several unsupported TCP options were specified in the Connect 1406 TLV, then the list of unsupported TCP options is returned. The 1407 list of unsupported TCP options MUST be padded with zeros to end 1408 on a 32 bits boundary. 1410 o Resource Exceeded (64): This error indicates that the Transport 1411 Converter does not have enough resources to perform the request. 1413 This error MUST be sent by the Transport Converter when it does 1414 not have sufficient resources to handle a new connection. The 1415 Transport Converter may indicate in the Value field the suggested 1416 delay (in seconds) that the Client SHOULD wait before soliciting 1417 the Transport Converter for a new proxied connection. A Value of 1418 zero corresponds to a default delay of at least 30 seconds. 1420 o Network Failure (65): This error indicates that the Transport 1421 Converter is experiencing a network failure to proxy the request. 1423 The Transport Converter MUST send this error code when it 1424 experiences forwarding issues to proxy a connection. The 1425 Transport Converter may indicate in the Value field the suggested 1426 delay (in seconds) that the Client SHOULD wait before soliciting 1427 the Transport Converter for a new proxied connection. A Value of 1428 zero corresponds to a default delay of at least 30 seconds. 1430 o Connection Reset (96): This error indicates that the final 1431 destination responded with an RST packet. The Value field MUST be 1432 set to zero. 1434 o Destination Unreachable (97): This error indicates that an ICMP 1435 destination unreachable, port unreachable, or network unreachable 1436 was received by the Transport Converter. The Value field MUST 1437 echo the Code field of the received ICMP message. 1439 Figure 24 summarizes the different error codes. 1441 +-------+------+-----------------------------------------------+ 1442 | Error | Hex | Description | 1443 +-------+------+-----------------------------------------------+ 1444 | 0 | 0x00 | Unsupported Version | 1445 | 1 | 0x01 | Malformed Message | 1446 | 2 | 0x02 | Unsupported Message | 1447 | 3 | 0x03 | Missing Cookie | 1448 | 32 | 0x20 | Not Authorized | 1449 | 33 | 0x21 | Unsupported TCP Option | 1450 | 64 | 0x40 | Resource Exceeded | 1451 | 65 | 0x41 | Network Failure | 1452 | 96 | 0x60 | Connection Reset | 1453 | 97 | 0x61 | Destination Unreachable | 1454 +-------+------+-----------------------------------------------+ 1456 Figure 24: Convert Error Values 1458 7. Compatibility of Specific TCP Options with the Conversion Service 1460 In this section, we discuss how several deployed standard track TCP 1461 options can be supported through the Convert Protocol. The other TCP 1462 options will be discussed in other documents. 1464 7.1. Base TCP Options 1466 Three TCP options were initially defined in [RFC0793]: End-of-Option 1467 List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size 1468 (Kind=2). The first two options are mainly used to pad the TCP 1469 header. There is no reason for a client to request a Transport 1470 Converter to specifically send these options towards the final 1471 destination. 1473 The Maximum Segment Size option (Kind=2) is used by a host to 1474 indicate the largest segment that it can receive over each 1475 connection. This value is function of the stack that terminates the 1476 TCP connection. There is no reason for a Client to request a 1477 Transport Converter to advertise a specific MSS value to a remote 1478 server. 1480 A Transport Converter MUST ignore options with Kind=0, 1 or 2 if they 1481 appear in a Connect TLV. It MUST NOT announce them in a Supported 1482 TCP Extensions TLV. 1484 7.2. Window Scale (WS) 1486 The Window Scale (WS) option (Kind=3) is defined in [RFC7323]. As 1487 for the MSS option, the window scale factor that is used for a 1488 connection strongly depends on the TCP stack that handles the 1489 connection. When a Transport Converter opens a TCP connection 1490 towards a remote server on behalf of a Client, it SHOULD use a WS 1491 option with a scaling factor that corresponds to the configuration of 1492 its stack. A local configuration MAY allow for WS option in the 1493 proxied message to be function of the scaling factor of the incoming 1494 connection. 1496 There is no benefit from a deployment viewpoint in enabling a Client 1497 of a Transport Converter to specifically request the utilization of 1498 the WS option (Kind=3) with a specific scaling factor towards a 1499 remote Server. For this reason, a Transport Converter MUST ignore 1500 option Kind=3 if it appears in a Connect TLV. It MUST NOT announce 1501 it in a Supported TCP Extensions TLV. 1503 7.3. Selective Acknowledgments 1505 Two distinct TCP options were defined to support selective 1506 acknowledgments in [RFC2018]. This first one, SACK Permitted 1507 (Kind=4), is used to negotiate the utilization of selective 1508 acknowledgments during the three-way handshake. The second one, SACK 1509 (Kind=5), carries the selective acknowledgments inside regular 1510 segments. 1512 The SACK Permitted option (Kind=4) MAY be advertised by a Transport 1513 Converter in the Supported TCP Extensions TLV. Clients connected to 1514 this Transport Converter MAY include the SACK Permitted option in the 1515 Connect TLV. 1517 The SACK option (Kind=5) cannot be used during the three-way 1518 handshake. For this reason, a Transport Converter MUST ignore option 1519 Kind=5 if it appears in a Connect TLV. It MUST NOT announce it in a 1520 TCP Supported Extensions TLV. 1522 7.4. Timestamp 1524 The Timestamp option [RFC7323] can be used during the three-way 1525 handshake to negotiate the utilization of timestamps during the TCP 1526 connection. It is notably used to improve round-trip-time 1527 estimations and to provide protection against wrapped sequence 1528 numbers (PAWS). As for the WS option, the timestamps are a property 1529 of a connection and there is limited benefit in enabling a client to 1530 request a Transport Converter to use the timestamp option when 1531 establishing a connection to a remote server. Furthermore, the 1532 timestamps that are used by TCP stacks are specific to each stack and 1533 there is no benefit in enabling a client to specify the timestamp 1534 value that a Transport Converter could use to establish a connection 1535 to a remote server. 1537 A Transport Converter MAY advertise the Timestamp option (Kind=8) in 1538 the TCP Supported Extensions TLV. The clients connected to this 1539 Transport Converter MAY include the Timestamp option in the Connect 1540 TLV but without any timestamp. 1542 7.5. Multipath TCP 1544 The Multipath TCP options are defined in [RFC6824]. [RFC6824] 1545 defines one variable length TCP option (Kind=30) that includes a sub- 1546 type field to support several Multipath TCP options. There are 1547 several operational use cases where clients would like to use 1548 Multipath TCP through a Transport Converter [IETFJ16]. However, none 1549 of these use cases require the Client to specify the content of the 1550 Multipath TCP option that the Transport Converter should send to a 1551 remote server. 1553 A Transport Converter which supports Multipath TCP conversion service 1554 MUST advertise the Multipath TCP option (Kind=30) in the Supported 1555 TCP Extensions TLV. Clients serviced by this Transport Converter may 1556 include the Multipath TCP option in the Connect TLV but without any 1557 content. 1559 7.6. TCP Fast Open 1561 The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413]. 1562 There are two different usages of this option that need to be 1563 supported by Transport Converters. The first utilization of the TCP 1564 Fast Open cookie option is to request a cookie from the server. In 1565 this case, the option is sent with an empty cookie by the client and 1566 the server returns the cookie. The second utilization of the TCP 1567 Fast Open cookie option is to send a cookie to the server. In this 1568 case, the option contains a cookie. 1570 A Transport Converter MAY advertise the TCP Fast Open cookie option 1571 (Kind=34) in the Supported TCP Extensions TLV. If a Transport 1572 Converter has advertised the support for TCP Fast Open in its 1573 Supported TCP Extensions TLV, it needs to be able to process two 1574 types of Connect TLV. If such a Transport Converter receives a 1575 Connect TLV with the TCP Fast Open cookie option that does not 1576 contain a cookie, it MUST add an empty TCP Fast Open cookie option in 1577 the SYN sent to the remote server. If such a Transport Converter 1578 receives a Connect TLV with the TCP Fast Open cookie option that 1579 contains a cookie, it MUST copy the TCP Fast Open cookie option in 1580 the SYN sent to the remote server. 1582 7.7. TCP-AO 1584 TCP-AO [RFC5925] provides a technique to authenticate all the packets 1585 exchanged over a TCP connection. Given the nature of this extension, 1586 it is unlikely that the applications that require their packets to be 1587 authenticated end-to-end would want their connections to pass through 1588 a converter. For this reason, we do not recommend the support of the 1589 TCP-AO option by Transport Converters. The only use cases where it 1590 could make sense to combine TCP-AO and the solution in this document 1591 are those where the TCP-AO-NAT extension [RFC6978] is in use. 1593 A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29) 1594 in the Supported TCP Extensions TLV. If a Transport Converter 1595 receives a Connect TLV that contains the TCP-AO option, it MUST 1596 reject the establishment of the connection with error code set to 1597 "Unsupported TCP Option", except if the TCP-AO-NAT option is used. 1599 8. Interactions with Middleboxes 1601 The Convert Protocol is designed to be used in networks that do not 1602 contain middleboxes that interfere with TCP. Under such conditions, 1603 it is assumed that the network provider ensures that all involved on- 1604 path nodes are not breaking TCP signals (e.g., strip TCP options, 1605 discard some SYNs, etc.). 1607 Nevertheless, and in order to allow for a robust service, this 1608 section describes how a Client can detect middlebox interference and 1609 stop using the Transport Converter affected by this interference. 1611 Internet measurements [IMC11] have shown that middleboxes can affect 1612 the deployment of TCP extensions. In this section, we focus the 1613 middleboxes that modify the payload since the Convert Protocol places 1614 its messages at the beginning of the bytestream. 1616 Consider a middlebox that removes the SYN payload. The Client can 1617 detect this problem by looking at the acknowledgment number field of 1618 the SYN+ACK returned by the Transport Converter. The Client MUST 1619 stop to use this Transport Converter given the middlebox 1620 interference. 1622 Consider now a middlebox that drops SYN/ACKs with a payload. The 1623 Client won't be able to establish a connection via the Transport 1624 Converter. The case of a middlebox that removes the payload of 1625 SYN+ACKs or from the packet that follows the SYN+ACK (but not the 1626 payload of SYN) can be detected by a Client. This is hinted by the 1627 absence of a valid Convert message in the response. 1629 As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect 1630 the operation of TFO if they assign different IP addresses to the 1631 same end host. Such CGNs could affect the operation of the cookie 1632 validation used by the Convert Protocol. As a reminder CGNs, enabled 1633 on the path between a Client and a Transport Converter, must adhere 1634 to the address preservation defined in [RFC6888]. See also the 1635 discussion in Section 7.1 of [RFC7413]. 1637 9. Security Considerations 1639 9.1. Privacy & Ingress Filtering 1641 The Transport Converter may have access to privacy-related 1642 information (e.g., subscriber credentials). The Transport Converter 1643 is designed to not leak such sensitive information outside a local 1644 domain. 1646 Given its function and its location in the network, a Transport 1647 Converter has access to the payload of all the packets that it 1648 processes. As such, it MUST be protected as a core IP router (e.g., 1649 [RFC1812]). 1651 Furthermore, ingress filtering policies MUST be enforced at the 1652 network boundaries [RFC2827]. 1654 This document assumes that all network attachments are managed by the 1655 same administrative entity. Therefore, enforcing anti-spoofing 1656 filters at these network ensures that hosts are not sending traffic 1657 with spoofed source IP addresses. 1659 9.2. Authorization 1661 The Convert Protocol is intended to be used in managed networks where 1662 end hosts can be identified by their IP address. 1664 Stronger mutual authentication schemes MUST be defined to use the 1665 Convert Protocol in more open network environments. One possibility 1666 is to use TLS to perform mutual authentication between the client and 1667 the Converter. That is, use TLS when a Client retrieves a Cookie 1668 from the Converter and rely on certificate-based client 1669 authentication, pre-shared key based [RFC4279] or raw public key 1670 based client authentication [RFC7250] to secure this connection. 1672 If the authentication succeeds, the Converter returns a cookie to the 1673 Client. Subsequent Connect messages will be authorized as a function 1674 of the content of the Cookie TLV. 1676 In deployments where network-assisted connections are not allowed 1677 between hosts of a domain (i.e., hairpinning), the Converter may be 1678 instructed to discard such connections. Hairpinned connections are 1679 thus rejected by the Transport Converter by returning an Error TLV 1680 set to "Not Authorized". Absent explicit configuration otherwise, 1681 hairpinning is enabled by the Converter (see Figure 25. 1683 <===Network Provider===> 1685 +----+ from X1:x1 to X2':x2' +-----+ X1':x1' 1686 | C1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+--- 1687 +----+ | v | 1688 | v | 1689 | v | 1690 | v | 1691 +----+ from X1':x1' to X2:x2 | v | X2':x2' 1692 | C2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+--- 1693 +----+ +-----+ 1694 Converter 1696 Note: X2':x2' may be equal to 1697 X2:x2 1699 Figure 25: Hairpinning Example 1701 See below for authorization considerations that are specific for 1702 Multipath TCP. 1704 9.3. Denial of Service 1706 Another possible risk is the amplification attacks since a Transport 1707 Converter sends a SYN towards a remote Server upon reception of a SYN 1708 from a Client. This could lead to amplification attacks if the SYN 1709 sent by the Transport Converter were larger than the SYN received 1710 from the Client or if the Transport Converter retransmits the SYN. 1711 To mitigate such attacks, the Transport Converter SHOULD rate limit 1712 the number of pending requests for a given Client. It SHOULD also 1713 avoid sending to remote Servers SYNs that are significantly longer 1714 than the SYN received from the Client. Finally, the Transport 1715 Converter SHOULD only retransmit a SYN to a Server after having 1716 received a retransmitted SYN from the corresponding Client. Means to 1717 protect against SYN flooding attacks should also be enabled (e.g., 1718 Section 3 of [RFC4987]). 1720 9.4. Traffic Theft 1722 Traffic theft is a risk if an illegitimate Converter is inserted in 1723 the path. Indeed, inserting an illegitimate Converter in the 1724 forwarding path allows traffic interception and can therefore provide 1725 access to sensitive data issued by or destined to a host. Converter 1726 discovery and configuration are out of scope of this document. 1728 9.5. Authentication Considerations 1730 The operator that manages the various network attachments (including 1731 the Transport Converters) can enforce authentication and 1732 authorization policies using appropriate mechanisms. For example, a 1733 non-exhaustive list of methods to achieve authorization is provided 1734 hereafter: 1736 o The network provider may enforce a policy based on the 1737 International Mobile Subscriber Identity (IMSI) to verify that a 1738 user is allowed to benefit from the TCP converter service. If 1739 that authorization fails, the Packet Data Protocol (PDP) context/ 1740 bearer will not be mounted. This method does not require any 1741 interaction with the Transport Converter for authorization 1742 matters. 1744 o The network provider may enforce a policy based upon Access 1745 Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG) 1746 to control the hosts that are authorized to communicate with a 1747 Transport Converter. These ACLs may be installed as a result of 1748 RADIUS exchanges, e.g., [I-D.boucadair-radext-tcpm-converter]. 1749 This method does not require any interaction with the Transport 1750 Converter for authorization matters. 1752 o A device that embeds a Transport Converter may also host a RADIUS 1753 client that will solicit an AAA server to check whether 1754 connections received from a given source IP address are authorized 1755 or not [I-D.boucadair-radext-tcpm-converter]. 1757 A first safeguard against the misuse of Transport Converter resources 1758 by illegitimate users (e.g., users with access networks that are not 1759 managed by the same provider that operates the Transport Converter) 1760 is the Transport Converter to reject Convert connections received on 1761 its Internet-facing interfaces. Only Convert connections received on 1762 the customer-facing interfaces of a Transport Converter will be 1763 accepted. 1765 10. IANA Considerations 1767 Note to the RFC Editor: Please replace "THISRFC" in the following 1768 sub-sections with the RFC number to be assigned to this document. 1770 10.1. Convert Service Name 1772 IANA is requested to assign a service name for the Convert Protocol 1773 from the "Service Name and Transport Protocol Port Number Registry" 1774 available at https://www.iana.org/assignments/service-names-port- 1775 numbers/service-names-port-numbers.xhtml. 1777 Service Name: convert 1778 Port Number: N/A 1779 Transport Protocol(s): TCP 1780 Description: 0-RTT TCP Convert Protocol 1781 Assignee: IESG 1782 Contact: IETF Chair 1783 Reference: THISRFC 1785 Clients may use this service name to fed the procedure defined in 1786 [RFC2782] to discover the IP address(es) and the port number used by 1787 the Transport Converters of a domain. 1789 10.2. The Convert Protocol (Convert) Parameters 1791 IANA is requested to create a new "The TCP Convert Protocol (Convert) 1792 Parameters" registry. 1794 The following subsections detail new registries within "The Convert 1795 Protocol (Convert) Parameters" registry. 1797 The Designated Expert is expected to ascertain the existence of 1798 suitable documentation as described in Section 4.6 of [RFC8126] and 1799 to verify that the document is permanently and publicly available. 1801 The Designated Expert is also expected to check the clarity of 1802 purpose and use of the requested code points. 1804 Also, criteria that should be applied by the Designated Experts 1805 includes determining whether the proposed registration duplicates 1806 existing functionality, whether it is likely to be of general 1807 applicability or whether it is useful only for a private use, and 1808 whether the registration description is clear. IANA must only accept 1809 registry updates to the 128-191 range (for both "Convert TLVs" and 1810 "Convert Error Messages" sub-registries) from the Designated Experts 1811 and should direct all requests for registration to the review mailing 1812 list. It is suggested that multiple Designated Experts be appointed. 1813 In cases where a registration decision could be perceived as creating 1814 a conflict of interest for a particular Expert, that Expert should 1815 defer to the judgment of the other Experts. 1817 10.2.1. Convert Versions 1819 IANA is requested to create the "Convert versions" sub-registry. New 1820 values are assigned via IETF Review (Section 4.8 of [RFC8126]). 1822 The initial values to be assigned at the creation of the registry are 1823 as follows: 1825 +---------+--------------------------------------+-------------+ 1826 | Version | Description | Reference | 1827 +---------+--------------------------------------+-------------+ 1828 | 0 | Reserved by this document | THISRFC | 1829 | 1 | Assigned by this document | THISRFC | 1830 +---------+--------------------------------------+-------------+ 1832 10.2.2. Convert TLVs 1834 IANA is requested to create the "Convert TLVs" sub-registry. The 1835 procedure for assigning values from this registry is as follows: 1837 o The values in the range 1-127 can be assigned via IETF Review. 1839 o The values in the range 128-191 can be assigned via Specification 1840 Required. 1842 o The values in the range 192-255 are reserved for Private Use. 1844 The initial values to be assigned at the creation of the registry are 1845 as follows: 1847 +---------+--------------------------------------+-------------+ 1848 | Code | Name | Reference | 1849 +---------+--------------------------------------+-------------+ 1850 | 0 | Reserved | THISRFC | 1851 | 1 | Info TLV | THISRFC | 1852 | 10 | Connect TLV | THISRFC | 1853 | 20 | Extended TCP Header TLV | THISRFC | 1854 | 21 | Supported TCP Extension TLV | THISRFC | 1855 | 22 | Cookie TLV | THISRFC | 1856 | 30 | Error TLV | THISRFC | 1857 +---------+--------------------------------------+-------------+ 1859 10.2.3. Convert Error Messages 1861 IANA is requested to create the "Convert Errors" sub-registry. Codes 1862 in this registry are assigned as a function of the error type. Four 1863 types are defined; the following ranges are reserved for each of 1864 these types: 1866 o Message validation and processing errors: 0-31 1868 o Client-side errors: 32-63 1870 o Transport Converter-side errors: 64-95 1872 o Errors caused by destination server: 96-127 1874 The procedure for assigning values from this sub-registry is as 1875 follows: 1877 o 0-127: Values in this range are assigned via IETF Review. 1879 o 128-191: Values in this range are assigned via Specification 1880 Required. 1882 o 192-255: Values in this range are reserved for Private Use. 1884 The initial values to be assigned at the creation of the registry are 1885 as follows: 1887 +-------+------+-----------------------------------+-----------+ 1888 | Error | Hex | Description | Reference | 1889 +-------+------+-----------------------------------+-----------+ 1890 | 0 | 0x00 | Unsupported Version | THISRFC | 1891 | 1 | 0x01 | Malformed Message | THISRFC | 1892 | 2 | 0x02 | Unsupported Message | THISRFC | 1893 | 3 | 0x03 | Missing Cookie | THISRFC | 1894 | 32 | 0x20 | Not Authorized | THISRFC | 1895 | 33 | 0x21 | Unsupported TCP Option | THISRFC | 1896 | 64 | 0x40 | Resource Exceeded | THISRFC | 1897 | 65 | 0x41 | Network Failure | THISRFC | 1898 | 96 | 0x60 | Connection Reset | THISRFC | 1899 | 97 | 0x61 | Destination Unreachable | THISRFC | 1900 +-------+------+-----------------------------------+-----------+ 1902 Figure 26: The Convert Error Codes 1904 11. References 1906 11.1. Normative References 1908 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1909 RFC 793, DOI 10.17487/RFC0793, September 1981, 1910 . 1912 [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP 1913 Selective Acknowledgment Options", RFC 2018, 1914 DOI 10.17487/RFC2018, October 1996, 1915 . 1917 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1918 Requirement Levels", BCP 14, RFC 2119, 1919 DOI 10.17487/RFC2119, March 1997, 1920 . 1922 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 1923 Defeating Denial of Service Attacks which employ IP Source 1924 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 1925 May 2000, . 1927 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1928 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 1929 2006, . 1931 [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address 1932 Translation (NAT) Behavioral Requirements for Unicast 1933 UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 1934 2007, . 1936 [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common 1937 Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, 1938 . 1940 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP 1941 Authentication Option", RFC 5925, DOI 10.17487/RFC5925, 1942 June 2010, . 1944 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1945 "TCP Extensions for Multipath Operation with Multiple 1946 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1947 . 1949 [RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa, 1950 A., and H. Ashida, "Common Requirements for Carrier-Grade 1951 NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888, 1952 April 2013, . 1954 [RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, 1955 "Special-Purpose IP Address Registries", BCP 153, 1956 RFC 6890, DOI 10.17487/RFC6890, April 2013, 1957 . 1959 [RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT 1960 Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013, 1961 . 1963 [RFC7323] Borman, D., Braden, B., Jacobson, V., and R. 1964 Scheffenegger, Ed., "TCP Extensions for High Performance", 1965 RFC 7323, DOI 10.17487/RFC7323, September 2014, 1966 . 1968 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1969 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1970 . 1972 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1973 Writing an IANA Considerations Section in RFCs", BCP 26, 1974 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1975 . 1977 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1978 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1979 May 2017, . 1981 11.2. Informative References 1983 [ANRW17] Trammell, B., Kuehlewind, M., De Vaere, P., Learmonth, I., 1984 and G. Fairhurst, "Tracking transport-layer evolution with 1985 PATHspider", Applied Networking Research Workshop 2017 1986 (ANRW17) , July 2017. 1988 [Fukuda2011] 1989 Fukuda, K., "An Analysis of Longitudinal TCP Passive 1990 Measurements (Short Paper)", Traffic Monitoring and 1991 Analysis. TMA 2011. Lecture Notes in Computer Science, vol 1992 6613. , 2011. 1994 [HotMiddlebox13b] 1995 Detal, G., Paasch, C., and O. Bonaventure, "Multipath in 1996 the Middle(Box)", HotMiddlebox'13 , December 2013, 1997 . 2000 [I-D.arkko-arch-low-latency] 2001 Arkko, J. and J. Tantsura, "Low Latency Applications and 2002 the Internet Architecture", draft-arkko-arch-low- 2003 latency-02 (work in progress), October 2017. 2005 [I-D.boucadair-mptcp-plain-mode] 2006 Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel, 2007 D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R., 2008 Vinapamula, S., Seo, S., Cloetens, W., Meyer, U., 2009 Contreras, L., and B. Peirens, "Extensions for Network- 2010 Assisted MPTCP Deployment Models", draft-boucadair-mptcp- 2011 plain-mode-10 (work in progress), March 2017. 2013 [I-D.boucadair-radext-tcpm-converter] 2014 Boucadair, M. and C. Jacquenet, "RADIUS Extensions for 2015 0-RTT TCP Converters", draft-boucadair-radext-tcpm- 2016 converter-02 (work in progress), April 2019. 2018 [I-D.boucadair-tcpm-dhc-converter] 2019 Boucadair, M., Jacquenet, C., and T. Reddy.K, "DHCP 2020 Options for 0-RTT TCP Converters", draft-boucadair-tcpm- 2021 dhc-converter-03 (work in progress), October 2019. 2023 [I-D.olteanu-intarea-socks-6] 2024 Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6", 2025 draft-olteanu-intarea-socks-6-08 (work in progress), 2026 November 2019. 2028 [I-D.peirens-mptcp-transparent] 2029 Peirens, B., Detal, G., Barre, S., and O. Bonaventure, 2030 "Link bonding with transparent Multipath TCP", draft- 2031 peirens-mptcp-transparent-00 (work in progress), July 2032 2016. 2034 [IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment", 2035 IETF Journal, Fall 2016 , n.d.. 2037 [IMC11] Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A., 2038 Handley, M., and T. Hideyuki, "Is it still possible to 2039 extend TCP?", Proceedings of the 2011 ACM SIGCOMM 2040 conference on Internet measurement conference , 2011. 2042 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", 2043 RFC 1812, DOI 10.17487/RFC1812, June 1995, 2044 . 2046 [RFC1919] Chatel, M., "Classical versus Transparent IP Proxies", 2047 RFC 1919, DOI 10.17487/RFC1919, March 1996, 2048 . 2050 [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and 2051 L. Jones, "SOCKS Protocol Version 5", RFC 1928, 2052 DOI 10.17487/RFC1928, March 1996, 2053 . 2055 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2056 specifying the location of services (DNS SRV)", RFC 2782, 2057 DOI 10.17487/RFC2782, February 2000, 2058 . 2060 [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. 2061 Shelby, "Performance Enhancing Proxies Intended to 2062 Mitigate Link-Related Degradations", RFC 3135, 2063 DOI 10.17487/RFC3135, June 2001, 2064 . 2066 [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key 2067 Ciphersuites for Transport Layer Security (TLS)", 2068 RFC 4279, DOI 10.17487/RFC4279, December 2005, 2069 . 2071 [RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and 2072 P. Roberts, "Issues with IP Address Sharing", RFC 6269, 2073 DOI 10.17487/RFC6269, June 2011, 2074 . 2076 [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix 2077 Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011, 2078 . 2080 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 2081 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 2082 DOI 10.17487/RFC6887, April 2013, 2083 . 2085 [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis, 2086 "Increasing TCP's Initial Window", RFC 6928, 2087 DOI 10.17487/RFC6928, April 2013, 2088 . 2090 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 2091 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 2092 Transport Layer Security (TLS) and Datagram Transport 2093 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 2094 June 2014, . 2096 [RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. 2097 Zimmermann, "A Roadmap for Transmission Control Protocol 2098 (TCP) Specification Documents", RFC 7414, 2099 DOI 10.17487/RFC7414, February 2015, 2100 . 2102 [RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and 2103 Operational Experience with Multipath TCP", RFC 8041, 2104 DOI 10.17487/RFC8041, January 2017, 2105 . 2107 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 2108 Better Connectivity Using Concurrency", RFC 8305, 2109 DOI 10.17487/RFC8305, December 2017, 2110 . 2112 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2113 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2114 . 2116 [RFC8548] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, 2117 Q., and E. Smith, "Cryptographic Protection of TCP Streams 2118 (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019, 2119 . 2121 [TS23501] 3GPP (3rd Generation Partnership Project), ., "Technical 2122 Specification Group Services and System Aspects; System 2123 Architecture for the 5G System; Stage 2 (Release 16)", 2124 2019, . 2127 Appendix A. Example Socket API Changes to Support the 0-RTT Convert 2128 Protocol 2130 A.1. Active Open (Client Side) 2132 On the client side, the support of the 0-RTT Converter protocol does 2133 not require any other changes than those identified in Appendix A of 2134 [RFC7413]. Those modifications are already supported by multiple TCP 2135 stacks. 2137 As an example, on Linux, a client can send the 0-RTT Convert message 2138 inside a SYN by using sendto with the MSG_FASTOPEN flag as shown in 2139 the example below: 2141 s = socket(AF_INET, SOCK_STREAM, 0); 2143 sendto(s, buffer, buffer_len, MSG_FASTOPEN, 2144 (struct sockaddr *) &server_addr, addr_len); 2146 The client side of the Linux TCP TFO can be used in two different 2147 modes depending on the host configuration (sysctl tcp_fastopen 2148 variable): 2150 o 0x1: (client) enables sending data in the opening SYN on the 2151 client. 2153 o 0x4: (client) send data in the opening SYN regardless of cookie 2154 availability and without a cookie option. 2156 By setting this configuration variable to 0x5, a Linux client using 2157 the above code would send data inside the SYN without using a TFO 2158 option. 2160 A.2. Passive Open (Converter Side) 2162 The Converter needs to enable the reception of data inside the SYN 2163 independently of the utilization of the TFO option. This implies 2164 that the Transport Converter application cannot rely on the TFO 2165 cookies to validate the reachability of the IP address that sent the 2166 SYN. It must rely on other techniques, such as the Cookie TLV 2167 described in this document, to verify this reachability. 2169 [RFC7413] suggested the utilization of a TCP_FASTOPEN socket option 2170 the enable the reception of SYNs containing data. Later, Appendix A 2171 of [RFC7413], mentioned: 2173 Traditionally, accept() returns only after a socket is connected. 2174 But, for a Fast Open connection, accept() returns upon receiving 2175 SYN with a valid Fast Open cookie and data, and the data is available 2176 to be read through, e.g., recvmsg(), read(). 2178 To support the 0-RTT Convert Protocol, this behavior should be 2179 modified as follows: 2181 Traditionally, accept() returns only after a socket is connected. 2182 But, for a Fast Open connection, accept() returns upon receiving a 2183 SYN with data, and the data is available to be read through, e.g., 2184 recvmsg(), read(). The application that receives such SYNs with data 2185 must be able to validate the reachability of the source of the SYN 2186 and also deal with replayed SYNs. 2188 The Linux server side can be configured with the following sysctls: 2190 o 0x2: (server) enables the server support, i.e., allowing data in a 2191 SYN packet to be accepted and passed to the application before 2192 3-way handshake finishes. 2194 o 0x200: (server) accept data-in-SYN w/o any cookie option present. 2196 However, this configuration is system-wide. This is convenient for 2197 typical Transport Converter deployments where no other applications 2198 relying on TFO are collocated on the same device. 2200 Recently, the TCP_FASTOPEN_NO_COOKIE socket option has been added to 2201 provide the same behavior on a per socket basis. This enables a 2202 single host to support both servers that require the TFO cookie and 2203 servers that do not use it. 2205 Acknowledgments 2207 Although they could disagree with the contents of the document, we 2208 would like to thank Joe Touch and Juliusz Chroboczek whose comments 2209 on the MPTCP mailing list have forced us to reconsider the design of 2210 the solution several times. 2212 We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha 2213 Nandugudi and Gregory Vander Schueren for their help in preparing 2214 this document. Nandini Ganesh provided valuable feedback about the 2215 handling of TFO and the error codes. Yuchung Cheng and Praveen 2216 Balasubramanian helped to clarify the discussion on supplying data in 2217 SYNs. Phil Eardley and Michael Scharf's helped to clarify different 2218 parts of the text. 2220 Many thanks to Mirja Kuehlewind for the detailed AD review. 2222 This document builds upon earlier documents that proposed various 2223 forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode], 2224 [I-D.peirens-mptcp-transparent] and [HotMiddlebox13b]. 2226 From [I-D.boucadair-mptcp-plain-mode]: 2228 Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi 2229 Nishida, and Christoph Paasch for their valuable comments. 2231 Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and 2232 Sri Gundavelli for the fruitful discussions in IETF#95 (Buenos 2233 Aires). 2235 Special thanks to Pierrick Seite, Yannick Le Goff, Fred Klamm, and 2236 Xavier Grall for their inputs. 2238 Thanks also to Olaf Schleusing, Martin Gysi, Thomas Zasowski, Andreas 2239 Burkhard, Silka Simmen, Sandro Berger, Michael Melloul, Jean-Yves 2240 Flahaut, Adrien Desportes, Gregory Detal, Benjamin David, Arun 2241 Srinivasan, and Raghavendra Mallya for the discussion. 2243 Contributors 2245 Bart Peirens contributed to an early version of the document. 2247 As noted above, this document builds on two previous documents. 2249 The authors of [I-D.boucadair-mptcp-plain-mode] were: 2251 o Mohamed Boucadair 2253 o Christian Jacquenet 2255 o Olivier Bonaventure 2257 o Denis Behaghel 2259 o Stefano Secci 2261 o Wim Henderickx 2263 o Robert Skog 2264 o Suresh Vinapamula 2266 o SungHoon Seo 2268 o Wouter Cloetens 2270 o Ullrich Meyer 2272 o Luis M. Contreras 2274 o Bart Peirens 2276 The authors of [I-D.peirens-mptcp-transparent] were: 2278 o Bart Peirens 2280 o Gregory Detal 2282 o Sebastien Barre 2284 o Olivier Bonaventure 2286 Change Log 2288 This section to be removed before publication. 2290 o 00 : initial version, designed to support Multipath TCP and TFO 2291 only 2293 o 00 to -01 : added section Section 7 describing the support of 2294 different standard tracks TCP options by Transport Converters, 2295 clarification of the IANA section, moved the SOCKS comparison to 2296 the appendix and various minor modifications 2298 o 01 to -02: Minor modifications 2300 o 02 to -03: Minor modifications 2302 o 03 to -04: Minor modifications 2304 o 04 to -05: Integrate a lot of feedback from implementers who have 2305 worked on client and server side implementations. The main 2306 modifications are the following : 2308 * TCP Fast Open is not strictly required anymore. Several 2309 implementers expressed concerns about this requirement. The 2310 TFO Cookie protects from some attack scenarios that affect open 2311 servers like web servers. The Convert Protocol is different 2312 and as discussed in RFC7413, there are different ways to 2313 protect from such attacks. Instead of using a TFO cookie 2314 inside the TCP options, which consumes precious space in the 2315 extended TCP header, this version supports the utilization of a 2316 Cookie that is placed in the SYN payload. This provides the 2317 same level of protection as a TFO Cookie in environments were 2318 such protection is required. 2320 * the Bootstrap procedure has been simplified based on feedback 2321 from implementers 2323 * Error messages are not included in RST segments anymore but 2324 sent in the bytestream. Implementers have indicated that 2325 processing such segments on clients was difficult on some 2326 platforms. This change simplifies client implementations. 2328 * Many minor editorial changes to clarify the text based on 2329 implementers feedback. 2331 o 05 to -06: Many clarifications to integrate the comments from the 2332 chairs in preparation to the WGLC: 2334 * Updated IANA policy to require "IETF Review" instead of 2335 "Standard Action" 2337 * Call out explicitly that data in SYNs are relayed by the 2338 Converter 2340 * Reiterate the scope 2342 * Hairpinning behavior can be disabled (policy-based) 2344 * Fix nits 2346 o 07: 2348 * Update the text about supplying data in SYNs to make it clear 2349 that a constraint defined in RFC793 is relaxed following the 2350 same rationale as in RFC7413. 2352 * Nits 2354 * Added Appendix A on example Socket API changes 2356 o 08: 2358 * Added short discussion on the termination of connections 2360 o 09: 2362 * Address various comments received during last call 2364 o 10-13: 2366 * Changes to address the comments from Phil: Add a new section to 2367 group data plane considerations in one place + add a new 2368 appendix with more details on address modes + rearrange the 2369 MPTCP text. 2371 o 14: fixed nits (the shepherd write-up) 2373 o 15: Rewrote parts of the text to address the detailed comments 2374 provided by M. Kuehlewind 2376 Authors' Addresses 2378 Olivier Bonaventure (editor) 2379 Tessares 2381 Email: Olivier.Bonaventure@tessares.net 2383 Mohamed Boucadair (editor) 2384 Orange 2385 Clos Courtel 2386 Rennes 35000 2387 France 2389 Email: mohamed.boucadair@orange.com 2391 Sri Gundavelli 2392 Cisco 2394 Email: sgundave@cisco.com 2396 SungHoon Seo 2397 Korea Telecom 2399 Email: sh.seo@kt.com 2400 Benjamin Hesmans 2401 Tessares 2403 Email: Benjamin.Hesmans@tessares.net