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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Stewart 3 Internet-Draft Netflix, Inc. 4 Intended status: Standards Track M. Tuexen 5 Expires: January 6, 2016 I. Ruengeler 6 Muenster Univ. of Appl. Sciences 7 July 5, 2015 9 Stream Control Transmission Protocol (SCTP) Network Address Translation 10 Support 11 draft-ietf-tsvwg-natsupp-08.txt 13 Abstract 15 The Stream Control Transmission Protocol (SCTP) provides a reliable 16 communications channel between two end-hosts in many ways similar to 17 the Transmission Control Protocol (TCP). With the widespread 18 deployment of Network Address Translators (NAT), specialized code has 19 been added to NAT for TCP that allows multiple hosts to reside behind 20 a NAT and yet use only a single globally unique IPv4 address, even 21 when two hosts (behind a NAT) choose the same port numbers for their 22 connection. This additional code is sometimes classified as Network 23 Address and Port Translation (NAPT). 25 This document describes the protocol extensions required for the SCTP 26 endpoints and the mechanisms for NATs necessary to provide similar 27 features of NAPT in the single-point and multi-point traversal 28 scenario. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on January 6, 2016. 47 Copyright Notice 49 Copyright (c) 2015 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 65 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 67 4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6 68 4.1. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . 6 69 4.1.1. Single Point Traversal . . . . . . . . . . . . . . . 6 70 4.1.2. Multi Point Traversal . . . . . . . . . . . . . . . . 7 71 4.2. Limitations of Classical NAPT for SCTP . . . . . . . . . 8 72 4.3. The SCTP Specific Variant of NAT . . . . . . . . . . . . 8 73 5. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 12 74 5.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 12 75 5.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 12 76 5.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 13 77 5.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 13 78 5.2.1. VTag and Port Number Collision Error Cause . . . . . 13 79 5.2.2. Missing State Error Cause . . . . . . . . . . . . . . 14 80 5.2.3. Port Number Collision Error Cause . . . . . . . . . . 15 81 5.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 15 82 5.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 16 83 5.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 16 84 6. Procedures for SCTP End Points and NATs . . . . . . . . . . . 17 85 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17 86 6.2. Association Setup Considerations . . . . . . . . . . . . 18 87 6.3. Handling of Internal Port Number and Verification Tag 88 Collisions . . . . . . . . . . . . . . . . . . . . . . . 18 89 6.4. Handling of Internal Port Number Collisions . . . . . . . 19 90 6.5. Handling of Missing State . . . . . . . . . . . . . . . . 20 91 6.6. Handling of Fragmented SCTP Packets . . . . . . . . . . . 22 92 6.7. Multi-Point Traversal Considerations . . . . . . . . . . 22 93 7. Various Examples of NAT Traversals . . . . . . . . . . . . . 22 94 7.1. Single-homed Client to Single-homed Server . . . . . . . 22 95 7.2. Single-homed Client to Multi-homed Server . . . . . . . . 25 96 7.3. Multihomed Client and Server . . . . . . . . . . . . . . 28 97 7.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 32 98 7.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 34 99 8. Socket API Considerations . . . . . . . . . . . . . . . . . . 39 100 8.1. Get or Set the NAT Friendliness 101 (SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . . 40 102 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 103 9.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 40 104 9.2. Three New Error Causes . . . . . . . . . . . . . . . . . 41 105 9.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 42 106 10. Security Considerations . . . . . . . . . . . . . . . . . . . 42 107 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 42 108 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 109 12.1. Normative References . . . . . . . . . . . . . . . . . . 43 110 12.2. Informative References . . . . . . . . . . . . . . . . . 43 111 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 113 1. Introduction 115 Stream Control Transmission Protocol [RFC4960] provides a reliable 116 communications channel between two end-hosts in many ways similar to 117 TCP [RFC0793]. With the widespread deployment of Network Address 118 Translators (NAT), specialized code has been added to NAT for TCP 119 that allows multiple hosts to reside behind a NAT using private 120 addresses (see [RFC6890]) and yet use only a single globally unique 121 IPv4 address, even when two hosts (behind a NAT) choose the same port 122 numbers for their connection. This additional code is sometimes 123 classified as Network Address and Port Translation (NAPT). Please 124 note that this document focuses on the case where the NAT maps 125 multiple private addresses to a single public address. To date, 126 specialized code for SCTP has not yet been added to most NATs so that 127 only true NAT is available. The end result of this is that only one 128 SCTP capable host can be behind a NAT. The only alternative for 129 supporting legacy NATs is to use UDP encapsulation as specified in 130 [RFC6951]. 132 This document describes an SCTP specific variant NAT and specific 133 packets and procedures to help NATs provide similar features of NAPT 134 in the single-point and multi-point traversal scenario. An SCTP 135 implementation supporting this extension will follow these procedures 136 to assure that in both single-homed and multi-homed cases a NAT will 137 maintain the proper state without needing to change port numbers. 139 It is possible and desirable to make these changes for a number of 140 reasons: 142 o It is desirable for SCTP internal end-hosts on multiple platforms 143 to be able to share a NAT's public IP address in the same way that 144 a TCP session can use a NAT. 146 o If a NAT does not need to change any data within an SCTP packet it 147 will reduce the processing burden of NAT'ing SCTP by NOT needing 148 to execute the CRC32c checksum required by SCTP. 150 o Not having to touch the IP payload makes the processing of ICMP 151 messages in NATs easier. 153 An SCTP-aware NAT will need to follow these procedures for generating 154 appropriate SCTP packet formats. 156 When considering this feature it is possible to have multiple levels 157 of support. At each level, the Internal Host, External Host and NAT 158 may or may not support the features described in this document. The 159 following table illustrates the results of the various combinations 160 of support and if communications can occur between two endpoints. 162 +---------------+------------+---------------+---------------+ 163 | Internal Host | NAT | External Host | Communication | 164 +---------------+------------+---------------+---------------+ 165 | Support | Support | Support | Yes | 166 | Support | Support | No Support | Limited | 167 | Support | No Support | Support | None | 168 | Support | No Support | No Support | None | 169 | No Support | Support | Support | Limited | 170 | No Support | Support | No Support | Limited | 171 | No Support | No Support | Support | None | 172 | No Support | No Support | No Support | None | 173 +---------------+------------+---------------+---------------+ 175 Table 1: Communication possibilities 177 From the table we can see that when a NAT does not support the 178 extension no communication can occur. This is because for the most 179 part of the current situation i.e. SCTP packets sent externally from 180 behind a NAT are discarded by the NAT. In some cases, where the NAT 181 supports the feature but one of the two external hosts does not 182 support the feature, communication may occur but in a limited way. 183 For example only one host may be able to have a connection when a 184 collision case occurs. 186 2. Conventions 188 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 189 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 190 document are to be interpreted as described in [RFC2119]. 192 3. Terminology 194 This document uses the following terms, which are depicted in 195 Figure 1. Familiarity with the terminology used in [RFC4960] and 196 [RFC5061] is assumed. 198 Private-Address (Priv-Addr): The private address that is known to 199 the internal host. 201 Internal-Port (Int-Port): The port number that is in use by the host 202 holding the Private-Address. 204 Internal-VTag (Int-VTag): The SCTP Verification Tag (VTag) that the 205 internal host has chosen for its communication. The VTag is a 206 unique 32-bit tag that must accompany any incoming SCTP packet for 207 this association to the Private-Address. 209 External-Address (Ext-Addr): The address that an internal host is 210 attempting to contact. 212 External-Port (Ext-Port): The port number of the peer process at the 213 External-Address. 215 External-VTag (Ext-VTag): The Verification Tag that the host holding 216 the External-Address has chosen for its communication. The VTag 217 is a unique 32-bit tag that must accompany any incoming SCTP 218 packet for this association to the External-Address. 220 Public-Address (Pub-Addr): The public address assigned to the NAT 221 box which it uses as a source address when sending packets towards 222 the External-Address. 224 Internal Network | External Network 225 | 226 Private | Public External 227 +---------+ Address | Address /--\/--\ Address +---------+ 228 | SCTP | +-----+ / \ | SCTP | 229 |end point|=========| NAT |=======| Internet |==========|end point| 230 | A | +-----+ \ / | B | 231 +---------+ Internal | \--/\--/ External+---------+ 232 Internal Port | Port External 233 VTag | VTag 235 Figure 1: Basic network setup 237 4. Motivation 239 4.1. SCTP NAT Traversal Scenarios 241 This section defines the notion of single and multi-point NAT 242 traversal. 244 4.1.1. Single Point Traversal 246 In this case, all packets in the SCTP association go through a single 247 NAT, as shown below: 249 Internal Network | External Network 250 | 251 +---------+ | /--\/--\ +---------+ 252 | SCTP | +-----+ / \ | SCTP | 253 |end point|=========| NAT |========= | Internet | ========|end point| 254 | A | +-----+ \ / | B | 255 +---------+ | \--/\--/ +---------+ 256 | 258 Single NAT scenario 260 A variation of this case is shown below, i.e., multiple NATs in a 261 single path: 263 Internal | External : Internal | External 264 | : | 265 +---------+ | : | /--\/--\ +---------+ 266 | SCTP | +-----+ : +-----+ / \ | SCTP | 267 |end point|==| NAT |=======:=======| NAT |==| Internet |==|end point| 268 | A | +-----+ : +-----+ \ / | B | 269 +---------+ | : | \--/\--/ +---------+ 270 | : | 272 Serial NATs scenario 274 In this single point traversal scenario, we must acknowledge that 275 while one of the main benefits of SCTP multi-homing is redundant 276 paths, the NAT function represents a single point of failure in the 277 path of the SCTP multi-home association. However, the rest of the 278 path may still benefit from path diversity provided by SCTP multi- 279 homing. 281 The two SCTP endpoints in this case can be either single-homed or 282 multi-homed. However, the important thing is that the NAT (or NATs) 283 in this case sees all the packets of the SCTP association. 285 4.1.2. Multi Point Traversal 287 This case involves multiple NATs and each NAT only sees some of the 288 packets in the SCTP association. An example is shown below: 290 Internal | External 291 +------+ /---\/---\ 292 +---------+ /=======|NAT A |=========\ / \ +---------+ 293 | SCTP | / +------+ \/ \ | SCTP | 294 |end point|/ ... | Internet |===|end point| 295 | A |\ \ / | B | 296 +---------+ \ +------+ / \ / +---------+ 297 \=======|NAT B |=========/ \---\/---/ 298 +------+ 299 | 301 Parallel NATs scenario 303 This case does NOT apply to a single-homed SCTP association (i.e., 304 BOTH endpoints in the association use only one IP address). The 305 advantage here is that the existence of multiple NAT traversal points 306 can preserve the path diversity of a multi-homed association for the 307 entire path. This in turn can improve the robustness of the 308 communication. 310 4.2. Limitations of Classical NAPT for SCTP 312 Using classical NAPT may result in changing one of the SCTP port 313 numbers during the processing which requires the recomputation of the 314 transport layer checksum. Whereas for UDP and TCP this can be done 315 very efficiently, for SCTP the checksum (CRC32c) over the entire 316 packet needs to be recomputed. This would considerably add to the 317 NAT computational burden, however hardware support may mitigate this 318 in some implementations. 320 An SCTP endpoint may have multiple addresses but only has a single 321 port number. To make multipoint traversal work, all the NATs 322 involved must recognize the packets they see as belonging to the same 323 SCTP association and perform port number translation in a consistent 324 way. One possible way of doing this is to use pre-defined table of 325 ports and addresses configured within each NAT. Other mechanisms 326 could make use of NAT to NAT communication. Such mechanisms are not 327 to be deployable on a wide scale base and thus not a recommended 328 solution. Therefore the SCTP variant of NAT has been developed. 330 4.3. The SCTP Specific Variant of NAT 332 In this section we assume that we have multiple SCTP capable hosts 333 behind a NAT which has one Public-Address. Furthermore we are 334 focusing in this section on the single point traversal scenario. 336 The modification of SCTP packets sent to the public Internet is easy. 337 The source address of the packet has to be replaced with the Public- 338 Address. It may also be necessary to establish some state in the NAT 339 box to handle incoming packets, which is discussed later. 341 For SCTP packets coming from the public Internet the destination 342 address of the packets has to be replaced with the Private-Address of 343 the host the packet has to be delivered to. The lookup of the 344 Private-Address is based on the External-VTag, External-Port, 345 External-Address, Internal-VTag and the Internal-Port. 347 For the SCTP NAT processing the NAT box has to maintain a table of 348 Internal-VTag, Internal-Port, Private-Address, External-VTag, 349 External-Port and whether the restart procedure is disabled or not. 350 An entry in that table is called a NAT state control block. The 351 function Create() obtains the just mentioned parameters and returns a 352 NAT-State control block. 354 The entries in this table fulfill some uniqueness conditions. There 355 must not be more than one entry with the same pair of Internal-Port 356 and External-Port. This rule can be relaxed, if all entries with the 357 same Internal-Port and External-Port have the support for the restart 358 procedure enabled. In this case there must be no more than one entry 359 with the same Internal-Port, External-Port and Ext-VTag and no more 360 than one entry with the same Internal-Port, External-Port and Int- 361 VTag. 363 The processing of outgoing SCTP packets containing an INIT-chunk is 364 described in the following figure. The scenario shown is valid for 365 all message flows in this section. 367 /--\/--\ 368 +--------+ +-----+ / \ +--------+ 369 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 370 +--------+ +-----+ \ / +--------+ 371 \--/\---/ 373 INIT[Initiate-Tag] 374 Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port 375 Ext-VTag=0 377 Create(Initiate-Tag, Int-Port, Priv-Addr, 0) 378 Returns(NAT-State control block) 380 Translate To: 382 INIT[Initiate-Tag] 383 Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port 384 Ext-VTag=0 386 Normally a NAT control block will be created. However, it is 387 possible that there is already a NAT control block with the same 388 External-Address, External-Port, Internal-Port, and Internal-VTag but 389 different Private-Address. In this case the INIT MUST be dropped by 390 the NAT and an ABORT MUST be sent back to the SCTP host with the 391 M-Bit set and an appropriate error cause (see Section 5.1.1 for the 392 format). The source address of the packet containing the ABORT chunk 393 MUST be the destination address of the packet containing the INIT 394 chunk. 396 It is also possible that a connection to External-Address and 397 External-Port exists without an Internal-VTag conflict but the 398 External-Address does not support the DISABLE_RESTART feature (noted 399 in the NAT control block when the prior connection was established). 400 In such a case the INIT SHOULD be dropped by the NAT and an ABORT 401 SHOULD be sent back to the SCTP host with the M-Bit set and an 402 appropriate error cause (see Section 5.1.1 for the format). 404 The processing of outgoing SCTP packets containing no INIT-chunk is 405 described in the following figure. 407 /--\/--\ 408 +--------+ +-----+ / \ +--------+ 409 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 410 +--------+ +-----+ \ / +--------+ 411 \--/\---/ 413 Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port 414 Ext-VTag 416 Translate To: 418 Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port 419 Ext-VTag 421 The processing of incoming SCTP packets containing INIT-ACK chunks is 422 described in the following figure. The Lookup() function getting as 423 input the Internal-VTag, Internal-Port, External-VTag (=0), External- 424 Port, and External-Address, returns the corresponding entry of the 425 NAT table and updates the External-VTag by substituting it with the 426 value of the Initiate-Tag of the INIT-ACK chunk. The wildcard 427 character signifies that the parameter's value is not considered in 428 the Lookup() function or changed in the Update() function, 429 respectively. 431 /--\/--\ 432 +--------+ +-----+ / \ +--------+ 433 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 434 +--------+ +-----+ \ / +--------+ 435 \--/\---/ 437 INIT-ACK[Initiate-Tag] 438 Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port 439 Int-VTag 441 Lookup(Int-VTag, Int-Port, *, 0, Ext-Port) 442 Update(*, *, *, Initiate-Tag, *) 444 Returns(NAT-State control block containing Private-Address) 446 INIT-ACK[Initiate-Tag] 447 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port 448 Int-VTag 450 In the case Lookup fails, the SCTP packet is dropped. The Update 451 routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK 452 chunk) in the NAT state control block. 454 The processing of incoming SCTP packets containing an ABORT or 455 SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the 456 following figure. 458 /--\/--\ 459 +--------+ +-----+ / \ +--------+ 460 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 461 +--------+ +-----+ \ / +--------+ 462 \--/\---/ 464 Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port 465 Ext-VTag 467 Lookup(0, Int-Port, *, Ext-VTag, Ext-Port) 469 Returns(NAT-State control block containing Private-Address) 471 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port 472 Ext-VTag 474 The processing of other incoming SCTP packets is described in the 475 following figure. 477 /--\/--\ 478 +--------+ +-----+ / \ +--------+ 479 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 480 +--------+ +-----+ \ / +--------+ 481 \--/\---/ 483 Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port 484 Int-VTag 486 Lookup(Int-VTag, Int-Port, *, *, Ext-Port) 488 Returns(NAT-State control block containing Local-Address) 490 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port 491 Int-VTag 493 For an incoming packet containing an INIT-chunk a table lookup is 494 made only based on the addresses and port numbers. If an entry with 495 an External-VTag of zero is found, it is considered a match and the 496 External-VTag is updated. 498 This allows the handling of INIT-collision through NAT. 500 5. Data Formats 502 This section defines the formats used to support NAT traversal. 503 Section 5.1 and Section 5.2 describe chunks and error causes sent by 504 NATs and received by SCTP end points. Section 5.3 describes 505 parameters sent by SCTP end points and used by NATs and SCTP end 506 points. 508 5.1. Modified Chunks 510 This section presents existing chunks defined in [RFC4960] that are 511 modified by this document. 513 5.1.1. Extended ABORT Chunk 515 0 1 2 3 516 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 1 517 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 518 | Type = 6 | Reserved |M|T| Length | 519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 520 \ \ 521 / zero or more Error Causes / 522 \ \ 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 524 The ABORT chunk is extended to add the new 'M-bit'. The M-bit 525 indicates to the receiver of the ABORT chunk that the chunk was not 526 generated by the peer SCTP endpoint, but instead by a middle box. 528 [NOTE: 530 ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA. 532 ] 534 5.1.2. Extended ERROR Chunk 536 0 1 2 3 537 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 1 538 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 539 | Type = 9 | Reserved |M|T| Length | 540 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 541 \ \ 542 / zero or more Error Causes / 543 \ \ 544 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 The ERROR chunk defined in [RFC4960] is extended to add the new 547 'M-bit'. The M-bit indicates to the receiver of the ERROR chunk that 548 the chunk was not generated by the peer SCTP endpoint, but instead by 549 a middle box. 551 [NOTE: 553 ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA. 555 ] 557 5.2. New Error Causes 559 This section defines the new error causes added by this document. 561 5.2.1. VTag and Port Number Collision Error Cause 563 0 1 2 3 564 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 1 565 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 566 | Cause Code = 0x00B0 | Cause Length = Variable | 567 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 568 \ Chunk / 569 / \ 570 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 571 Cause Code: 2 bytes (unsigned integer) 572 This field holds the IANA defined cause code for the VTag and Port 573 Number Collision Error Cause. The suggested value of this field 574 for IANA is 0x00B0. 576 Cause Length: 2 bytes (unsigned integer) 577 This field holds the length in bytes of the error cause. The 578 value MUST be the length of the Cause-Specific Information plus 4. 580 Chunk: variable length 581 The Cause-Specific Information is filled with the chunk that 582 caused this error. This can be an INIT, INIT-ACK, or ASCONF 583 chunk. Note that if the entire chunk will not fit in the ERROR 584 chunk or ABORT chunk being sent then the bytes that do not fit are 585 truncated. 587 [NOTE: 589 ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. 591 ] 593 5.2.2. Missing State Error Cause 595 0 1 2 3 596 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 1 597 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 598 | Cause Code = 0x00B1 | Cause Length = Variable | 599 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 600 \ Incoming Packet / 601 / \ 602 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 604 Cause Code: 2 bytes (unsigned integer) 605 This field holds the IANA defined cause code for the Missing State 606 Error Cause. The suggested value of this field for IANA is 607 0x00B1. 609 Cause Length: 2 bytes (unsigned integer) 610 This field holds the length in bytes of the error cause. The 611 value MUST be the length of the Cause-Specific Information plus 4. 613 Incoming Packet: variable length 614 The Cause-Specific Information is filled with the IPv4 or IPv6 615 packet that caused this error. The IPv4 or IPv6 header MUST be 616 included. Note that if the packet will not fit in the ERROR chunk 617 or ABORT chunk being sent then the bytes that do not fit are 618 truncated. 620 [NOTE: 622 ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. 624 ] 626 5.2.3. Port Number Collision Error Cause 628 0 1 2 3 629 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 1 630 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 631 | Cause Code = 0x00B2 | Cause Length = Variable | 632 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 633 \ chunk / 634 / \ 635 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 637 Cause Code: 2 bytes (unsigned integer) 638 This field holds the IANA defined cause code for the Port Number 639 Collision Error Cause. The suggested value of this field for IANA 640 is 0x00B2. 642 Cause Length: 2 bytes (unsigned integer) 643 This field holds the length in bytes of the error cause. The 644 value MUST be the length of the Cause-Specific Information plus 4. 646 Chunk: variable length 647 The Cause-Specific Information is filled with the chunk that 648 caused this error. This can be an INIT, INIT-ACK, or ASCONF 649 chunk. Note that if the entire chunk will not fit in the ERROR 650 chunk or ABORT chunk being sent then the bytes that do not fit are 651 truncated. 653 [NOTE: 655 ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA. 657 ] 659 5.3. New Parameters 661 This section defines new parameters and their valid appearance 662 defined by this document. 664 5.3.1. Disable Restart Parameter 666 This parameter is used to indicate that the RESTART procedure is 667 requested to be disabled. Both endpoints of an association MUST 668 include this parameter in the INIT chunk and INIT-ACK chunk when 669 establishing an association and MUST include it in the ASCONF chunk 670 when adding an address to successfully disable the restart procedure. 672 0 1 2 3 673 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 1 674 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 675 | Type = 0xC007 | Length = 4 | 676 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 678 Parameter Type: 2 bytes (unsigned integer) 679 This field holds the IANA defined parameter type for the Disable 680 Restart Parameter. The suggested value of this field for IANA is 681 0xC007. 683 Parameter Length: 2 bytes (unsigned integer) 684 This field holds the length in bytes of the parameter. The value 685 MUST be 4. 687 [NOTE: 689 ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA. 691 ] 693 This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and 694 MUST NOT appear in any other chunk. 696 5.3.2. VTags Parameter 698 This parameter is used to help a NAT recover from state loss. 700 0 1 2 3 701 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 1 702 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 703 | Parameter Type = 0xC008 | Parameter Length = 16 | 704 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 705 | ASCONF-Request Correlation ID | 706 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 707 | Internal Verification Tag | 708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 709 | External Verification Tag | 710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 711 Parameter Type: 2 bytes (unsigned integer) 712 This field holds the IANA defined parameter type for the VTags 713 Parameter. The suggested value of this field for IANA is 0xC008. 715 Parameter Length: 2 bytes (unsigned integer) 716 This field holds the length in bytes of the parameter. The value 717 MUST be 16. 719 ASCONF-Request Correlation ID: 4 bytes (unsigned integer) 720 This is an opaque integer assigned by the sender to identify each 721 request parameter. The receiver of the ASCONF Chunk will copy 722 this 32-bit value into the ASCONF Response Correlation ID field of 723 the ASCONF-ACK response parameter. The sender of the ASCONF can 724 use this same value in the ASCONF-ACK to find which request the 725 response is for. Note that the receiver MUST NOT change this 726 32-bit value. 728 Internal Verification Tag: 4 bytes (unsigned integer) 729 The Verification Tag that the internal host has chosen for its 730 communication. The Verification Tag is a unique 32-bit tag that 731 must accompany any incoming SCTP packet for this association to 732 the Private-Address. 734 External Verification Tag: 4 bytes (unsigned integer) The 735 Verification Tag that the host holding the External-Address has 736 chosen for its communication. The VTag is a unique 32-bit tag 737 that must accompany any incoming SCTP packet for this association 738 to the External-Address. 740 [NOTE: 742 ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA. 744 ] 746 This parameter MAY appear in ASCONF chunks and MUST NOT appear in any 747 other chunk. 749 6. Procedures for SCTP End Points and NATs 751 6.1. Overview 753 When an SCTP endpoint is behind an SCTP-aware NAT a number of 754 problems may arise as it tries to communicate with its peer: 756 o More than one host behind a NAT may pick the same VTag and source 757 port when talking to the same peer server. This creates a 758 situation where the NAT will not be able to tell the two 759 associations apart. This situation is discussed in Section 6.3. 761 o When an SCTP endpoint is a server communicating with multiple 762 peers and the peers are behind the same NAT, then the two 763 endpoints cannot be distinguished by the server. This case is 764 discussed in Section 6.4. 766 o A restart of a NAT during a conversation could cause a loss of its 767 state. This problem and its solution is discussed in Section 6.5. 769 o An SCTP endpoint may be behind two NATs providing redundancy. The 770 method to set up this scenario is discussed in Section 6.7. 772 Each of these mechanisms requires additional chunks and parameters, 773 defined in this document, and possibly modified handling procedures 774 from those specified in [RFC4960]. 776 6.2. Association Setup Considerations 778 The association setup procedure defined in [RFC4960] allows multi- 779 homed SCTP end points to exchange its IP-addresses by using IPv4 or 780 IPv6 address parameters in the INIT and INIT-ACK chunks. However, 781 this can't be used when NATs are present. 783 Every association MUST initially be set up single-homed. There MUST 784 NOT be any IPv4 Address parameter, IPv6 Address parameter, or 785 Supported Address Types parameter in the INIT-chunk. The INIT-ACK 786 chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address 787 parameter. 789 If the association should finally be multi-homed, the procedure in 790 Section 6.7 MUST be used. 792 The INIT and INIT-ACK chunk SHOULD contain the Disable Restart 793 parameter defined in Section 5.3.1. 795 6.3. Handling of Internal Port Number and Verification Tag Collisions 797 Consider the case where two hosts in the Private-Address space want 798 to set up an SCTP association with the same server running on the 799 same host in the Internet. This means that the External-Port and the 800 External-Address are the same. If they both choose the same 801 Internal-Port and Internal-VTag, the NAT box cannot distinguish 802 between incoming packets anymore. But this is very unlikely. The 803 Internal-VTags are chosen at random and if the Internal-Ports are 804 also chosen from the ephemeral port range at random this gives a 805 46-bit random number which has to match. In the TCP-like NAPT case 806 the NAT box can control the 16-bit Natted Port and therefore avoid 807 collisions deterministically. 809 The same can happen when an INIT-ACK chunk or an ASCONF chunk is 810 processed by the NAT. 812 However, in this unlikely event the NAT box MUST send an ABORT chunk 813 with the M-bit set if the collision is triggered by an INIT or INIT- 814 ACK chunk or send an ERROR chunk with the M-bit set if the collision 815 is triggered by an ASCONF chunk. The M-bit is a new bit defined by 816 this document to express to SCTP that the source of this packet is a 817 "middle" box, not the peer SCTP endpoint (see Section 5.1.1). If a 818 packet containing an INIT-ACK chunk triggers the collision, the 819 corresponding packet containing the ABORT chunk MUST contain the same 820 source and destination address and port numbers as the packet 821 containing the INIT-ACK chunk. In the other two cases, the source 822 and destination address and port numbers MUST be swapped. 824 The sender of the packet containing the INIT chunk or the receiver of 825 the INIT-ACK chunk, upon reception of an ABORT chunk with M-bit set 826 and the appropriate error cause code for colliding NAT table state is 827 included, MUST reinitiate the association setup procedure after 828 choosing a new initiate tag, if the association is in COOKIE-WAIT 829 state. In any other state, the SCTP endpoint MUST NOT respond. 831 The sender of the ASCONF chunk, upon reception of an ERROR chunk with 832 M-bit set, MUST stop adding the path to the association. 834 The sender of the ERROR or ABORT chunk MUST include the error cause 835 with cause code 'VTag and Port Number Collision' (see Section 5.2.1). 837 6.4. Handling of Internal Port Number Collisions 839 When two SCTP hosts are behind an SCTP-aware NAT it is possible that 840 two SCTP hosts in the Private-Address space will want to set up an 841 SCTP association with the same server running on the same host in the 842 Internet. For the NAT, appropriate tracking may be performed by 843 assuring that the VTags are unique between the two hosts. 845 But for the external SCTP server on the Internet this means that the 846 External-Port and the External-Address are the same. If they both 847 have chosen the same Internal-Port the server cannot distinguish 848 between both associations based on the address and port numbers. For 849 the server it looks like the association is being restarted. To 850 overcome this limitation the client sends a Disable Restart parameter 851 in the INIT-chunk. 853 When the server receives this parameter it MUST do the following: 855 o Include a Disable Restart parameter in the INIT-ACK to inform the 856 client that it will support the feature. 858 o Disable the restart procedures defined in [RFC4960] for this 859 association. 861 Servers that support this feature will need to be capable of 862 maintaining multiple connections to what appears to be the same peer 863 (behind the NAT) differentiated only by the VTags. 865 The NAT, when processing the INIT-ACK, should note in its internal 866 table that the association supports the Disable Restart extension. 867 This note is used when establishing future associations (i.e. when 868 processing an INIT from an internal host) to decide if the connection 869 should be allowed. The NAT MUST do the following when processing an 870 INIT: 872 o If the INIT is destined to an external address and port for which 873 the NAT has no outbound connection, allow the INIT creating an 874 internal mapping table. 876 o If the INIT matches the external address and port of an already 877 existing connection, validate that the external server supports 878 the Disable Restart feature, if it does allow the INIT to be 879 forwarded. 881 o If the external server does not support the Disable Restart 882 extension the NAT MUST send an ABORT with the M-bit set. 884 The 'Port Number Collision' error cause (see Section 5.2.3) MUST be 885 included in the ABORT chunk. 887 If the collision is triggered by an ASCONF chunk, a packet containing 888 an ERROR chunk with the 'Port Number Collision' error cause MUST be 889 sent back. 891 6.5. Handling of Missing State 893 If the NAT box receives a packet from the internal network for which 894 the lookup procedure does not find an entry in the NAT table, a 895 packet containing an ERROR chunk is sent back with the M-bit set. 896 The source address of the packet containing the ERROR chunk MUST be 897 the destination address of the incoming SCTP packet. The 898 verification tag is reflected and the T-bit is set. Please note that 899 such a packet containing an ERROR chunk SHOULD NOT be sent if the 900 received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK 901 chunk. An ERROR chunk MUST NOT be sent if the received packet 902 contains an ERROR chunk with the M-bit set. 904 When sending the ERROR chunk, the new error cause Missing state (see 905 Section 5.2.2) MUST be included and the new M-bit of the ERROR chunk 906 MUST be set (see Section 5.1.2). 908 Upon reception of this ERROR chunk by an SCTP endpoint the receiver 909 SHOULD take the following actions: 911 o Validate that the verification tag is reflected by looking at the 912 VTag that would have been included in the outgoing packet. 914 o Validate that the peer of the SCTP association supports the 915 dynamic address extension, if it does not discard the incoming 916 ERROR chunk. 918 o Generate a new ASCONF chunk containing the VTags parameter (see 919 Section 5.3.2) and the Disable Restart parameter if the 920 association is using the disabled restart feature. By processing 921 this packet the NAT can recover the appropriate state. The 922 procedures for generating an ASCONF chunk can be found in 923 [RFC5061]. 925 If the NAT box receives a packet for which it has no NAT table entry 926 and the packet contains an ASCONF chunk with the VTags parameter, the 927 NAT box MUST update its NAT table according to the verification tags 928 in the VTags parameter and the optional Disable Restart parameter. 930 The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either 931 add the address and respond with an acknowledgment, if the address is 932 new to the association (following all procedures defined in 933 [RFC5061]). Or, if the address is already part of the association, 934 the SCTP endpoint MUST NOT respond with an error, but instead should 935 respond with an ASCONF-ACK chunk acknowledging the address but take 936 no action (since the address is already in the association). 938 Note that it is possible that upon receiving an ASCONF chunk 939 containing the VTags parameter the NAT will realize that it has an 940 'Internal Port Number and Verification Tag collision'. In such a 941 case the NAT MUST send an ERROR chunk with the error cause code set 942 to 'VTag and Port Number Collision' (see Section 5.2.1). 944 If an SCTP endpoint receives an ERROR with 'Internal Port Number and 945 Verification Tag collision' as the error cause and the packet in the 946 Error Chunk contains an ASCONF with the VTags parameter, careful 947 examination of the association is required. The endpoint MUST do the 948 following: 950 o Validate that the verification tag is reflected by looking at the 951 VTag that would have been included in the outgoing packet. 953 o Validate that the peer of the SCTP association supports the 954 dynamic address extension, if it does not discard the incoming 955 ERROR chunk. 957 o If the association is attempting to add an address (i.e. following 958 the procedures in Section 6.7) then the endpoint MUST-NOT consider 959 the address part of the association and SHOULD make no further 960 attempt to add the address (i.e. cancel any ASCONF timers and 961 remove any record of the path), since the NAT has a VTag collision 962 and the association cannot easily create a new VTag (as it would 963 if the error occurred when sending an INIT). 965 o If the endpoint has no other path, i. e. the procedure was 966 executed due to missing a state in the NAT, then the endpoint MUST 967 abort the association. This would occur only if the local NAT 968 restarted and accepted a new association before attempting to 969 repair the missing state (Note that this is no different than what 970 happens to all TCP connections when a NAT looses its state). 972 6.6. Handling of Fragmented SCTP Packets 974 A NAT box MUST support IP reassembly of received fragmented SCTP 975 packets. The fragments may arrive in any order. 977 When an SCTP packet has to be fragmented by the NAT box and the IP 978 header forbids fragmentation a corresponding ICMP packet SHOULD be 979 sent. 981 6.7. Multi-Point Traversal Considerations 983 If a multi-homed SCTP endpoint behind a NAT connects to a peer, it 984 SHOULD first set up the association single-homed with only one 985 address causing the first NAT to populate its state. Then it SHOULD 986 add each IP address using ASCONF chunks sent via their respective 987 NATs. The address to add is the wildcard address and the lookup 988 address SHOULD also contain the VTags parameter and optionally the 989 Disable Restart parameter as illustrated above. 991 7. Various Examples of NAT Traversals 993 Please note that this section is informational only. 995 7.1. Single-homed Client to Single-homed Server 997 The internal client starts the association with the external server 998 via a four-way-handshake. Host A starts by sending an INIT chunk. 1000 /--\/--\ 1001 +--------+ +-----+ / \ +--------+ 1002 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 1003 +--------+ +-----+ \ / +--------+ 1004 \--/\---/ 1005 +---------+--------+-----------+----------+--------+ 1006 NAT | Int | Int | Priv | Ext | Ext | 1007 | VTag | Port | Addr | VTag | Port | 1008 +---------+--------+--- -------+----------+--------+ 1010 INIT[Initiate-Tag = 1234] 1011 10.0.0.1:1 ------> 100.0.0.1:2 1012 Ext-VTtag = 0 1014 A NAT entry is created, the source address is substituted and the 1015 packet is sent on: 1017 NAT creates entry: 1018 +---------+--------+-----------+----------+--------+ 1019 NAT | Int | Int | Priv | Ext | Ext | 1020 | VTag | Port | Addr | VTag | Port | 1021 +---------+--------+-----------+----------+--------+ 1022 | 1234 | 1 | 10.0.0.1 | 0 | 2 | 1023 +---------+--------+-----------+----------+--------+ 1025 INIT[Initiate-Tag = 1234] 1026 101.0.0.1:1 --------------------------> 100.0.0.1:2 1027 Ext-VTtag = 0 1029 Host B receives the INIT and sends an INIT-ACK with the NAT's 1030 external address as destination address. 1032 /--\/--\ 1033 +--------+ +-----+ / \ +--------+ 1034 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 1035 +--------+ +-----+ \ / +--------+ 1036 \--/\---/ 1038 INIT-ACK[Initiate-Tag = 5678] 1039 101.0.0.1:1 <------------------------- 100.0.0.1:2 1040 Int-VTag = 1234 1042 NAT updates entry: 1043 +---------+--------+-----------+----------+--------+ 1044 NAT | Int | Int | Priv | Ext | Ext | 1045 | VTag | Port | Addr | VTag | Port | 1046 +---------+--------+-----------+----------+--------+ 1047 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1048 +---------+--------+-----------+----------+--------+ 1050 INIT-ACK[Initiate-Tag = 5678] 1051 10.0.0.1:1 <------ 100.0.0.1:2 1052 Int-VTag = 1234 1054 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- 1055 ACK. 1057 /--\/--\ 1058 +--------+ +-----+ / \ +--------+ 1059 | Host A | <------> | NAT | <------> | Internet | <------> | Host B | 1060 +--------+ +-----+ \ / +--------+ 1061 \--/\---/ 1063 COOKIE-ECHO 1064 10.0.0.1:1 ------> 100.0.0.1:2 1065 Ext-VTag = 5678 1067 COOKIE-ECHO 1068 101.0.0.1:1 -------------------------> 100.0.0.1:2 1069 Ext-VTag = 5678 1071 COOKIE-ACK 1072 101.0.0.1:1 <------------------------- 100.0.0.1:2 1073 Int-VTag = 1234 1075 COOKIE-ACK 1076 10.0.0.1:1 <------ 100.0.0.1:2 1077 Int-VTag = 1234 1079 7.2. Single-homed Client to Multi-homed Server 1081 The internal client is single-homed whereas the external server is 1082 multi-homed. The client (Host A) sends an INIT like in the single- 1083 homed case. 1085 +--------+ 1086 /--\/--\ /-|Router 1| \ 1087 +------+ +-----+ / \ / +--------+ \ +------+ 1088 | Host | <-----> | NAT | <-> | Internet | == =| Host | 1089 | A | +-----+ \ / \ +--------+ / | B | 1090 +------+ \--/\--/ \-|Router 2|-/ +------+ 1091 +--------+ 1093 +---------+--------+-----------+----------+--------+ 1094 NAT | Int | Int | Priv | Ext | Ext | 1095 | VTag | Port | Addr | VTag | Port | 1096 +---------+--------+-----------+----------+--------+ 1098 INIT[Initiate-Tag = 1234] 1099 10.0.0.1:1 ---> 100.0.0.1:2 1100 Ext-VTag = 0 1102 NAT creates entry: 1104 +---------+--------+-----------+----------+--------+ 1105 NAT | Int | Int | Priv | Ext | Ext | 1106 | VTag | Port | Addr | VTag | Port | 1107 +---------+--------+-----------+----------+--------+ 1108 | 1234 | 1 | 10.0.0.1 | 0 | 2 | 1109 +---------+--------+-----------+----------+--------+ 1111 INIT[Initiate-Tag = 1234] 1112 101.0.0.1:1 ----------------------------> 100.0.0.1:2 1113 Ext-VTag = 0 1115 The server (Host B) includes its two addresses in the INIT-ACK chunk, 1116 which results in two NAT entries. 1118 +--------+ 1119 /--\/--\ /-|Router 1| \ 1120 +------+ +-----+ / \ / +--------+ \ +------+ 1121 | Host | <-----> | NAT | <-> | Internet | == =| Host | 1122 | A | +-----+ \ / \ +--------+ / | B | 1123 +------+ \--/\--/ \-|Router 2|-/ +------+ 1124 +--------+ 1126 INIT-ACK[Initiate-tag = 5678, IP-Addr = 100.1.0.1] 1127 101.0.0.1:1 <---------------------------- 100.0.0.1:2 1128 Int-VTag = 1234 1130 NAT does need to change the table for second address: 1132 +---------+--------+-----------+----------+--------+ 1133 NAT | Int | Int | Priv | Ext | Ext | 1134 | VTag | Port | Addr | VTag | Port | 1135 +---------+--------+-----------+----------+--------+ 1136 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1137 +---------+--------+-----------+----------+--------+ 1139 INIT-ACK[Initiate-Tag = 5678] 1140 10.0.0.1:1 <--- 100.0.0.1:2 1141 Int-VTag = 1234 1143 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- 1144 ACK. 1146 +--------+ 1147 /--\/--\ /-|Router 1| \ 1148 +------+ +-----+ / \ / +--------+ \ +------+ 1149 | Host | <-----> | NAT | <-> | Internet | == =| Host | 1150 | A | +-----+ \ / \ +--------+ / | B | 1151 +------+ \--/\--/ \-|Router 2|-/ +------+ 1152 +--------+ 1154 COOKIE-ECHO 1155 10.0.0.1:1 ---> 100.0.0.1:2 1156 ExtVTag = 5678 1158 COOKIE-ECHO 1159 101.0.0.1:1 ----------------------------> 100.0.0.1:2 1160 Ext-VTag = 5678 1162 COOKIE-ACK 1163 101.0.0.1:1 <---------------------------- 100.0.0.1:2 1164 Int-VTag = 1234 1166 COOKIE-ACK 1167 10.0.0.1:1 <--- 100.0.0.1:2 1168 Int-VTag = 1234 1170 7.3. Multihomed Client and Server 1172 The client (Host A) sends an INIT to the server (Host B), but does 1173 not include the second address. 1175 +-------+ 1176 /--| NAT 1 |--\ /--\/--\ 1177 +------+ / +-------+ \ / \ +--------+ 1178 | Host |=== ====| Internet |====| Host B | 1179 | A | \ +-------+ / \ / +--------+ 1180 +------+ \--| NAT 2 |--/ \--/\--/ 1181 +-------+ 1183 +---------+--------+-----------+----------+--------+ 1184 NAT 1 | Int | Int | Priv | Ext | Ext | 1185 | VTag | Port | Addr | VTag | Port | 1186 +---------+--------+--- -------+----------+--------+ 1188 INIT[Initiate-Tag = 1234] 1189 10.0.0.1:1 --------> 100.0.0.1:2 1190 Ext-VTag = 0 1192 NAT 1 creates entry: 1194 +---------+--------+-----------+----------+--------+ 1195 NAT 1 | Int | Int | Priv | Ext | Ext | 1196 | VTag | Port | Addr | VTag | Port | 1197 +---------+--------+-----------+----------+--------+ 1198 | 1234 | 1 | 10.0.0.1 | 0 | 2 | 1199 +---------+--------+-----------+----------+--------+ 1201 INIT[Initiate-Tag = 1234] 1202 101.0.0.1:1 -----------------------> 100.0.0.1:2 1203 ExtVTag = 0 1205 Host B includes its second address in the INIT-ACK, which results in 1206 two NAT entries in NAT 1. 1208 +-------+ 1209 /--------| NAT 1 |--------\ /--\/--\ 1210 +------+ / +-------+ \ / \ +--------+ 1211 | Host |=== ====| Internet |===| Host B | 1212 | A | \ +-------+ / \ / +--------+ 1213 +------+ \--------| NAT 2 |--------/ \--/\--/ 1214 +-------+ 1216 INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1] 1217 101.0.0.1:1 <------------------------- 100.0.0.1:2 1218 Int-VTag = 1234 1220 NAT 1 does not need to update the table for second address: 1222 +---------+--------+-----------+----------+--------+ 1223 NAT 1 | Int | Int | Priv | Ext | Ext | 1224 | VTag | Port | Addr | VTag | Port | 1225 +---------+--------+-----------+----------+--------+ 1226 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1227 +---------+--------+-----------+----------+--------+ 1229 INIT-ACK[Initiate-Tag = 5678] 1230 10.0.0.1:1 <---------100.0.0.1:2 1231 Int-VTag = 1234 1233 The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE- 1234 ACK. 1236 +-------+ 1237 /--------| NAT 1 |--------\ /--\/--\ 1238 +------+ / +-------+ \ / \ +--------+ 1239 | Host |=== ====| Internet |===| Host B | 1240 | A | \ +-------+ / \ / +--------+ 1241 +------+ \--------| NAT 2 |--------/ \--/\--/ 1242 +-------+ 1244 COOKIE-ECHO 1245 10.0.0.1:1 --------> 100.0.0.1:2 1246 Ext-VTag = 5678 1248 COOKIE-ECHO 1249 101.0.0.1:1 --------------------> 100.0.0.1:2 1250 Ext-VTag = 5678 1252 COOKIE-ACK 1253 101.0.0.1:1 <-------------------- 100.0.0.1:2 1254 Int-VTag = 1234 1256 COOKIE-ACK 1257 10.0.0.1:1 <------- 100.0.0.1:2 1258 Int-VTag = 1234 1260 Host A announces its second address in an ASCONF chunk. The address 1261 parameter contains an undefined address (0) to indicate that the 1262 source address should be added. The lookup address parameter within 1263 the ASCONF chunk will also contain the pair of VTags (external and 1264 internal) so that the NAT may populate its table completely with this 1265 single packet. 1267 +-------+ 1268 /--------| NAT 1 |--------\ /--\/--\ 1269 +------+ / +-------+ \ / \ +--------+ 1270 | Host |=== ====| Internet |===| Host B | 1271 | A | \ +-------+ / \ / +--------+ 1272 +------+ \--------| NAT 2 |--------/ \--/\--/ 1273 +-------+ 1275 ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678] 1276 10.1.0.1:1 --------> 100.1.0.1:2 1277 Ext-VTag = 5678 1279 NAT 2 creates complete entry: 1281 +---------+--------+-----------+----------+--------+ 1282 NAT 2 | Int | Int | Priv | Ext | Ext | 1283 | VTag | Port | Addr | VTag | Port | 1284 +---------+--------+-----------+----------+--------+ 1285 | 1234 | 1 | 10.1.0.1 | 5678 | 2 | 1286 +---------+--------+-----------+----------+--------+ 1288 ASCONF [ADD-IP,Int-VTag=1234, Ext-VTag = 5678] 1289 101.1.0.1:1 -----------------------> 100.1.0.1:2 1290 Ext-VTag = 5678 1292 ASCONF-ACK 1293 101.1.0.1:1 <----------------------- 100.1.0.1:2 1294 Int-VTag = 1234 1296 ASCONF-ACK 1297 10.1.0.1:1 <----- 100.1.0.1:2 1298 Int-VTag = 1234 1300 7.4. NAT Loses Its State 1302 Association is already established between Host A and Host B, when 1303 the NAT loses its state and obtains a new public address. Host A 1304 sends a DATA chunk to Host B. 1306 /--\/--\ 1307 +--------+ +-----+ / \ +--------+ 1308 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | 1309 +--------+ +-----+ \ / +--------+ 1310 \--/\--/ 1312 +---------+--------+-----------+----------+--------+ 1313 NAT | Int | Int | Priv | Ext | Ext | 1314 | VTag | Port | Addr | VTag | Port | 1315 +---------+--------+-----------+----------+--------+ 1316 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1317 +---------+--------+-----------+----------+--------+ 1319 DATA 1320 10.0.0.1:1 ----------> 100.0.0.1:2 1321 Ext-VTag = 5678 1323 The NAT box cannot find entry for the association. It sends ERROR 1324 message with the M-Bit set and the cause "NAT state missing". 1326 /--\/--\ 1327 +--------+ +-----+ / \ +--------+ 1328 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | 1329 +--------+ +-----+ \ / +--------+ 1330 \--/\--/ 1332 ERROR [M-Bit, NAT state missing] 1333 10.0.0.1:1 <---------- 100.0.0.1:2 1334 Ext-VTag = 5678 1336 On reception of the ERROR message, Host A sends an ASCONF chunk 1337 indicating that the former information has to be deleted and the 1338 source address of the actual packet added. 1340 /--\/--\ 1341 +--------+ +-----+ / \ +--------+ 1342 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | 1343 +--------+ +-----+ \ / +--------+ 1344 \--/\--/ 1346 ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678] 1347 10.0.0.1:1 ----------> 100.1.0.1:2 1348 Ext-VTag = 5678 1350 +---------+--------+-----------+----------+--------+ 1351 NAT | Int | Int | Priv | Ext | Ext | 1352 | VTag | Port | Addr | VTag | Port | 1353 +---------+--------+-----------+----------+--------+ 1354 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1355 +---------+--------+-----------+----------+--------+ 1357 ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678] 1358 102.1.0.1:1 ---------------------> 100.1.0.1:2 1359 Ext-VTag = 5678 1361 Host B adds the new source address and deletes all former entries. 1363 /--\/--\ 1364 +--------+ +-----+ / \ +--------+ 1365 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B | 1366 +--------+ +-----+ \ / +--------+ 1367 \--/\--/ 1369 ASCONF-ACK 1370 102.1.0.1:1 <--------------------- 100.1.0.1:2 1371 Int-VTag = 1234 1373 ASCONF-ACK 1374 10.1.0.1:1 <---------- 100.1.0.1:2 1375 Int-VTag = 1234 1377 DATA 1378 10.0.0.1:1 ----------> 100.0.0.1:2 1379 Ext-VTag = 5678 1380 DATA 1381 102.1.0.1:1 ---------------------> 100.1.0.1:2 1382 Ext-VTag = 5678 1384 7.5. Peer-to-Peer Communication 1386 If two hosts are behind NATs, they have to get knowledge of the 1387 peer's public address. This can be achieved with a so-called 1388 rendezvous server. Afterwards the destination addresses are public, 1389 and the association is set up with the help of the INIT collision. 1390 The NAT boxes create their entries according to their internal peer's 1391 point of view. Therefore, NAT A's Internal-VTag and Internal-Port 1392 are NAT B's External-VTag and External-Port, respectively. The 1393 naming of the verification tag in the packet flow is done from the 1394 sending peer's point of view. 1396 Internal | External External | Internal 1397 | | 1398 | /--\/---\ | 1399 +--------+ +-------+ / \ +-------+ +--------+ 1400 | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | 1401 +--------+ +-------+ \ / +-------+ +--------+ 1402 | \--/\---/ | 1404 NAT-Tables 1405 +---------+--------+-----------+----------+--------+ 1406 NAT A | Int | Int | Priv | Ext | Ext | 1407 | VTag | Port | Addr | VTag | Port | 1408 +---------+--------+--- -------+----------+--------+ 1410 +---------+--------+-----------+----------+--------+ 1411 NAT B | Int | Int | Priv | Ext | Ext | 1412 | v-tag | port | addr | v-tag | port | 1413 +---------+--------+--- -------+----------+--------+ 1415 INIT[Initiate-Tag = 1234] 1416 10.0.0.1:1 --> 100.0.0.1:2 1417 Ext-VTag = 0 1419 NAT A creates entry: 1421 +---------+--------+-----------+----------+--------+ 1422 NAT A | Int | Int | Priv | Ext | Ext | 1423 | VTag | Port | Addr | VTag | Port | 1424 +---------+--------+-----------+----------+--------+ 1425 | 1234 | 1 | 10.0.0.1 | 0 | 2 | 1426 +---------+--------+-----------+----------+--------+ 1428 INIT[Initiate-Tag = 1234] 1429 101.0.0.1:1 ----------------> 100.0.0.1:2 1430 Ext-VTag = 0 1432 NAT B processes INIT, but cannot find an entry. The SCTP packet is 1433 silently discarded and leaves the NAT table of NAT B unchanged. 1435 +---------+--------+-----------+----------+--------+ 1436 NAT B | Int | Int | Priv | Ext | Ext | 1437 | VTag | Port | Addr | VTag | Port | 1438 +---------+--------+-----------+----------+--------+ 1440 Now Host B sends INIT, which is processed by NAT B. Its parameters 1441 are used to create an entry. 1443 Internal | External External | Internal 1444 | | 1445 | /--\/---\ | 1446 +--------+ +-------+ / \ +-------+ +--------+ 1447 | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | 1448 +--------+ +-------+ \ / +-------+ +--------+ 1449 | \--/\---/ | 1451 INIT[Initiate-Tag = 5678] 1452 101.0.0.1:1 <-- 10.1.0.1:2 1453 Ext-VTag = 0 1455 +---------+--------+-----------+----------+--------+ 1456 NAT B | Int | Int | Priv | Ext | Ext | 1457 | VTag | Port | Addr | VTag | Port | 1458 +---------+--------+-----------+----------+--------+ 1459 | 5678 | 2 | 10.1.0.1 | 0 | 1 | 1460 +---------+--------+-----------+----------+--------+ 1462 INIT[Initiate-Tag = 5678] 1463 101.0.0.1:1 <--------------- 100.0.0.1:2 1464 Ext-VTag = 0 1466 NAT A processes INIT. As the outgoing INIT of Host A has already 1467 created an entry, the entry is found and updated: 1469 Internal | External External | Internal 1470 | | 1471 | /--\/---\ | 1472 +--------+ +-------+ / \ +-------+ +--------+ 1473 | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | 1474 +--------+ +-------+ \ / +-------+ +--------+ 1475 | \--/\---/ | 1477 VTag != Int-VTag, but Ext-VTag == 0, find entry. 1478 +---------+--------+-----------+----------+--------+ 1479 NAT A | Int | Int | Priv | Ext | Ext | 1480 | VTag | Port | Addr | VTag | Port | 1481 +---------+--------+-----------+----------+--------+ 1482 | 1234 | 1 | 10.0.0.1 | 5678 | 2 | 1483 +---------+--------+-----------+----------+--------+ 1485 INIT[Initiate-tag = 5678] 1486 10.0.0.1:1 <-- 100.0.0.1:2 1487 Ext-VTag = 0 1489 Host A send INIT-ACK, which can pass through NAT B: 1491 Internal | External External | Internal 1492 | | 1493 | /--\/---\ | 1494 +--------+ +-------+ / \ +-------+ +--------+ 1495 | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | 1496 +--------+ +-------+ \ / +-------+ +--------+ 1497 | \--/\---/ | 1499 INIT-ACK[Initiate-Tag = 1234] 1500 10.0.0.1:1 -->; 100.0.0.1:2 1501 Ext-VTag = 5678 1503 INIT-ACK[Initiate-Tag = 1234] 1504 101.0.0.1:1 ----------------> 100.0.0.1:2 1505 Ext-VTag = 5678 1507 NAT B updates entry: 1509 +---------+--------+-----------+----------+--------+ 1510 NAT B | Int | Int | Priv | Ext | Ext | 1511 | VTag | Port | Addr | VTag | Port | 1512 +---------+--------+-----------+----------+--------+ 1513 | 5678 | 2 | 10.1.0.1 | 1234 | 1 | 1514 +---------+--------+-----------+----------+--------+ 1516 INIT-ACK[Initiate-Tag = 1234] 1517 101.0.0.1:1 --> 10.1.0.1:2 1518 Ext-VTag = 5678 1520 The lookup for COOKIE-ECHO and COOKIE-ACK is successful. 1522 Internal | External External | Internal 1523 | | 1524 | /--\/---\ | 1525 +--------+ +-------+ / \ +-------+ +--------+ 1526 | Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B | 1527 +--------+ +-------+ \ / +-------+ +--------+ 1528 | \--/\---/ | 1530 COOKIE-ECHO 1531 101.0.0.1:1 <-- 10.1.0.1:2 1532 Ext-VTag = 1234 1534 COOKIE-ECHO 1535 101.0.0.1:1 <------------- 100.0.0.1:2 1536 Ext-VTag = 1234 1538 COOKIE-ECHO 1539 10.0.0.1:1 <-- 100.0.0.1:2 1540 Ext-VTag = 1234 1542 COOKIE-ACK 1543 10.0.0.1:1 --> 100.0.0.1:2 1544 Ext-VTag = 5678 1546 COOKIE-ACK 1547 101.0.0.1:1 ----------------> 100.0.0.1:2 1548 Ext-VTag = 5678 1550 COOKIE-ACK 1551 101.0.0.1:1 --> 10.1.0.1:2 1552 Ext-VTag = 5678 1554 8. Socket API Considerations 1556 This section describes how the socket API defined in [RFC6458] is 1557 extended to provide a way for the application to control NAT 1558 friendliness. 1560 Please note that this section is informational only. 1562 A socket API implementation based on [RFC6458] is extended by 1563 supporting one new read/write socket option. 1565 8.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) 1567 This socket option uses the option_level IPPROTO_SCTP and the 1568 option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the 1569 NAT friendliness for future associations and retrieve the value for 1570 future and specific ones. 1572 struct sctp_assoc_value { 1573 sctp_assoc_t assoc_id; 1574 uint32_t assoc_value; 1575 }; 1577 assoc_id: This parameter is ignored for one-to-one style sockets. 1578 For one-to-many style sockets the application may fill in an 1579 association identifier or SCTP_FUTURE_ASSOC for this query. It is 1580 an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id. 1582 assoc_value: A non-zero value indicates a NAT-friendly mode. 1584 9. IANA Considerations 1586 [NOTE to RFC-Editor: 1588 "RFCXXXX" is to be replaced by the RFC number you assign this 1589 document. 1591 ] 1593 [NOTE to RFC-Editor: 1595 The suggested values for the chunk type and the chunk parameter 1596 types are tentative and to be confirmed by IANA. 1598 ] 1600 This document (RFCXXXX) is the reference for all registrations 1601 described in this section. The suggested changes are described 1602 below. 1604 9.1. New Chunk Flags for Two Existing Chunk Types 1606 As defined in [RFC6096] two chunk flags have to be assigned by IANA 1607 for the ERROR chunk. The suggested value for the T bit is 0x01 and 1608 for the M bit is 0x02. 1610 This requires an update of the "ERROR Chunk Flags" registry for SCTP: 1612 ERROR Chunk Flags 1614 +------------------+-----------------+-----------+ 1615 | Chunk Flag Value | Chunk Flag Name | Reference | 1616 +------------------+-----------------+-----------+ 1617 | 0x01 | T bit | [RFCXXXX] | 1618 | 0x02 | M bit | [RFCXXXX] | 1619 | 0x04 | Unassigned | | 1620 | 0x08 | Unassigned | | 1621 | 0x10 | Unassigned | | 1622 | 0x20 | Unassigned | | 1623 | 0x40 | Unassigned | | 1624 | 0x80 | Unassigned | | 1625 +------------------+-----------------+-----------+ 1627 As defined in [RFC6096] one chunk flag has to be assigned by IANA for 1628 the ABORT chunk. The suggested value of the M bit is 0x02. 1630 This requires an update of the "ABORT Chunk Flags" registry for SCTP: 1632 ABORT Chunk Flags 1634 +------------------+-----------------+-----------+ 1635 | Chunk Flag Value | Chunk Flag Name | Reference | 1636 +------------------+-----------------+-----------+ 1637 | 0x01 | T bit | [RFC4960] | 1638 | 0x02 | M bit | [RFCXXXX] | 1639 | 0x04 | Unassigned | | 1640 | 0x08 | Unassigned | | 1641 | 0x10 | Unassigned | | 1642 | 0x20 | Unassigned | | 1643 | 0x40 | Unassigned | | 1644 | 0x80 | Unassigned | | 1645 +------------------+-----------------+-----------+ 1647 9.2. Three New Error Causes 1649 Three error causes have to be assigned by IANA. It is suggested to 1650 use the values given below. 1652 This requires three additional lines in the "Error Cause Codes" 1653 registry for SCTP: 1655 Error Cause Codes 1657 +-------+--------------------------------+-----------+ 1658 | Value | Cause Code | Reference | 1659 +-------+--------------------------------+-----------+ 1660 | 176 | VTag and Port Number Collision | [RFCXXXX] | 1661 | 177 | Missing State | [RFCXXXX] | 1662 | 178 | Port Number Collision | [RFCXXXX] | 1663 +-------+--------------------------------+-----------+ 1665 9.3. Two New Chunk Parameter Types 1667 Two chunk parameter types have to be assigned by IANA. It is 1668 suggested to use the values given below. IANA should assign these 1669 values from the pool of parameters with the upper two bits set to 1670 '11'. 1672 This requires two additional lines in the "Chunk Parameter Types" 1673 registry for SCTP: 1675 Chunk Parameter Types 1677 +----------+--------------------------+-----------+ 1678 | ID Value | Chunk Parameter Type | Reference | 1679 +----------+--------------------------+-----------+ 1680 | 49159 | Disable Restart (0xC007) | [RFCXXXX] | 1681 | 49160 | VTags (0xC008) | [RFCXXXX] | 1682 +----------+--------------------------+-----------+ 1684 10. Security Considerations 1686 State maintenance within a NAT is always a subject of possible Denial 1687 Of Service attacks. This document recommends that at a minimum a NAT 1688 runs a timer on any SCTP state so that old association state can be 1689 cleaned up. 1691 For SCTP end-points, this document does not add any additional 1692 security considerations to the ones given in [RFC4960], [RFC4895], 1693 and [RFC5061]. In particular, SCTP is protected by the verification 1694 tags and the usage of [RFC4895] against off-path attackers. 1696 11. Acknowledgments 1698 The authors wish to thank Jason But, Gorry Fairhurst, Bryan Ford, 1699 David Hayes, Alfred Hines, Henning Peters, Timo Voelker, Dan Wing, 1700 and Qiaobing Xie for their invaluable comments. 1702 12. References 1704 12.1. Normative References 1706 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1707 Requirement Levels", BCP 14, RFC 2119, March 1997. 1709 [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, 1710 "Authenticated Chunks for the Stream Control Transmission 1711 Protocol (SCTP)", RFC 4895, August 2007. 1713 [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 1714 4960, September 2007. 1716 [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. 1717 Kozuka, "Stream Control Transmission Protocol (SCTP) 1718 Dynamic Address Reconfiguration", RFC 5061, September 1719 2007. 1721 [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission 1722 Protocol (SCTP) Chunk Flags Registration", RFC 6096, 1723 January 2011. 1725 12.2. Informative References 1727 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 1728 793, September 1981. 1730 [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. 1731 Yasevich, "Sockets API Extensions for the Stream Control 1732 Transmission Protocol (SCTP)", RFC 6458, December 2011. 1734 [RFC6890] Cotton, M., Vegoda, L., Bonica, R., and B. Haberman, 1735 "Special-Purpose IP Address Registries", BCP 153, RFC 1736 6890, April 2013. 1738 [RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream 1739 Control Transmission Protocol (SCTP) Packets for End-Host 1740 to End-Host Communication", RFC 6951, May 2013. 1742 Authors' Addresses 1744 Randall R. Stewart 1745 Netflix, Inc. 1746 Chapin, SC 29036 1747 US 1749 Email: randall@lakerest.net 1750 Michael Tuexen 1751 Muenster University of Applied Sciences 1752 Stegerwaldstrasse 39 1753 48565 Steinfurt 1754 DE 1756 Email: tuexen@fh-muenster.de 1758 Irene Ruengeler 1759 Muenster University of Applied Sciences 1760 Stegerwaldstrasse 39 1761 48565 Steinfurt 1762 DE 1764 Email: i.ruengeler@fh-muenster.de