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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group F. Templin, Ed. 3 Internet-Draft Boeing Research & Technology 4 Intended status: Informational July 4, 2012 5 Expires: January 5, 2013 7 The Subnetwork Encapsulation and Adaptation Layer (SEAL) 8 draft-templin-intarea-seal-46.txt 10 Abstract 12 For the purpose of this document, a subnetwork is defined as a 13 virtual topology configured over a connected IP network routing 14 region and bounded by encapsulating border nodes. These virtual 15 topologies are manifested by tunnels that may span multiple IP and/or 16 sub-IP layer forwarding hops, and can introduce failure modes due to 17 packet duplication, packet reordering, source address spoofing and 18 traversal of links with diverse Maximum Transmission Units (MTUs). 19 This document specifies a Subnetwork Encapsulation and Adaptation 20 Layer (SEAL) that addresses these issues. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on January 5, 2013. 39 Copyright Notice 41 Copyright (c) 2012 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 58 1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6 59 2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7 60 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9 61 4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10 62 4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 10 63 4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 11 64 4.3. SEAL Header and Trailer Format . . . . . . . . . . . . . . 12 65 4.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 14 66 4.4.1. IP Protocol Constants . . . . . . . . . . . . . . . . 14 67 4.4.2. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14 68 4.4.3. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 15 69 4.4.4. Pre-Encapsulation . . . . . . . . . . . . . . . . . . 16 70 4.4.5. SEAL Encapsulation and Segmentation . . . . . . . . . 17 71 4.4.6. Outer Encapsulation . . . . . . . . . . . . . . . . . 18 72 4.4.7. Path Probing and ETE Reachability Verification . . . . 19 73 4.4.8. Processing ICMP Messages . . . . . . . . . . . . . . . 19 74 4.4.9. IPv4 Middlebox Reassembly Testing . . . . . . . . . . 20 75 4.4.10. Stateful MTU Determination . . . . . . . . . . . . . . 22 76 4.4.11. Detecting Path MTU Changes . . . . . . . . . . . . . . 22 77 4.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 22 78 4.5.1. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 22 79 4.5.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 23 80 4.5.3. Decapsulation and Re-Encapsulation . . . . . . . . . . 23 81 4.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 24 82 4.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 25 83 4.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 27 84 5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 29 85 6. End System Requirements . . . . . . . . . . . . . . . . . . . 29 86 7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 29 87 8. Nested Encapsulation Considerations . . . . . . . . . . . . . 30 88 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 89 10. Security Considerations . . . . . . . . . . . . . . . . . . . 31 90 11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 31 91 12. Implementation Status . . . . . . . . . . . . . . . . . . . . 32 92 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32 93 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32 94 14.1. Normative References . . . . . . . . . . . . . . . . . . . 32 95 14.2. Informative References . . . . . . . . . . . . . . . . . . 33 96 Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37 97 Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 37 98 Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 38 99 Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 38 100 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39 102 1. Introduction 104 As Internet technology and communication has grown and matured, many 105 techniques have developed that use virtual topologies (including 106 tunnels of one form or another) over an actual network that supports 107 the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual 108 topologies have elements that appear as one hop in the virtual 109 topology, but are actually multiple IP or sub-IP layer hops. These 110 multiple hops often have quite diverse properties that are often not 111 even visible to the endpoints of the virtual hop. This introduces 112 failure modes that are not dealt with well in current approaches. 114 The use of IP encapsulation (also known as "tunneling") has long been 115 considered as the means for creating such virtual topologies. 116 However, the encapsulation headers often include insufficiently 117 provisioned per-packet identification values. This can present 118 issues for duplicate packet detection and detection of packet 119 reordering within the subnetwork. IP encapsulation also allows an 120 attacker to produce encapsulated packets with spoofed source 121 addresses even if the source address in the encapsulating header 122 cannot be spoofed. A denial-of-service vector that is not possible 123 in non-tunneled subnetworks is therefore presented. 125 Additionally, the insertion of an outer IP header reduces the 126 effective path MTU visible to the inner network layer. When IPv6 is 127 used as the encapsulation protocol, original sources expect to be 128 informed of the MTU limitation through IPv6 Path MTU discovery 129 (PMTUD) [RFC1981]. When IPv4 is used, this reduced MTU can be 130 accommodated through the use of IPv4 fragmentation, but unmitigated 131 in-the-network fragmentation has been found to be harmful through 132 operational experience and studies conducted over the course of many 133 years [FRAG][FOLK][RFC4963]. Additionally, classical IPv4 PMTUD 134 [RFC1191] has known operational issues that are exacerbated by in- 135 the-network tunnels [RFC2923][RFC4459]. 137 The following subsections present further details on the motivation 138 and approach for addressing these issues. 140 1.1. Motivation 142 Before discussing the approach, it is necessary to first understand 143 the problems. In both the Internet and private-use networks today, 144 IP is ubiquitously deployed as the Layer 3 protocol. The primary 145 functions of IP are to provide for routing, addressing, and a 146 fragmentation and reassembly capability used to accommodate links 147 with diverse MTUs. While it is well known that the IP address space 148 is rapidly becoming depleted, there is a lesser-known but growing 149 consensus that other IP protocol limitations have already or may soon 150 become problematic. 152 First, the Internet historically provided no means for discerning 153 whether the source addresses of IP packets are authentic. This 154 shortcoming is being addressed more and more through the deployment 155 of site border router ingress filters [RFC2827], however the use of 156 encapsulation provides a vector for an attacker to circumvent 157 filtering for the encapsulated packet even if filtering is correctly 158 applied to the encapsulation header. Secondly, the IP header does 159 not include a well-behaved identification value unless the source has 160 included a fragment header for IPv6 or unless the source permits 161 fragmentation for IPv4. These limitations preclude an efficient 162 means for routers to detect duplicate packets and packets that have 163 been re-ordered within the subnetwork. 165 For IPv4 encapsulation, when fragmentation is permitted the header 166 includes a 16-bit Identification field, meaning that at most 2^16 167 unique packets with the same (source, destination, protocol)-tuple 168 can be active in the Internet at the same time 169 [I-D.ietf-intarea-ipv4-id-update]. (When middleboxes such as Network 170 Address Translators (NATs) re-write the Identification field to 171 random values, the number of unique packets is even further reduced.) 172 Due to the escalating deployment of high-speed links, however, these 173 numbers have become too small by several orders of magnitude for high 174 data rate packet sources such as tunnel endpoints [RFC4963]. 176 Furthermore, there are many well-known limitations pertaining to IPv4 177 fragmentation and reassembly - even to the point that it has been 178 deemed "harmful" in both classic and modern-day studies (see above). 179 In particular, IPv4 fragmentation raises issues ranging from minor 180 annoyances (e.g., in-the-network router fragmentation [RFC1981]) to 181 the potential for major integrity issues (e.g., mis-association of 182 the fragments of multiple IP packets during reassembly [RFC4963]). 184 As a result of these perceived limitations, a fragmentation-avoiding 185 technique for discovering the MTU of the forward path from a source 186 to a destination node was devised through the deliberations of the 187 Path MTU Discovery Working Group (PMTUDWG) during the late 1980's 188 through early 1990's (see Appendix D). In this method, the source 189 node provides explicit instructions to routers in the path to discard 190 the packet and return an ICMP error message if an MTU restriction is 191 encountered. However, this approach has several serious shortcomings 192 that lead to an overall "brittleness" [RFC2923]. 194 In particular, site border routers in the Internet have been known to 195 discard ICMP error messages coming from the outside world. This is 196 due in large part to the fact that malicious spoofing of error 197 messages in the Internet is trivial since there is no way to 198 authenticate the source of the messages [RFC5927]. Furthermore, when 199 a source node that requires ICMP error message feedback when a packet 200 is dropped due to an MTU restriction does not receive the messages, a 201 path MTU-related black hole occurs. This means that the source will 202 continue to send packets that are too large and never receive an 203 indication from the network that they are being discarded. This 204 behavior has been confirmed through documented studies showing clear 205 evidence of PMTUD failures for both IPv4 and IPv6 in the Internet 206 today [TBIT][WAND][SIGCOMM]. 208 The issues with both IP fragmentation and this "classical" PMTUD 209 method are exacerbated further when IP tunneling is used [RFC4459]. 210 For example, an ingress tunnel endpoint (ITE) may be required to 211 forward encapsulated packets into the subnetwork on behalf of 212 hundreds, thousands, or even more original sources. If the ITE 213 allows IP fragmentation on the encapsulated packets, persistent 214 fragmentation could lead to undetected data corruption due to 215 Identification field wrapping and/or reassembly congestion at the 216 ETE. If the ITE instead uses classical IP PMTUD it must rely on ICMP 217 error messages coming from the subnetwork that may be suspect, 218 subject to loss due to filtering middleboxes, or insufficiently 219 provisioned for translation into error messages to be returned to the 220 original sources. 222 Although recent works have led to the development of a robust end-to- 223 end MTU determination scheme [RFC4821], they do not excuse tunnels 224 from accounting for the encapsulation overhead they add to packets. 225 Moreover, in current practice existing tunneling protocols mask the 226 MTU issues by selecting a "lowest common denominator" MTU that may be 227 much smaller than necessary for most paths and difficult to change at 228 a later date. Therefore, a new approach to accommodate tunnels over 229 links with diverse MTUs is necessary. 231 1.2. Approach 233 For the purpose of this document, a subnetwork is defined as a 234 virtual topology configured over a connected network routing region 235 and bounded by encapsulating border nodes. Example connected network 236 routing regions include Mobile Ad hoc Networks (MANETs), enterprise 237 networks and the global public Internet itself. Subnetwork border 238 nodes forward unicast and multicast packets over the virtual topology 239 across multiple IP and/or sub-IP layer forwarding hops that may 240 introduce packet duplication and/or traverse links with diverse 241 Maximum Transmission Units (MTUs). 243 This document introduces a Subnetwork Encapsulation and Adaptation 244 Layer (SEAL) for tunneling inner network layer protocol packets over 245 IP subnetworks that connect Ingress and Egress Tunnel Endpoints 246 (ITEs/ETEs) of border nodes. It provides a modular specification 247 designed to be tailored to specific associated tunneling protocols. 248 A transport-mode of operation is also possible, and described in 249 Appendix C. 251 SEAL provides a mid-layer encapsulation that accommodates links with 252 diverse MTUs, and allows routers in the subnetwork to perform 253 efficient duplicate packet and packet reordering detection. The 254 encapsulation further ensures data origin authentication, packet 255 header integrity and anti-replay in environments in which these 256 functions are necessary. 258 SEAL treats tunnels that traverse the subnetwork as ordinary links 259 that must support network layer services. Moreover, SEAL provides 260 dynamic mechanisms to ensure a maximal path MTU over the tunnel. 261 This is in contrast to static approaches which avoid MTU issues by 262 selecting a lowest common denominator MTU value that may be overly 263 conservative for the vast majority of tunnel paths and difficult to 264 change even when larger MTUs become available. 266 The following sections provide the SEAL normative specifications, 267 while the appendices present non-normative additional considerations. 269 2. Terminology and Requirements 271 The following terms are defined within the scope of this document: 273 subnetwork 274 a virtual topology configured over a connected network routing 275 region and bounded by encapsulating border nodes. 277 IP 278 used to generically refer to either Internet Protocol (IP) 279 version, i.e., IPv4 or IPv6. 281 Ingress Tunnel Endpoint (ITE) 282 a virtual interface over which an encapsulating border node (host 283 or router) sends encapsulated packets into the subnetwork. 285 Egress Tunnel Endpoint (ETE) 286 a virtual interface over which an encapsulating border node (host 287 or router) receives encapsulated packets from the subnetwork. 289 ETE Link Path 290 a subnetwork path from an ITE to an ETE beginning with an 291 underlying link of the ITE as the first hop. Note that, if the 292 ITE's interface connection to the underlying link assigns multiple 293 IP addresses, each address represents a separate ETE link path. 295 inner packet 296 an unencapsulated network layer protocol packet (e.g., IPv4 297 [RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any 298 outer encapsulations are added. Internet protocol numbers that 299 identify inner packets are found in the IANA Internet Protocol 300 registry [RFC3232]. SEAL protocol packets that incur an 301 additional layer of SEAL encapsulation are also considered inner 302 packets. 304 outer IP packet 305 a packet resulting from adding an outer IP header (and possibly 306 other outer headers) to a SEAL-encapsulated inner packet. 308 packet-in-error 309 the leading portion of an invoking data packet encapsulated in the 310 body of an error control message (e.g., an ICMPv4 [RFC0792] error 311 message, an ICMPv6 [RFC4443] error message, etc.). 313 Packet Too Big (PTB) message 314 a control plane message indicating an MTU restriction (e.g., an 315 ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4 316 "Fragmentation Needed" message [RFC0792], etc.). 318 Don't Fragment (DF) bit 319 a bit that indicates whether the packet may be fragmented by the 320 network. The DF bit is explicitly included in the IPv4 header 321 [RFC0791] and may be set to '0' to allow fragmentation or '1' to 322 disallow fragmentation. The bit is absent from the IPv6 header 323 [RFC2460], but implicitly set to '1'. 325 The following abbreviations correspond to terms used within this 326 document and/or elsewhere in common Internetworking nomenclature: 328 ETE - Egress Tunnel Endpoint 330 HLEN - the length of the SEAL header plus outer headers 332 ICV - Integrity Check Vector 334 ITE - Ingress Tunnel Endpoint 336 MTU - Maximum Transmission Unit 338 SCMP - the SEAL Control Message Protocol 339 SDU - SCMP Destination Unreachable message 341 SPP - SCMP Parameter Problem message 343 SPTB - SCMP Packet Too Big message 345 SEAL - Subnetwork Encapsulation and Adaptation Layer 347 TE - Tunnel Endpoint (i.e., either ingress or egress) 349 VET - Virtual Enterprise Traversal 351 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 352 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 353 document are to be interpreted as described in [RFC2119]. When used 354 in lower case (e.g., must, must not, etc.), these words MUST NOT be 355 interpreted as described in [RFC2119], but are rather interpreted as 356 they would be in common English. 358 3. Applicability Statement 360 SEAL was originally motivated by the specific case of subnetwork 361 abstraction for Mobile Ad hoc Networks (MANETs), however the domain 362 of applicability also extends to subnetwork abstractions over 363 enterprise networks, ISP networks, SOHO networks, the global public 364 Internet itself, and any other connected network routing region. 365 SEAL, along with the Virtual Enterprise Traversal (VET) 366 [I-D.templin-intarea-vet] tunnel virtual interface abstraction, are 367 the functional building blocks for the Internet Routing Overlay 368 Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in 369 Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139] 370 architectures. 372 SEAL provides a network sublayer for encapsulation of an inner 373 network layer packet within outer encapsulating headers. SEAL can 374 also be used as a sublayer within a transport layer protocol data 375 payload, where transport layer encapsulation is typically used for 376 Network Address Translator (NAT) traversal as well as operation over 377 subnetworks that give preferential treatment to certain "core" 378 Internet protocols (e.g., TCP, UDP, etc.). The SEAL header is 379 processed the same as for IPv6 extension headers, i.e., it is not 380 part of the outer IP header but rather allows for the creation of an 381 arbitrarily extensible chain of headers in the same way that IPv6 382 does. 384 To accommodate MTU diversity, the Ingress Tunnel Endpoint (ITE) may 385 need to perform a small amount of fragmentation which the Egress 386 Tunnel Endpoint (ETE) must reassemble. The ETE further acts as a 387 passive observer that informs the ITE of any packet size limitations. 388 This allows the ITE to return appropriate PMTUD feedback even if the 389 network path between the ITE and ETE filters ICMP messages. 391 SEAL further provides mechanisms to ensure data origin 392 authentication, packet header integrity, and anti-replay. The SEAL 393 framework is therefore similar to the IP Security (IPsec) 394 Authentication Header (AH) [RFC4301][RFC4302], however it provides 395 only minimal hop-by-hop authenticating services while leaving full 396 data integrity, authentication and confidentiality services as an 397 end-to-end consideration. While SEAL performs data origin 398 authentication, the origin site must also perform the necessary 399 ingress filtering in order to provide full source address 400 verification [I-D.ietf-savi-framework]. 402 In many aspects, SEAL also very closely resembles the Generic Routing 403 Encapsulation (GRE) framework [RFC1701]. SEAL can therefore be 404 applied in the same use cases that are traditionally addressed by 405 GRE, and can also provide additional capabilities as described in 406 this document. 408 4. SEAL Specification 410 The following sections specify the operation of SEAL: 412 4.1. VET Interface Model 414 SEAL is an encapsulation sublayer used within VET non-broadcast, 415 multiple access (NBMA) tunnel virtual interfaces. Each VET interface 416 is configured over one or more underlying interfaces attached to 417 subnetwork links. The VET interface connects an ITE to one or more 418 ETE "neighbors" via tunneling across an underlying subnetwork, where 419 the tunnel neighbor relationship may be either unidirectional or 420 bidirectional. 422 A unidirectional tunnel neighbor relationship allows the near end ITE 423 to send data packets forward to the far end ETE, while the ETE only 424 returns control messages when necessary. A bidirectional tunnel 425 neighbor relationship is one over which both TEs can exchange both 426 data and control messages. 428 Implications of the VET unidirectional and bidirectional models are 429 discussed in [I-D.templin-intarea-vet]. 431 4.2. SEAL Model of Operation 433 SEAL-enabled ITEs encapsulate each inner packet in a SEAL header and 434 any outer header encapsulations as shown in Figure 1: 436 +--------------------+ 437 ~ outer IP header ~ 438 +--------------------+ 439 ~ other outer hdrs ~ 440 +--------------------+ 441 ~ SEAL Header ~ 442 +--------------------+ +--------------------+ 443 | | --> | | 444 ~ Inner ~ --> ~ Inner ~ 445 ~ Packet ~ --> ~ Packet ~ 446 | | --> | | 447 +--------------------+ +----------+---------+ 449 Figure 1: SEAL Encapsulation 451 The ITE inserts the SEAL header according to the specific tunneling 452 protocol. For simple encapsulation of an inner network layer packet 453 within an outer IP header, the ITE inserts the SEAL header following 454 the outer IP header and before the inner packet as: IP/SEAL/{inner 455 packet}. 457 For encapsulations over transports such as UDP, the ITE inserts the 458 SEAL header following the outer transport layer header and before the 459 inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the 460 UDP header is seen as an "other outer header" as depicted in 461 Figure 1. 463 SEAL supports both "nested" tunneling and "re-encapsulating" 464 tunneling. Nested tunneling occurs when a first tunnel is 465 encapsulated within a second tunnel, which may then further be 466 encapsulated within additional tunnels. Nested tunneling can be 467 useful, and stands in contrast to "recursive" tunneling which is an 468 anomalous condition incurred due to misconfiguration or a routing 469 loop. Considerations for nested tunneling are discussed in Section 4 470 of [RFC2473]. 472 Re-encapsulating tunneling occurs when a packet arrives at a first 473 ETE, which then acts as an ITE to re-encapsulate and forward the 474 packet to a second ETE connected to the same subnetwork. In that 475 case each ITE/ETE transition represents a segment of a bridged path 476 between the ITE nearest the source and the ETE nearest the 477 destination. Combinations of nested and re-encapsulating tunneling 478 are also naturally supported by SEAL. 480 The SEAL ITE considers each underlying interface as the ingress 481 attachment point to a subnetwork link path to the ETE. The ITE 482 therefore may experience different path MTUs on different ETE link 483 paths. 485 Finally, the SEAL ITE ensures that the inner network layer protocol 486 will see a minimum MTU of 1500 bytes over each ETE link path 487 regardless of the outer network layer protocol version, i.e., even if 488 a small amount of fragmentation and reassembly are necessary. This 489 is necessary to avoid path MTU "black holes" for the minimum MTU 490 configured by the vast majority of links in the Internet. 492 4.3. SEAL Header and Trailer Format 494 The SEAL header is formatted as follows: 496 0 1 2 3 497 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 1 498 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 499 |VER|C|A|R|L|I|V|X|M| Offset | NEXTHDR | LINK_ID |LEVEL| 500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 501 | Identification (optional) | 502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 | Integrity Check Vector (ICV) (optional) | 504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 506 Figure 2: SEAL Header Format 508 VER (2) 509 a 2-bit version field. This document specifies Version 0 of the 510 SEAL protocol, i.e., the VER field encodes the value 0. 512 C (1) 513 the "Control/Data" bit. Set to 1 by the ITE in SEAL Control 514 Message Protocol (SCMP) control messages, and set to 0 in ordinary 515 data packets. 517 A (1) 518 the "Acknowledgement Requested" bit. Set to 1 by the ITE in SEAL 519 data packets for which it wishes to receive an explicit 520 acknowledgement from the ETE. 522 R (1) 523 the "Redirects Permitted" bit. For data packets, set to 1 by the 524 ITE to inform the ETE that the source is accepting Redirects (see: 525 [I-D.templin-intarea-vet]). 527 L (1) 528 the "Rate Limit" bit for IPv4 ETE link paths. Reserved for future 529 use for IPv6 ETE link paths. 531 I (1) 532 the "Identification Included" bit. 534 V (1) 535 the "ICV included" bit. 537 X (1) a 1-bit reserved field. 539 M (1) the "More Segments" bit. Set to 1 in a non-final segment and 540 set to 0 in the final segment of the SEAL packet. 542 Offset (6) a 6-bit Offset field. Set to 0 in the first segment of a 543 segmented SEAL packet. Set to an integral number of 32 byte 544 blocks in subsequent segments (e.g., an Offset of 10 indicates a 545 block that begins at the 320th byte in the packet). 547 NEXTHDR (8) an 8-bit field that encodes the next header Internet 548 Protocol number the same as for the IPv4 protocol and IPv6 next 549 header fields. 551 LINK_ID (5) 552 a 5-bit link identification value, set to a unique value by the 553 ITE for each link path over which it will send encapsulated 554 packets to the ETE (up to 32 link paths per ETE are therefore 555 supported). Note that, if the ITE's interface connection to the 556 underlying link assigns multiple IP addresses, each address 557 represents a separate ETE link path that must be assigned a 558 separate LINK_ID. 560 LEVEL (3) 561 a 3-bit nesting level; use to limit the number of tunnel nesting 562 levels. Set to an integer value up to 7 in the innermost SEAL 563 encapsulation, and decremented by 1 for each successive additional 564 SEAL encapsulation nesting level. Up to 8 levels of nesting are 565 therefore supported. 567 Identification (32) 568 an optional 32-bit per-packet identification field; present when 569 I==1. Set to a monotonically-incrementing 32-bit value for each 570 SEAL packet transmitted to this ETE, beginning with 0. 572 Integrity Check Vector (ICV) (32) 573 an optional 32-bit header integrity check value; present when 574 V==1. Covers the leading 128 bytes of the packet beginning with 575 the SEAL header. The value 128 is chosen so that at least the 576 SEAL header as well as the inner packet network and transport 577 layer headers are covered by the integrity check. 579 4.4. ITE Specification 581 4.4.1. IP Protocol Constants 583 SEAL observes the IP protocol constants for minimum link MTU (MINMTU) 584 and minimum reassembly buffer (MINMRU) sizes. 586 For IPv6, the ITE sets MINMTU=1280 and MINMRU=1500 (see: [RFC2460]). 588 For IPv4, the ITE sets both MINMTU=576 and MINMRU=576 (see: 589 [RFC1122]) i.e., even though the true MINMTU for IPv4 is only 68 590 bytes (see: [RFC0791]). 592 4.4.2. Tunnel Interface MTU 594 The tunnel interface must present a constant MTU value to the inner 595 network layer as the size for admission of inner packets into the 596 interface. Since VET NBMA tunnel virtual interfaces may support a 597 large set of ETE link paths that accept widely varying maximum packet 598 sizes, however, a number of factors should be taken into 599 consideration when selecting a tunnel interface MTU. 601 Due to the ubiquitous deployment of standard Ethernet and similar 602 networking gear, the nominal Internet cell size has become 1500 603 bytes; this is the de facto size that end systems have come to expect 604 will either be delivered by the network without loss due to an MTU 605 restriction on the path or a suitable ICMP Packet Too Big (PTB) 606 message returned. When large packets sent by end systems incur 607 additional encapsulation at an ITE, however, they may be dropped 608 silently within the tunnel since the network may not always deliver 609 the necessary PTBs [RFC2923]. The ITE should therefore set a tunnel 610 interface MTU of at least 1500 bytes. 612 The inner network layer protocol consults the tunnel interface MTU 613 when admitting a packet into the interface. For non-SEAL inner IPv4 614 packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet 615 is larger than the tunnel interface MTU the inner IPv4 layer uses 616 IPv4 fragmentation to break the packet into fragments no larger than 617 the tunnel interface MTU. The ITE then admits each fragment into the 618 interface as an independent packet. 620 For all other inner packets, the inner network layer admits the 621 packet if it is no larger than the tunnel interface MTU; otherwise, 622 it drops the packet and sends a PTB error message to the source with 623 the MTU value set to the tunnel interface MTU. The message contains 624 as much of the invoking packet as possible without the entire message 625 exceeding the network layer MINMTU size. 627 The ITE can alternatively set an indefinite MTU on the tunnel 628 interface such that all inner packets are admitted into the interface 629 regardless of their size. For ITEs that host applications that use 630 the tunnel interface directly, this option must be carefully 631 coordinated with protocol stack upper layers since some upper layer 632 protocols (e.g., TCP) derive their packet sizing parameters from the 633 MTU of the outgoing interface and as such may select too large an 634 initial size. This is not a problem for upper layers that use 635 conservative initial maximum segment size estimates and/or when the 636 tunnel interface can reduce the upper layer's maximum segment size, 637 e.g., by reducing the size advertised in the MSS option of outgoing 638 TCP messages (sometimes known as "MSS clamping"). 640 In light of the above considerations, the ITE should configure an 641 indefinite MTU on tunnel *router* interfaces so that subnetwork 642 adaptation is handled from within the interface. The ITE can instead 643 set a smaller MTU on tunnel *host* interfaces (e.g., the smallest MTU 644 among all of the underlying links minus the size of the encapsulation 645 headers) but should not set an MTU smaller than 1500 bytes. 647 4.4.3. Tunnel Neighbor Soft State 649 The tunnel virtual interface maintains a number of soft state 650 variables for each ETE and for each ETE link path. 652 When per-packet identification is required, the ITE maintains a per 653 ETE window of Identification values for the packets it has recently 654 sent to this ETE. The ITE then sets a variable "USE_ID" to TRUE, and 655 includes an Identification in each packet it sends to this ETE; 656 otherwise, it sets USE_ID to FALSE. 658 When data origin authentication and integrity checking is required, 659 the ITE also maintains a per ETE integrity check vector (ICV) 660 calculation algorithm and a symmetric secret key to calculate the ICV 661 in each packet it will send to this ETE. The ITE then sets a 662 variable "USE_ICV" to TRUE, and includes an ICV in each packet it 663 sends to this ETE; otherwise, it sets USE_ICV to FALSE. 665 For IPv4 ETE link paths, the ITE further maintains a variable 666 "RATE_LIMIT" initialized to FALSE. If the link path subsequently 667 exhibits unavoidable IPv4 fragmentation the ETE sets RATE_LIMIT to 668 TRUE. 670 For each ETE link path, the ITE must also account for encapsulation 671 header lengths. The ITE therefore maintains the per ETE link path 672 constant values "SHLEN" set to the length of the SEAL header, "THLEN" 673 set to the length of the outer encapsulating transport layer headers 674 (or 0 if outer transport layer encapsulation is not used), "IHLEN" 675 set to the length of the outer IP layer header, and "HLEN" set to 676 (SHLEN+THLEN+IHLEN). (The ITE must include the length of the 677 uncompressed headers even if header compression is enabled when 678 calculating these lengths.) In addition, the ITE maintains a per ETE 679 link path variable "PATH_MTU" initialized to the maximum of 1500 680 bytes and the MTU of the underlying link minus HLEN. (Thereafter, 681 the ITE must not reduce PATH_MTU to a value smaller than 1500 bytes.) 683 The ITE may instead maintain the packet sizing variables and 684 constants as per ETE (rather than per ETE link path) values. In that 685 case, the values reflect the lowest-common-denominator size across 686 all of the ETE's link paths. 688 4.4.4. Pre-Encapsulation 690 For each inner packet admitted into the tunnel interface, if the 691 packet is itself a SEAL packet (i.e., one with the port number for 692 SEAL in the transport layer header or one with the protocol number 693 for SEAL in the IP layer header) and the LEVEL field of the SEAL 694 header contains the value 0, the ITE silently discards the packet. 696 Otherwise, for non-atomic inner packets (i.e., a non-SEAL IPv4 packet 697 with DF==0 in the IP header), if the packet is larger than (MINMTU- 698 HLEN) bytes the ITE fragments the packet into N roughly equal-length 699 pieces, where N is minimized and each fragment is significantly 700 smaller than (MINMTU-HLEN) to allow for additional encapsulations in 701 the path. The ITE then submits each inner fragment for SEAL 702 encapsulation as specified in Section 4.4.5. 704 For atomic inner packets, if the packet is larger than (MINMTU-HLEN) 705 but no larger than 1500 bytes the ITE must ensure that it will 706 traverse the tunnel. For SEAL packets, the ITE first sends an SCMP 707 PTB (SPTB) message toward the previous hop SEAL ITE (see: Section 708 4.6.1.1) with the MTU field set to (MINMTU-HLEN) subject to rate 709 limiting. For non-SEAL IPv6 packets, the ITE sends an ICMPv6 PTB 710 message with the MTU field set to (MINMTU-HLEN) toward the original 711 source subject to rate limiting. (For IPv4 SEAL packets with DF==0, 712 the ITE should set DF=1 and re-calculate the IPv4 header checksum 713 before generating the PTB message in order to avoid bogon filters.) 714 After sending the (S)PTB message, the ITE submits the packet for SEAL 715 segmentation as specifed in Section 4.4.5. 717 For atomic inner packets larger than 1500 bytes, if the packet is no 718 larger than PATH_MTU for the corresponding ETE link path, the ITE 719 submits it for SEAL encapsulation. Otherwise, the ITE drops the 720 packet and sends a PTB error message toward the source address of the 721 inner packet. For SEAL packets, the ITE sends an SPTB message to the 722 previous hop (see: Section 4.6.1.1) with the MTU field set to 723 PATH_MTU. Otherwise, the ITE sends an ordinary PTB message 724 appropriate to the inner protocol version with the MTU field set to 725 PATH_MTU. (For IPv4 SEAL packets with DF==0, the ITE should set DF=1 726 and re-calculate the IPv4 header checksum before generating the PTB 727 message in order to avoid bogon filters.) After sending the (S)PTB 728 message, the ITE discards the inner packet. 730 4.4.5. SEAL Encapsulation and Segmentation 732 For each inner packet/fragment submitted for SEAL encapsulation, the 733 ITE next encapsulates the packet in a SEAL header formatted as 734 specified in Section 4.3. The SEAL header includes an Identification 735 field when USE_ID is TRUE, followed by an ICV field when USE_ICV is 736 TRUE. 738 The ITE next sets C=0 in the SEAL header. The ITE also sets A=1 if 739 necessary for ETE reachability determination (see: Section 4.4.6) or 740 for stateful MTU determination (see Section 4.4.9). Otherwise, the 741 ITE sets A=0. Next, when RATE_LIMIT is TRUE the ITE sets L=1; 742 otherwise, it sets L=0. The ITE also sets X=0. 744 The ITE then sets R=1 if redirects are permitted (see: 745 [I-D.templin-intarea-vet]). (Note that if this process is entered 746 via re-encapsulation (see: Section 4.5.4), R is instead copied from 747 the SEAL header of the re-encapsulated packet. This implies that the 748 R value is propagated across a re-encapsulating chain of ITE/ETEs.) 750 The ITE then sets LINK_ID to the value assigned to the underlying ETE 751 link path, and sets NEXTHDR to the protocol number corresponding to 752 the address family of the encapsulated inner packet. For example, 753 the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets 754 [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213], 755 '80' for encapsulated OSI/CLNP packets [RFC1070], etc. 757 Next, if the inner packet is not itself a SEAL packet the ITE sets 758 LEVEL to an integer value between 0 and 7 as a specification of the 759 number of additional layers of nested SEAL encapsulations permitted. 760 If the inner packet is a SEAL packet that is undergoing nested 761 encapsulation, the ITE instead sets LEVEL to the value that appears 762 in the inner packet's SEAL header minus 1. If the inner packet is 763 undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL 764 value from the SEAL header of the packet to be re-encapsulated. 766 Next, if the inner packet is no larger than (MINMTU-HLEN) or larger 767 than 1500, the ITE sets (M=0; Offset=0). Otherwise, the ITE breaks 768 the inner packet into a minimum number of non-overlapping segments 769 that are no larger than (MINMTU-HLEN) bytes then appends a clone of 770 the SEAL header from the first segment onto the head of each 771 additional segment. The ITE then sets (M=1; Offset=0) in the first 772 segment, sets (M=0/1; Offset=i) in the second segment, sets (M=0/1; 773 Offset=j) in the third segment (if needed), etc., then finally sets 774 (M=0; Offset=k) in the final segment (where i, j, k, etc. are the 775 number of 32 byte blocks that preceded this segment). 777 When USE_ID is FALSE, the ITE next sets I=0. Otherwise, the ITE sets 778 I=1 and writes a monotonically-increasing integer value for this ETE 779 in the Identification field beginning with 0 in the first packet 780 transmitted. (For SEAL packets that have been split into multiple 781 pieces, the ITE writes the same Identification value in each piece.) 783 When USE_ICV is FALSE, the ITE next sets V=0. Otherwise, the ITE 784 sets V=1 and calculates the packet header ICV value using an 785 algorithm agreed on by the ITE and ETE. When data origin 786 authentication is required, the algorithm uses a symmetric secret key 787 so that the ETE can verify that the ICV was generated by the ITE. 788 Beginning with the SEAL header, the ITE calculates the ICV over the 789 leading 128 bytes of the packet (or up to the end of the packet if 790 there are fewer than 128 bytes) and places result in the ICV field. 791 (For SEAL packets that have been split into two pieces, each piece 792 calculates its own ICV value.) 794 The ITE then adds the outer encapsulating headers as specified in 795 Section 4.4.6. 797 4.4.6. Outer Encapsulation 799 Following SEAL encapsulation, the ITE next encapsulates each segment 800 in the requisite outer transport (when necessary) and IP layer 801 headers. When a transport layer header is included, the ITE writes 802 the port number for SEAL in the transport destination service port 803 field and writes the protocol number of the transport protocol in the 804 outer IP header protocol field. Otherwise, the ITE writes the 805 protocol number for SEAL in the outer IP header protocol field. 807 The ITE then sets the other fields of the outer transport and IP 808 layer headers as specified in Sections 5.5.4 and 5.5.5 809 of[I-D.templin-intarea-vet]. If this process is entered via re- 810 encapsulation (see: Section 4.5.4), the ITE instead follows the re- 811 encapsulation procedures specified in Section 5.5.6 of 812 [I-D.templin-intarea-vet]. 814 For IPv4 ETE link paths, the ITE sets DF=0 in the IPv4 header to 815 allow the packet to be fragmented if it encounters a restricting 816 link. (For IPv6 link paths, the DF bit is implicitly set to 1.) 818 The ITE then sends each outer packet via the underlying link 819 corresponding to LINK_ID. For IPv4 ETE link paths with 820 RATE_LIMIT=TRUE, the ITE sends the packet subject to rate limiting so 821 that the IPv4 Identification value is not repeated within the IPv4 822 Maximum Segment Lifetime (i.e., 120 seconds) [RFC1122]. 824 4.4.7. Path Probing and ETE Reachability Verification 826 All SEAL data packets sent by the ITE are considered implicit probes. 827 SEAL data packets will elicit an SCMP message from the ETE if it 828 needs to acknowledge a probe and/or report an error condition. SEAL 829 data packets may also be dropped by either the ETE or a router on the 830 path, which will return an ICMP message. 832 The ITE can also send an SCMP Router/Neighbor Solicitation message to 833 elicit an SCMP Router/Neighbor Advertisement response (see: 834 [I-D.templin-intarea-vet]) as verification that the ETE is still 835 reachable via a specific link path. 837 The ITE processes ICMP messages as specified in Section 4.4.7. 839 The ITE processes SCMP messages as specified in Section 4.6.2. 841 4.4.8. Processing ICMP Messages 843 When the ITE sends SEAL packets, it may receive ICMP error messages 844 [RFC0792][RFC4443] from an ordinary router within the subnetwork. 845 Each ICMP message includes an outer IP header, followed by an ICMP 846 header, followed by a portion of the SEAL data packet that generated 847 the error (also known as the "packet-in-error") beginning with the 848 outer IP header. 850 The ITE should process ICMPv4 Protocol Unreachable messages and 851 ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header 852 type encountered" as a hint that the ETE does not implement the SEAL 853 protocol. The ITE can also process other ICMP messages that do not 854 include sufficient information in the packet-in-error as a hint that 855 the ETE link path may be failing. Specific actions that the ITE may 856 take in these cases are out of scope. 858 For other ICMP messages, the ITE should use any outer header 859 information available as a first-pass authentication filter (e.g., to 860 determine if the source of the message is within the same 861 administrative domain as the ITE) and discards the message if first 862 pass filtering fails. 864 Next, the ITE examines the packet-in-error beginning with the SEAL 865 header. If the value in the Identification field (if present) is not 866 within the window of packets the ITE has recently sent to this ETE, 867 or if the value in the SEAL header ICV field (if present) is 868 incorrect, the ITE discards the message. 870 Next, if the received ICMP message is a PTB the ITE sets the 871 temporary variable "PMTU" for this ETE link path to the MTU value in 872 the PTB message. If PMTU==0, the ITE consults a plateau table (e.g., 873 as described in [RFC1191]) to determine PMTU based on the length 874 field in the outer IP header of the packet-in-error. For example, if 875 the ITE receives a PTB message with MTU==0 and length 4KB, it can set 876 PMTU=2KB. If the ITE subsequently receives a PTB message with MTU==0 877 and length 2KB, it can set PMTU=1792, etc. to a minimum value of 878 PMTU=(1500+HLEN). If the ITE is performing stateful MTU 879 determination for this ETE link path (see Section 4.4.9), the ITE 880 next sets PATH_MTU=MAX((PMTU-HLEN), 1500). 882 If the ICMP message was not discarded, the ITE then transcribes it 883 into a message to return to the previous hop. If the inner packet 884 was a SEAL data packet, the ITE transcribes the ICMP message into an 885 SCMP message. Otherwise, the ITE transcribes the ICMP message into a 886 message appropriate for the inner protocol version. 888 To transcribe the message, the ITE extracts the inner packet from 889 within the ICMP message packet-in-error field and uses it to generate 890 a new message corresponding to the type of the received ICMP message. 891 For SCMP messages, the ITE generates the message the same as 892 described for ETE generation of SCMP messages in Section 4.6.1. For 893 (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field. 895 The ITE finally forwards the transcribed message to the previous hop 896 toward the inner source address. 898 4.4.9. IPv4 Middlebox Reassembly Testing 900 The ITE can perform a qualification exchange to ensure that the 901 subnetwork correctly delivers fragments to the ETE. This procedure 902 can be used, e.g., to determine whether there are middleboxes on the 903 path that violate the [RFC1812], Section 5.2.6 requirement that: "A 904 router MUST NOT reassemble any datagram before forwarding it". 906 The ITE should use knowledge of its topological arrangement as an aid 907 in determining when middlebox reassembly testing is necessary. For 908 example, if the ITE is aware that the ETE is located somewhere in the 909 public Internet, middlebox reassembly testing should not be 910 necessary. If the ITE is aware that the ETE is located behind a NAT 911 or a firewall, however, then middlebox reassembly testing is 912 recommended. 914 The ITE can perform a middlebox reassembly test by selecting a data 915 packet to be used as a probe. While performing the test with real 916 data packets, the ITE should select only inner packets that are no 917 larger than (1500-HLEN) bytes for testing purposes. The ITE can also 918 construct a dummy probe packet instead of using ordinary SEAL data 919 packets. 921 To generate a dummy probe packet, the ITE creates a packet buffer 922 beginning with the same outer headers, SEAL header and inner network 923 layer header that would appear in an ordinary data packet, then pads 924 the packet with random data to a length that is at least 128 bytes 925 but no longer than (1500-HLEN) bytes. The ITE then writes the value 926 '0' in the inner network layer TTL (for IPv4) or Hop Limit (for IPv6) 927 field. 929 The ITE then sets (C=0; R=0) in the SEAL header of the probe packet 930 and sets the NEXTHDR field to the inner network layer protocol type. 931 (The ITE may also set A=1 if it requires a positive acknowledgement; 932 otherwise, it sets A=0.) Next, the ITE sets LINK_ID and LEVEL to the 933 appropriate values for this ETE link path, sets Identification and 934 I=1 (when USE_ID is TRUE), then finally calculates the ICV and sets 935 V=1(when USE_ICV is TRUE). 937 The ITE then encapsulates the probe packet in the appropriate outer 938 headers, splits it into two outer IPv4 fragments, then sends both 939 fragments over the same ETE link path. 941 The ITE should send a series of probe packets (e.g., 3-5 probes with 942 1sec intervals between tests) instead of a single isolated probe in 943 case of packet loss. If the ETE returns an SCMP PTB message with MTU 944 != 0, then the ETE link path correctly supports fragmentation; 945 otherwise, the ITE enables stateful MTU determination for this ETE 946 link path as specified in Section 4.4.9. 948 (Examples of middleboxes that may perform reassembly include stateful 949 NATs and firewalls. Such devices could still allow for stateless MTU 950 determination if they gather the fragments of a fragmented IPv4 SEAL 951 data packet for packet analysis purposes but then forward the 952 fragments on to the final destination rather than forwarding the 953 reassembled packet.) 955 4.4.10. Stateful MTU Determination 957 SEAL supports a stateless MTU determination capability, however the 958 ITE may in some instances wish to impose a stateful MTU limit on a 959 particular ETE link path. For example, when the ETE is situated 960 behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8) 961 it is imperative that fragmentation be avoided. In other instances 962 (e.g., when the ETE link path includes performance-constrained 963 links), the ITE may deem it necessary to cache a conservative static 964 MTU in order to avoid sending large packets that would only be 965 dropped due to an MTU restriction somewhere on the path. 967 To determine a static MTU value, the ITE can send a series of dummy 968 probe packets of various sizes to the ETE with A=1 in the SEAL header 969 and DF=1 in the outer IP header. The ITE can then cache the size 'S' 970 of the largest packet for which it receives a probe reply from the 971 ETE by setting PATH_MTU=MAX((S-HLEN), 1500) for this ETE link path. 973 For example, the ITE could send probe packets of 4KB, followed by 974 2KB, followed by 1792 bytes, etc. While probing, the ITE processes 975 any ICMP PTB message it receives as a potential indication of probe 976 failure then discards the message. 978 4.4.11. Detecting Path MTU Changes 980 When stateful MTU determination is used, the ITE can periodically 981 reset PATH_MTU and/or re-probe the path to determine whether PATH_MTU 982 has increased. If the path still has a too-small MTU, the ITE will 983 receive a PTB message that reports a smaller size. 985 For IPv4 ETE link paths, when the path correctly implements 986 fragmentation and RATE_LIMIT is TRUE, the ITE can periodically reset 987 RATE_LIMIT=FALSE to determine whether the path still requires rate 988 limiting. If the ITE receives an SPTB message it should again set 989 RATE_LIMIT=TRUE. 991 4.5. ETE Specification 993 4.5.1. Tunnel Neighbor Soft State 995 When data origin authentication and integrity checking is required, 996 the ETE maintains a per-ITE ICV calculation algorithm and a symmetric 997 secret key to verify the ICV. When per-packet identification is 998 required, the ETE also maintains a window of Identification values 999 for the packets it has recently received from this ITE. 1001 When the tunnel neighbor relationship is bidirectional, the ETE 1002 further maintains a per ETE link path mapping of outer IP and 1003 transport layer addresses to the LINK_ID that appears in packets 1004 received from the ITE. 1006 4.5.2. IP-Layer Reassembly 1008 The ETE should maintain conservative reassembly cache high- and low- 1009 water marks. When the size of the reassembly cache exceeds this 1010 high-water mark, the ETE should actively discard stale incomplete 1011 reassemblies (e.g., using an Active Queue Management (AQM) strategy) 1012 until the size falls below the low-water mark. The ETE should also 1013 actively discard any pending reassemblies that clearly have no 1014 opportunity for completion, e.g., when a considerable number of new 1015 fragments have arrived before a fragment that completes a pending 1016 reassembly arrives. 1018 The ETE processes non-SEAL IP packets as specified in the normative 1019 references, i.e., it performs any necessary IP reassembly then 1020 discards the packet if it is larger than the reassembly buffer size 1021 or delivers the (fully-reassembled) packet to the appropriate upper 1022 layer protocol module. 1024 For SEAL packets, the ITE performs any necessary IP reassembly then 1025 submits the packet for SEAL decapsulation as specified in Section 1026 4.5.3. (Note that if the packet is larger than the reassembly buffer 1027 size, the ITE still returns the leading portion of the (partially) 1028 reassembled packet.) 1030 4.5.3. Decapsulation and Re-Encapsulation 1032 For each SEAL packet accepted for decapsulation, when I==1 the ETE 1033 first examines the Identification field. If the Identification is 1034 not within the window of acceptable values for this ITE, the ETE 1035 silently discards the packet. 1037 Next, if V==1 the ETE verifies the ICV value (with the ICV field 1038 itself reset to 0) and silently discards the packet if the value is 1039 incorrect. 1041 Next, if the packet arrived as multiple IPv4 fragments and L ==0, the 1042 ETE sends an SPTB message back to the ITE with MTU set to the size of 1043 the largest fragment received minus HLEN (see: Section 4.6.1.1). 1045 Next, if the packet arrived as multiple IP fragments and the inner 1046 packet is larger than 1500 bytes, the ETE silently discards the 1047 packet; otherwise, it continues to process the packet. 1049 Next, if there is an incorrect value in a SEAL header field (e.g., an 1050 incorrect "VER" field value), the ETE discards the packet. If the 1051 SEAL header has C==0, the ETE also returns an SCMP "Parameter 1052 Problem" (SPP) message (see Section 4.6.1.2). 1054 Next, if the SEAL header has C==1, the ETE processes the packet as an 1055 SCMP packet as specified in Section 4.6.2. Otherwise, the ETE 1056 continues to process the packet as a SEAL data packet. 1058 Next, if the SEAL header has (M==1 || Offset!==0) the ETE checks to 1059 see if the other segments of this already-segmented SEAL packet have 1060 arrived, i.e., by looking for additional segments that have the same 1061 outer IP source address, destination address, source transport port 1062 number (if present) and SEAL Identification value. If the other 1063 segments have already arrived, the ETE discards the SEAL header and 1064 other outer headers from the non-initial segments and appends them 1065 onto the end of the first segment. Otherwise, the ETE caches the 1066 segment for at most 60 seconds while awaiting the arrival of its 1067 partners. To support this process, the ETE must be able to buffer 1068 segmented SEAL packets up to (1500+HLEN) bytes in length. 1070 Next, if the SEAL header in the (reassembled) packet has A==1, the 1071 ETE sends an SPTB message back to the ITE with MTU=0 (see: Section 1072 4.6.1.1). 1074 Finally, the ETE discards the outer headers and processes the inner 1075 packet according to the header type indicated in the SEAL NEXTHDR 1076 field. If the inner (TTL / Hop Limit) field encodes the value 0, the 1077 ETE silently discards the packet. Otherwise, if the next hop toward 1078 the inner destination address is via a different interface than the 1079 SEAL packet arrived on, the ETE discards the SEAL header and delivers 1080 the inner packet either to the local host or to the next hop 1081 interface if the packet is not destined to the local host. 1083 If the next hop is on the same interface the SEAL packet arrived on, 1084 however, the ETE submits the packet for SEAL re-encapsulation 1085 beginning with the specification in Section 4.4.3 above and without 1086 decrementing the value in the inner (TTL / Hop Limit) field. In this 1087 process, the packet remains within the tunnel (i.e., it does not exit 1088 and then re-enter the tunnel); hence, the packet is not discarded if 1089 the LEVEL field in the SEAL header contains the value 0. 1091 4.6. The SEAL Control Message Protocol (SCMP) 1093 SEAL provides a companion SEAL Control Message Protocol (SCMP) that 1094 uses the same message types and formats as for the Internet Control 1095 Message Protocol for IPv6 (ICMPv6) [RFC4443]. As for ICMPv6, each 1096 SCMP message includes a 32-bit header and a variable-length body. 1097 The ITE encapsulates the SCMP message in a SEAL header and outer 1098 headers as shown in Figure 3: 1100 +--------------------+ 1101 ~ outer IP header ~ 1102 +--------------------+ 1103 ~ other outer hdrs ~ 1104 +--------------------+ 1105 ~ SEAL Header ~ 1106 +--------------------+ +--------------------+ 1107 | SCMP message header| --> | SCMP message header| 1108 +--------------------+ +--------------------+ 1109 | | --> | | 1110 ~ SCMP message body ~ --> ~ SCMP message body ~ 1111 | | --> | | 1112 +--------------------+ +--------------------+ 1114 SCMP Message SCMP Packet 1115 before encapsulation after encapsulation 1117 Figure 3: SCMP Message Encapsulation 1119 The following sections specify the generation, processing and 1120 relaying of SCMP messages. 1122 4.6.1. Generating SCMP Error Messages 1124 ETEs generate SCMP error messages in response to receiving certain 1125 SEAL data packets using the format shown in Figure 4: 1127 0 1 2 3 1128 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 1129 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1130 | Type | Code | Checksum | 1131 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1132 | Type-Specific Data | 1133 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1134 | As much of the invoking SEAL data packet as possible | 1135 ~ (beginning with the SEAL header) without the SCMP ~ 1136 | packet exceeding MINMTU bytes (*) | 1138 (*) also known as the "packet-in-error" 1140 Figure 4: SCMP Error Message Format 1142 The error message includes the 32-bit SCMP message header, followed 1143 by a 32-bit Type-Specific Data field, followed by the leading portion 1144 of the invoking SEAL data packet beginning with the SEAL header as 1145 the "packet-in-error". The packet-in-error includes as much of the 1146 invoking packet as possible extending to a length that would not 1147 cause the entire SCMP packet following outer encapsulation to exceed 1148 MINMTU bytes. 1150 When the ETE processes a SEAL data packet for which the 1151 Identification and ICV values are correct but an error must be 1152 returned, it prepares an SCMP error message as shown in Figure 4. 1153 The ETE sets the Type and Code fields to the same values that would 1154 appear in the corresponding ICMPv6 message [RFC4443], but calculates 1155 the Checksum beginning with the SCMP message header using the 1156 algorithm specified for ICMPv4 in [RFC0792]. 1158 The ETE next encapsulates the SCMP message in the requisite SEAL and 1159 outer headers as shown in Figure 3. During encapsulation, the ETE 1160 sets the outer destination address/port numbers of the SCMP packet to 1161 the values associated with the ITE and sets the outer source address/ 1162 port numbers to its own outer address/port numbers. 1164 The ETE then sets (C=1; A=0; R=0; L=0; X=0; M=0; Offset=0) in the 1165 SEAL header, then sets I, V, NEXTHDR and LEVEL to the same values 1166 that appeared in the SEAL header of the data packet. If the neighbor 1167 relationship between the ITE and ETE is unidirectional, the ETE next 1168 sets the LINK_ID field to the same value that appeared in the SEAL 1169 header of the data packet. Otherwise, the ETE sets the LINK_ID field 1170 to the value it would use in sending a SEAL packet to this ITE. 1172 When I==1, the ETE next sets the Identification field to an 1173 appropriate value for the ITE. If the neighbor relationship between 1174 the ITE and ETE is unidirectional, the ETE sets the Identification 1175 field to the same value that appeared in the SEAL header of the data 1176 packet. Otherwise, the ETE sets the Identification field to the 1177 value it would use in sending the next SEAL packet to this ITE. 1179 When V==1, the ETE then calculates and sets the ICV field the same as 1180 specified for SEAL data packet encapsulation in Section 4.4.4. 1182 Finally, the ETE sends the resulting SCMP packet to the ITE the same 1183 as specified for SEAL data packets in Section 4.4.5. 1185 The following sections describe additional considerations for various 1186 SCMP error messages: 1188 4.6.1.1. Generating SCMP Packet Too Big (SPTB) Messages 1190 An ETE generates an SCMP "Packet Too Big" (SPTB) message when it 1191 receives a SEAL data packet that arrived as multiple outer IPv4 1192 fragments and for which L==0. The ETE prepares the SPTB message the 1193 same as for the corresponding ICMPv6 PTB message, and writes the 1194 length of the largest outer IP fragment received minus HLEN in the 1195 MTU field of the message. 1197 The ETE also generates an SPTB message when it accepts a SEAL 1198 protocol data packet with A==1 in the SEAL header. The ETE prepares 1199 the SPTB message the same as above, except that it writes the value 0 1200 in the MTU field. 1202 4.6.1.2. Generating Other SCMP Error Messages 1204 An ETE generates an SCMP "Destination Unreachable" (SDU) message 1205 under the same circumstances that an IPv6 system would generate an 1206 ICMPv6 Destination Unreachable message. 1208 An ETE generates an SCMP "Parameter Problem" (SPP) message when it 1209 receives a SEAL packet with an incorrect value in the SEAL header. 1211 TEs generate other SCMP message types using methods and procedures 1212 specified in other documents. For example, SCMP message types used 1213 for tunnel neighbor coordinations are specified in VET 1214 [I-D.templin-intarea-vet]. 1216 4.6.2. Processing SCMP Error Messages 1218 An ITE may receive SCMP messages with C==1 in the SEAL header after 1219 sending packets to an ETE. The ITE first verifies that the outer 1220 addresses of the SCMP packet are correct, and (when I==1) that the 1221 Identification field contains an acceptable value. The ITE next 1222 verifies that the SEAL header fields are set correctly as specified 1223 in Section 4.6.1. When V==1, the ITE then verifies the ICV value. 1224 The ITE next verifies the Checksum value in the SCMP message header. 1225 If any of these values are incorrect, the ITE silently discards the 1226 message; otherwise, it processes the message as follows: 1228 4.6.2.1. Processing SCMP PTB Messages 1230 After an ITE sends a SEAL data packet to an ETE, it may receive an 1231 SPTB message with a packet-in-error containing the leading portion of 1232 the packet (see: Section 4.6.1.1). For IP SPTB messages with MTU==0, 1233 the ITE processes the message as confirmation that the ETE received a 1234 SEAL data packet with A==1 in the SEAL header. The ITE then discards 1235 the message. 1237 For SPTB messages with MTU != 0, the ITE processes the message as an 1238 indication of a packet size limitation as follows. If the inner 1239 packet is itself a SEAL packet, and the inner packet length is less 1240 than 1500, the ITE reduces its MINMTU value for this ITE. If the 1241 inner packet is a non-SEAL IPv4 packet and the inner packet length is 1242 less than 1500, the ITE instead sets RATE_LIMIT=1. For all other 1243 cases, if the inner packet length is larger than 1500 and the MTU 1244 value is not substantially less than 1500 bytes, the value is likely 1245 to reflect the true MTU of the restricting link on the path to the 1246 ETE; otherwise, a router on the path may be generating runt 1247 fragments. 1249 In that case, the ITE can consult a plateau table (e.g., as described 1250 in [RFC1191]) to rewrite the MTU value to a reduced size. For 1251 example, if the ITE receives an IPv4 SPTB message with MTU==256 and 1252 inner packet length 4KB, it can rewrite the MTU to 2KB. If the ITE 1253 subsequently receives an IPv4 SPTB message with MTU==256 and inner 1254 packet length 2KB, it can rewrite the MTU to 1792, etc., to a minimum 1255 of 1500 bytes. If the ITE is performing stateful MTU determination 1256 for this ETE link path, it then writes the new MTU value minus HLEN 1257 in PATH_MTU. 1259 The ITE then checks its forwarding tables to discover the previous 1260 hop toward the source address of the inner packet. If the previous 1261 hop is reached via the same tunnel interface the SPTB message arrived 1262 on, the ITE relays the message to the previous hop. In order to 1263 relay the message, the first writes zero in the Identification and 1264 ICV fields of the SEAL header within the packet-in-error. The ITE 1265 next rewrites the outer SEAL header fields with values corresponding 1266 to the previous hop and recalculates the ICV using the ICV 1267 calculation parameters associated with the previous hop. Next, the 1268 ITE replaces the SPTB's outer headers with headers of the appropriate 1269 protocol version and fills in the header fields as specified in 1270 Sections 5.5.4-5.5.6 of [I-D.templin-intarea-vet], where the 1271 destination address/port correspond to the previous hop and the 1272 source address/port correspond to the ITE. The ITE then sends the 1273 message to the previous hop the same as if it were issuing a new SPTB 1274 message. (Note that, in this process, the values within the SEAL 1275 header of the packet-in-error are meaningless to the previous hop and 1276 therefore cannot be used by the previous hop for authentication 1277 purposes.) 1279 If the previous hop is not reached via the same tunnel interface, the 1280 ITE instead transcribes the message into a format appropriate for the 1281 inner packet (i.e., the same as described for transcribing ICMP 1282 messages in Section 4.4.7) and sends the resulting transcribed 1283 message to the original source. (NB: if the inner packet within the 1284 SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set 1285 DF=1 and re-calculate the IPv4 header checksum while transcribing the 1286 message in order to avoid bogon filters.) The ITE then discards the 1287 SPTB message. 1289 Note that the ITE may receive an SPTB message from another ITE that 1290 is at the head end of a nested level of encapsulation. The ITE has 1291 no security associations with this nested ITE, hence it should 1292 consider this SPTB message the same as if it had received an ICMP PTB 1293 message from an ordinary router on the path to the ETE. That is, the 1294 ITE should examine the packet-in-error field of the SPTB message and 1295 only process the message if it is able to recognize the packet as one 1296 it had previously sent. 1298 4.6.2.2. Processing Other SCMP Error Messages 1300 An ITE may receive an SDU message with an appropriate code under the 1301 same circumstances that an IPv6 node would receive an ICMPv6 1302 Destination Unreachable message. The ITE either transcribes or 1303 relays the message toward the source address of the inner packet 1304 within the packet-in-error the same as specified for SPTB messages in 1305 Section 4.6.2.1. 1307 An ITE may receive an SPP message when the ETE receives a SEAL packet 1308 with an incorrect value in the SEAL header. The ITE should examine 1309 the SEAL header within the packet-in-error to determine whether a 1310 different setting should be used in subsequent packets, but does not 1311 relay the message further. 1313 TEs process other SCMP message types using methods and procedures 1314 specified in other documents. For example, SCMP message types used 1315 for tunnel neighbor coordinations are specified in VET 1316 [I-D.templin-intarea-vet]. 1318 5. Link Requirements 1320 Subnetwork designers are expected to follow the recommendations in 1321 Section 2 of [RFC3819] when configuring link MTUs. 1323 6. End System Requirements 1325 End systems are encouraged to implement end-to-end MTU assurance 1326 (e.g., using Packetization Layer PMTUD per [RFC4821]) even if the 1327 subnetwork is using SEAL. 1329 7. Router Requirements 1331 Routers within the subnetwork are expected to observe the router 1332 requirements found in the normative references, including the 1333 implementation of IP fragmentation and reassembly [RFC1812][RFC2460] 1334 as well as the generation of ICMP messages [RFC0792][RFC4443]. 1336 8. Nested Encapsulation Considerations 1338 SEAL supports nested tunneling for up to 8 layers of encapsulation. 1339 In this model, the SEAL ITE has a tunnel neighbor relationship only 1340 with ETEs at its own nesting level, i.e., it does not have a tunnel 1341 neighbor relationship with other ITEs, nor with ETEs at other nesting 1342 levels. 1344 Therefore, when an ITE 'A' within an inner nesting level needs to 1345 return an error message to an ITE 'B' within an outer nesting level, 1346 it generates an ordinary ICMP error message the same as if it were an 1347 ordinary router within the subnetwork. 'B' can then perform message 1348 validation as specified in Section 4.4.7, but full message origin 1349 authentication is not possible. 1351 Since ordinary ICMP messages are used for coordinations between ITEs 1352 at different nesting levels, nested SEAL encapsulations should only 1353 be used when the ITEs are within a common administrative domain 1354 and/or when there is no ICMP filtering middlebox such as a firewall 1355 or NAT between them. An example would be a recursive nesting of 1356 mobile networks, where the first network receives service from an 1357 ISP, the second network receives service from the first network, the 1358 third network receives service from the second network, etc. 1360 NB: As an alternative, the SCMP protocol could be extended to allow 1361 ITE 'A' to return an SCMP message to ITE 'B' rather than return an 1362 ICMP message. This would conceptually allow the control messages to 1363 pass through firewalls and NATs, however it would give no more 1364 message origin authentication assurance than for ordinary ICMP 1365 messages. It was therefore determined that the complexity of 1366 extending the SCMP protocol was of little value within the context of 1367 the anticipated use cases for nested encapsulations. 1369 9. IANA Considerations 1371 The IANA is instructed to allocate a System Port number for "SEAL" in 1372 the 'port-numbers' registry for the TCP, UDP, DCCP and SCTP 1373 protocols. 1375 The IANA is further instructed to allocate an IP protocol number for 1376 "SEAL" in the "protocol-numbers" registry. 1378 Considerations for port and protocol number assignments appear in 1379 [RFC2780][RFC5226][RFC6335]. 1381 10. Security Considerations 1383 SEAL provides a segment-by-segment data origin authentication and 1384 anti-replay service across the (potentially) multiple segments of a 1385 re-encapsulating tunnel. It further provides a segment-by-segment 1386 integrity check of the headers of encapsulated packets, but does not 1387 verify the integrity of the rest of the packet beyond the headers 1388 unless fragmentation is unavoidable. SEAL therefore considers full 1389 message integrity checking, authentication and confidentiality as 1390 end-to-end considerations in a manner that is compatible with 1391 securing mechanisms such as TLS/SSL [RFC5246]. 1393 An amplification/reflection/buffer overflow attack is possible when 1394 an attacker sends IP fragments with spoofed source addresses to an 1395 ETE in an attempt to clog the ETE's reassembly buffer and/or cause 1396 the ETE to generate a stream of SCMP messages returned to a victim 1397 ITE. The SCMP message ICV, Identification, as well as the inner 1398 headers of the packet-in-error, provide mitigation for the ETE to 1399 detect and discard SEAL segments with spoofed source addresses. 1401 The SEAL header is sent in-the-clear the same as for the outer IP and 1402 other outer headers. In this respect, the threat model is no 1403 different than for IPv6 extension headers. Unlike IPv6 extension 1404 headers, however, the SEAL header can be protected by an integrity 1405 check that also covers the inner packet headers. 1407 Security issues that apply to tunneling in general are discussed in 1408 [RFC6169]. 1410 11. Related Work 1412 Section 3.1.7 of [RFC2764] provides a high-level sketch for 1413 supporting large tunnel MTUs via a tunnel-level segmentation and 1414 reassembly capability to avoid IP level fragmentation. 1416 Section 3 of [RFC4459] describes inner and outer fragmentation at the 1417 tunnel endpoints as alternatives for accommodating the tunnel MTU. 1419 Section 4 of [RFC2460] specifies a method for inserting and 1420 processing extension headers between the base IPv6 header and 1421 transport layer protocol data. The SEAL header is inserted and 1422 processed in exactly the same manner. 1424 IPsec/AH is [RFC4301][RFC4301] is used for full message integrity 1425 verification between tunnel endpoints, whereas SEAL only ensures 1426 integrity for the inner packet headers. The AYIYA proposal 1427 [I-D.massar-v6ops-ayiya] uses similar means for providing message 1428 authentication and integrity. 1430 The concepts of path MTU determination through the report of 1431 fragmentation and extending the IPv4 Identification field were first 1432 proposed in deliberations of the TCP-IP mailing list and the Path MTU 1433 Discovery Working Group (MTUDWG) during the late 1980's and early 1434 1990's. An historical analysis of the evolution of these concepts, 1435 as well as the development of the eventual PMTUD mechanism, appears 1436 in Appendix D of this document. 1438 12. Implementation Status 1440 An early implementation of the first revision of SEAL [RFC5320] is 1441 available at: http://isatap.com/seal/pre-rfc5320.txt 1443 13. Acknowledgments 1445 The following individuals are acknowledged for helpful comments and 1446 suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver 1447 Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, 1448 Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph 1449 Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci, 1450 Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob 1451 Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt 1452 Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark 1453 Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin 1454 Whittle, James Woodyatt, and members of the Boeing Research & 1455 Technology NST DC&NT group. 1457 Discussions with colleagues following the publication of [RFC5320] 1458 have provided useful insights that have resulted in significant 1459 improvements to this, the Second Edition of SEAL. 1461 Path MTU determination through the report of fragmentation was first 1462 proposed by Charles Lynn on the TCP-IP mailing list in 1987. 1463 Extending the IP identification field was first proposed by Steve 1464 Deering on the MTUDWG mailing list in 1989. 1466 14. References 1468 14.1. Normative References 1470 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 1471 September 1981. 1473 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 1474 RFC 792, September 1981. 1476 [RFC1122] Braden, R., "Requirements for Internet Hosts - 1477 Communication Layers", STD 3, RFC 1122, October 1989. 1479 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1480 Requirement Levels", BCP 14, RFC 2119, March 1997. 1482 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1483 (IPv6) Specification", RFC 2460, December 1998. 1485 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 1486 Neighbor Discovery (SEND)", RFC 3971, March 2005. 1488 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 1489 Message Protocol (ICMPv6) for the Internet Protocol 1490 Version 6 (IPv6) Specification", RFC 4443, March 2006. 1492 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1493 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1494 September 2007. 1496 14.2. Informative References 1498 [FOLK] Shannon, C., Moore, D., and k. claffy, "Beyond Folklore: 1499 Observations on Fragmented Traffic", December 2002. 1501 [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", 1502 October 1987. 1504 [I-D.ietf-intarea-ipv4-id-update] 1505 Touch, J., "Updated Specification of the IPv4 ID Field", 1506 draft-ietf-intarea-ipv4-id-update-05 (work in progress), 1507 May 2012. 1509 [I-D.ietf-savi-framework] 1510 Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, 1511 "Source Address Validation Improvement Framework", 1512 draft-ietf-savi-framework-06 (work in progress), 1513 January 2012. 1515 [I-D.massar-v6ops-ayiya] 1516 Massar, J., "AYIYA: Anything In Anything", 1517 draft-massar-v6ops-ayiya-02 (work in progress), July 2004. 1519 [I-D.templin-aero] 1520 Templin, F., "Asymmetric Extended Route Optimization 1521 (AERO)", draft-templin-aero-08 (work in progress), 1522 February 2012. 1524 [I-D.templin-intarea-vet] 1525 Templin, F., "Virtual Enterprise Traversal (VET)", 1526 draft-templin-intarea-vet-33 (work in progress), 1527 December 2011. 1529 [I-D.templin-ironbis] 1530 Templin, F., "The Internet Routing Overlay Network 1531 (IRON)", draft-templin-ironbis-10 (work in progress), 1532 December 2011. 1534 [MTUDWG] "IETF MTU Discovery Working Group mailing list, 1535 gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1536 1989 - February 1995.". 1538 [RFC0994] International Organization for Standardization (ISO) and 1539 American National Standards Institute (ANSI), "Final text 1540 of DIS 8473, Protocol for Providing the Connectionless- 1541 mode Network Service", RFC 994, March 1986. 1543 [RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP 1544 MTU discovery options", RFC 1063, July 1988. 1546 [RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as 1547 a subnetwork for experimentation with the OSI network 1548 layer", RFC 1070, February 1989. 1550 [RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum 1551 options", RFC 1146, March 1990. 1553 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 1554 November 1990. 1556 [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic 1557 Routing Encapsulation (GRE)", RFC 1701, October 1994. 1559 [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", 1560 RFC 1812, June 1995. 1562 [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery 1563 for IP version 6", RFC 1981, August 1996. 1565 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 1566 October 1996. 1568 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 1569 IPv6 Specification", RFC 2473, December 1998. 1571 [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", 1572 RFC 2675, August 1999. 1574 [RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A. 1575 Malis, "A Framework for IP Based Virtual Private 1576 Networks", RFC 2764, February 2000. 1578 [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For 1579 Values In the Internet Protocol and Related Headers", 1580 BCP 37, RFC 2780, March 2000. 1582 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 1583 Defeating Denial of Service Attacks which employ IP Source 1584 Address Spoofing", BCP 38, RFC 2827, May 2000. 1586 [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", 1587 RFC 2923, September 2000. 1589 [RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by 1590 an On-line Database", RFC 3232, January 2002. 1592 [RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on 1593 link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, 1594 August 2002. 1596 [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., 1597 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 1598 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 1599 RFC 3819, July 2004. 1601 [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and 1602 More-Specific Routes", RFC 4191, November 2005. 1604 [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms 1605 for IPv6 Hosts and Routers", RFC 4213, October 2005. 1607 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1608 Internet Protocol", RFC 4301, December 2005. 1610 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, 1611 December 2005. 1613 [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- 1614 Network Tunneling", RFC 4459, April 2006. 1616 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 1617 Discovery", RFC 4821, March 2007. 1619 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 1620 Errors at High Data Rates", RFC 4963, July 2007. 1622 [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common 1623 Mitigations", RFC 4987, August 2007. 1625 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1626 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1627 May 2008. 1629 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1630 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1632 [RFC5320] Templin, F., "The Subnetwork Encapsulation and Adaptation 1633 Layer (SEAL)", RFC 5320, February 2010. 1635 [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) 1636 Schemes", RFC 5445, March 2009. 1638 [RFC5720] Templin, F., "Routing and Addressing in Networks with 1639 Global Enterprise Recursion (RANGER)", RFC 5720, 1640 February 2010. 1642 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. 1644 [RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and 1645 Addressing in Networks with Global Enterprise Recursion 1646 (RANGER) Scenarios", RFC 6139, February 2011. 1648 [RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security 1649 Concerns with IP Tunneling", RFC 6169, April 2011. 1651 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1652 Cheshire, "Internet Assigned Numbers Authority (IANA) 1653 Procedures for the Management of the Service Name and 1654 Transport Protocol Port Number Registry", BCP 165, 1655 RFC 6335, August 2011. 1657 [SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU 1658 Discovery Behavior", November 2010. 1660 [TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring 1661 Interactions Between Transport Protocols and Middleboxes", 1662 October 2004. 1664 [TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List, 1665 http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1666 1987 - May 1990.". 1668 [WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and 1669 Debugging Path MTU Discovery Failures", October 2005. 1671 Appendix A. Reliability 1673 Although a SEAL tunnel may span an arbitrarily-large subnetwork 1674 expanse, the IP layer sees the tunnel as a simple link that supports 1675 the IP service model. Links with high bit error rates (BERs) (e.g., 1676 IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] 1677 to increase packet delivery ratios, while links with much lower BERs 1678 typically omit such mechanisms. Since SEAL tunnels may traverse 1679 arbitrarily-long paths over links of various types that are already 1680 either performing or omitting ARQ as appropriate, it would therefore 1681 be inefficient to require the tunnel endpoints to also perform ARQ. 1683 Appendix B. Integrity 1685 The SEAL header includes an integrity check field that covers the 1686 SEAL header and at least the inner packet headers. This provides for 1687 header integrity verification on a segment-by-segment basis for a 1688 segmented re-encapsulating tunnel path. 1690 Fragmentation and reassembly schemes must also consider packet- 1691 splicing errors, e.g., when two fragments from the same packet are 1692 concatenated incorrectly, when a fragment from packet X is 1693 reassembled with fragments from packet Y, etc. The primary sources 1694 of such errors include implementation bugs and wrapping IPv4 ID 1695 fields. 1697 In particular, the IPv4 16-bit ID field can wrap with only 64K 1698 packets with the same (src, dst, protocol)-tuple alive in the system 1699 at a given time [RFC4963]. When the IPv4 ID field is re-written by a 1700 middlebox such as a NAT or Firewall, ID field wrapping can occur with 1701 even fewer packets alive in the system. 1703 When outer IPv4 fragmentation is unavoidable, SEAL institutes rate 1704 limiting so that the number of packets admitted into the tunnel by 1705 the ITE does not exceed the number of unique packets that may be 1706 alive within the Internet. 1708 Appendix C. Transport Mode 1710 SEAL can also be used in "transport-mode", e.g., when the inner layer 1711 comprises upper-layer protocol data rather than an encapsulated IP 1712 packet. For instance, TCP peers can negotiate the use of SEAL (e.g., 1713 by inserting an unspecified 'SEAL_OPTION' TCP option during 1714 connection establishment) for the carriage of protocol data 1715 encapsulated as IP/SEAL/TCP. In this sense, the "subnetwork" becomes 1716 the entire end-to-end path between the TCP peers and may potentially 1717 span the entire Internet. 1719 If both TCPs agree on the use of SEAL, their protocol messages will 1720 be carried as IP/SEAL/TCP and the connection will be serviced by the 1721 SEAL protocol using TCP (instead of an encapsulating tunnel endpoint) 1722 as the transport layer protocol. The SEAL protocol for transport 1723 mode otherwise observes the same specifications as for Section 4. 1725 Appendix D. Historic Evolution of PMTUD 1727 The topic of Path MTU discovery (PMTUD) saw a flurry of discussion 1728 and numerous proposals in the late 1980's through early 1990. The 1729 initial problem was posed by Art Berggreen on May 22, 1987 in a 1730 message to the TCP-IP discussion group [TCP-IP]. The discussion that 1731 followed provided significant reference material for [FRAG]. An IETF 1732 Path MTU Discovery Working Group [MTUDWG] was formed in late 1989 1733 with charter to produce an RFC. Several variations on a very few 1734 basic proposals were entertained, including: 1736 1. Routers record the PMTUD estimate in ICMP-like path probe 1737 messages (proposed in [FRAG] and later [RFC1063]) 1739 2. The destination reports any fragmentation that occurs for packets 1740 received with the "RF" (Report Fragmentation) bit set (Steve 1741 Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal) 1743 3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw 1744 RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990) 1746 4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30, 1747 1990) 1749 5. Fragmentation avoidance by setting "IP_DF" flag on all packets 1750 and retransmitting if ICMPv4 "fragmentation needed" messages 1751 occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191] 1752 by Mogul and Deering). 1754 Option 1) seemed attractive to the group at the time, since it was 1755 believed that routers would migrate more quickly than hosts. Option 1756 2) was a strong contender, but repeated attempts to secure an "RF" 1757 bit in the IPv4 header from the IESG failed and the proponents became 1758 discouraged. 3) was abandoned because it was perceived as too 1759 complicated, and 4) never received any apparent serious 1760 consideration. Proposal 5) was a late entry into the discussion from 1761 Steve Deering on Feb. 24th, 1990. The discussion group soon 1762 thereafter seemingly lost track of all other proposals and adopted 1763 5), which eventually evolved into [RFC1191] and later [RFC1981]. 1765 In retrospect, the "RF" bit postulated in 2) is not needed if a 1766 "contract" is first established between the peers, as in proposal 4) 1767 and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on 1768 Feb 19. 1990. These proposals saw little discussion or rebuttal, and 1769 were dismissed based on the following the assertions: 1771 o routers upgrade their software faster than hosts 1773 o PCs could not reassemble fragmented packets 1775 o Proteon and Wellfleet routers did not reproduce the "RF" bit 1776 properly in fragmented packets 1778 o Ethernet-FDDI bridges would need to perform fragmentation (i.e., 1779 "translucent" not "transparent" bridging) 1781 o the 16-bit IP_ID field could wrap around and disrupt reassembly at 1782 high packet arrival rates 1784 The first four assertions, although perhaps valid at the time, have 1785 been overcome by historical events. The final assertion is addressed 1786 by the mechanisms specified in SEAL. 1788 Author's Address 1790 Fred L. Templin (editor) 1791 Boeing Research & Technology 1792 P.O. Box 3707 1793 Seattle, WA 98124 1794 USA 1796 Email: fltemplin@acm.org