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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-18) exists of draft-ietf-dots-architecture-05 == Outdated reference: A later version (-09) exists of draft-ietf-sfc-use-case-mobility-07 == Outdated reference: A later version (-08) exists of draft-irtf-nwcrg-network-coding-taxonomy-07 == Outdated reference: A later version (-25) exists of draft-mm-wg-effect-encrypt-22 Summary: 0 errors (**), 0 flaws (~~), 6 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force D. Dolson 3 Internet-Draft 4 Intended status: Informational J. Snellman 5 Expires: September 2, 2018 6 M. Boucadair 7 C. Jacquenet 8 Orange 9 March 1, 2018 11 An Inventory of Transport-centric Functions Provided by Middleboxes 12 draft-dolson-transport-middlebox-02 14 Abstract 16 This document summarizes benefits that operators perceive to be 17 provided by intermediary devices that provide functions apart from 18 normal IP forwarding. Such intermediary devices are often called 19 "middleboxes". 21 RFC3234 defines a taxonomy of middleboxes and issues in the Internet. 22 Most of those middleboxes utilize or modify application-layer data. 23 This document primarily focuses on devices that observe and act on 24 information carried in the transport layer, and especially 25 information carried in TCP packets. 27 A primary goal of this document is to provide information to working 28 groups developing new transport protocols, to aid understanding of 29 what might be gained or lost by design decisions that may affect (or 30 be affected by) middlebox operation. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on September 2, 2018. 49 Copyright Notice 51 Copyright (c) 2018 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (https://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 67 1.1. Operator Perspective . . . . . . . . . . . . . . . . . . 3 68 1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 1.3. Requirements Language . . . . . . . . . . . . . . . . . . 5 70 2. Measurements . . . . . . . . . . . . . . . . . . . . . . . . 5 71 2.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 5 72 2.2. Round Trip Times . . . . . . . . . . . . . . . . . . . . 6 73 2.3. Measuring Packet Reordering . . . . . . . . . . . . . . . 7 74 2.4. Throughput and Bottleneck Identification . . . . . . . . 7 75 2.5. Congestion Responsiveness . . . . . . . . . . . . . . . . 7 76 2.6. Attack Detection . . . . . . . . . . . . . . . . . . . . 8 77 2.7. Packet Corruption . . . . . . . . . . . . . . . . . . . . 8 78 2.8. Application-Layer Measurements . . . . . . . . . . . . . 9 79 3. Functions Beyond Measurement: A Few Examples . . . . . . . . 9 80 3.1. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 81 3.2. Firewall . . . . . . . . . . . . . . . . . . . . . . . . 9 82 3.3. DDoS Scrubbing . . . . . . . . . . . . . . . . . . . . . 10 83 3.4. Implicit Identification . . . . . . . . . . . . . . . . . 11 84 3.5. Performance-Enhancing Proxies . . . . . . . . . . . . . . 11 85 3.6. Network Coding . . . . . . . . . . . . . . . . . . . . . 12 86 3.7. Network-Assisted Bandwidth Aggregation . . . . . . . . . 12 87 3.8. Prioritization and Differentiated Services . . . . . . . 13 88 3.9. Measurement-Based Shaping . . . . . . . . . . . . . . . . 13 89 3.10. Fairness to End-User Quota . . . . . . . . . . . . . . . 14 90 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 91 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 92 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 93 6.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 14 94 6.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 15 95 6.3. More Information Can Improve Security . . . . . . . . . . 15 96 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 97 7.1. Normative References . . . . . . . . . . . . . . . . . . 15 98 7.2. Informative References . . . . . . . . . . . . . . . . . 16 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 101 1. Introduction 103 At IETF97, at a meeting regarding the Path Layer UDP Substrate (PLUS) 104 protocol, a request was made for documentation about the benefits 105 that might be provided by permitting middleboxes to have some 106 visibility to transport-layer information. 108 From RFC3234 [RFC3234], "A middlebox is defined as any intermediary 109 device performing functions other than the normal, standard functions 110 of an IP router on the datagram path between a source host and 111 destination host." 113 Middleboxes are usually (but not exclusively) deployed at locations 114 permitting observation of bidirectional traffic flows. Such 115 locations are typically points where stub networks connect to the 116 Internet; e.g.,: 118 o Where a residential or business customer connects to its service 119 provider(s), which may include multi-homing. 121 o On the Gi interface where a GGSN connects to a PDN (see section 122 3.1 of [RFC6459]). 124 o For the purposes of this document (and consistent with the RFC3234 125 definition), middlebox functions may be found in routers and 126 switches in addition to dedicated devices. 128 The QUIC working group and PLUS BoF are debating the appropriate 129 amount of information that end-points should expose to on-path 130 network middleboxes and human trouble-shooters. (Some information 131 used for debugging is discussed in .) This document itemizes a variety of 133 features provided by middleboxes and by ad hoc analysis performed by 134 operators using packet analyzers. 136 Many of the techniques described in this document require stateful 137 analysis of transport streams. A generic state machine is described 138 in [I-D.trammell-plus-statefulness]. 140 1.1. Operator Perspective 142 The Internet is complicated, and network operators are tasked with 143 providing the network abstraction between end-points. Network 144 operators are often the ones first called upon when applications fail 145 to function properly, often with user reports about application 146 behaviors (not about packet behaviors). Therefore it isn't 147 surprising that operators (wanting to be helpful) desire some 148 visibility into flow information to identify how well the problem 149 flows are progressing and how well other flows are progressing. 151 Advanced service functions (e.g., NATs, firewalls, etc.) are widely 152 used to achieve various objectives such as IP address sharing, 153 firewalling, avoiding covert channels, detecting and protect against 154 ever increasing DDoS attacks, etc. 156 These sophisticated service functions are located in the network but 157 also in service platforms, or intermediate entities (e.g., CDNs). 158 Maintenance and diagnostics are complicated, particularly when 159 responsibility is shared among various players. 161 Network Providers are challenged to deliver differentiated services 162 as a competitive business advantage, while mastering the complexity 163 of the applications, (continuously) evaluating the impacts on 164 middleboxes, and enhancing customer's quality of experience. 166 Despite the complexity, removing all those functions is not an option 167 because they are used to address constraints that are often typical 168 of the current (and changing) Internet situation. Operators must 169 deal with constraints such as global IPv4 address depletion and must 170 support a plethora of services with different QoS, security, 171 robustness, etc. requirements. Furthermore, environment-specific 172 designs may require a number of service functions, such as those 173 needed at the Gi interface of a mobile infrastructure 174 [I-D.ietf-sfc-use-case-mobility]. 176 1.2. Scope 178 Although many middleboxes observe and manipulate application-layer 179 content (e.g., session boarder controllers [RFC5853]) they are out of 180 scope for this document, the aim being to describe middleboxes using 181 transport-layer features. An earlier document 182 [I-D.mm-wg-effect-encrypt] describes the impact of pervasive 183 encryption of application-layer data on network monitoring, 184 protecting and troubleshooting. 186 This document is not intended to recommend (or prohibit) middlebox 187 deployment. Many operators have found the value provided by 188 middleboxes to outweigh the added cost and complexity; this document 189 attempts to provide that perspective as a reference in discussion of 190 protocol design trade-offs. 192 This document is not intended to discuss issues related to 193 middleboxes. These issues are well-known and are recorded in 194 existing documents such as [RFC3234] and [RFC6269]. This document 195 aims to elaborate on the motivations leading operators to enable such 196 functions in spite of complications. 198 This document takes an operator perspective that measurement and 199 management of transport connections is of benefit to both parties: 200 for the end-user to receive better quality of experience, and for the 201 network operator to improve resource usage, the former being a 202 consequence of the latter. 204 This document does not discuss whether exposing some data to on-path 205 devices for network assistance purposes can be achieved by using in- 206 band or out-of-band mechanisms. 208 1.3. Requirements Language 210 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 211 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 212 document are to be interpreted as described in RFC 2119 [RFC2119]. 214 2. Measurements 216 A number of measurements can be made by network devices that are 217 either in-line with the traffic (responsible for forwarding) or 218 receiving off-line copy of traffic from a tap or file capture. These 219 measurements can be used either by automated systems, or for manual 220 network troubleshooting purposes (e.g., using packet analysis tools). 221 The automated systems can further be classified as monitoring systems 222 that compute performance indicators for large numbers of connections 223 and generate aggregated reports from them, and active systems that 224 make decisions on how to handle specific packets based on these 225 performance indicators. 227 Long-term trends in these measurements can aid an operator in 228 capacity planning. Short-term anomalies revealed by these 229 measurements can identify network breakages, attacks in progress, or 230 misbehaving devices/applications. 232 2.1. Packet Loss 234 It is very useful for an operator to be able to detect and isolate 235 packet loss in a network. 237 Network problems and under-provisioning can be detected if packet 238 loss is measurable. TCP packet loss can be detected by observing 239 gaps in sequence numbers, retransmitted sequence numbers, and SACK 240 options. Packet loss can be detected per direction. 242 Gaps indicate loss upstream of the tap point; retransmissions 243 indicate loss downstream of the tap. Selective acknowledgements 244 (SACKs) can be used to detect either upstream or downstream packet 245 loss (although some care needs to be taken to avoid mis-identifying 246 packet reordering as packet loss), and to distinguish between 247 upstream vs. downstream losses. 249 Packet loss measurements on both sides of the measurement point are 250 an important component in precisely diagnosing insufficiently 251 dimensioned devices or links in networks. Additionally, since packet 252 losses are one of the two main ways for congestion to manifest (the 253 other being queueing delay), packet loss is an important measurement 254 for any middlebox that needs to make traffic handling decisions based 255 on observed levels of congestion. 257 2.2. Round Trip Times 259 The ability to measure partial-path round-trip times is valuable in 260 diagnosing network issues. Knowing if latency is poor on one side of 261 the observation point or the other provides more information than is 262 available at either end-point, which can only observe full round-trip 263 times. 265 A TCP packet stream can be used to measure the round-trip time on 266 each side of the measurement point. During the connection handshake, 267 the SYN, SYNACK, and ACK timings can be used to establish a baseline 268 RTT in each direction. Once the connection is established, the RTT 269 between the server and the measurement point can only reliably be 270 determined using TCP timestamps. On the side between the measurement 271 point and the client, the exact timing of data segments and ACKs can 272 be used as an alternative. For this latter method to be accurate 273 when packet loss is present, the connection must use selective 274 acknowledgements. 276 In many networks, congestion will show up as increasing packet 277 queueing, and congestion-induced packet loss will only happen in 278 extreme cases. RTTs will also show up as a much smoother signal than 279 the discrete packet loss events. This makes RTTs a good way to 280 identify individual subscribers for whom the network is a bottleneck 281 at a given time, or geographical sites (such as cellular towers) that 282 are experiencing large scale congestion. 284 The main limit of RTT measurement as a congestion signal is the 285 difficulty of reliably distinguishing between the data segments being 286 queued vs. the ACKs being queued. 288 2.3. Measuring Packet Reordering 290 If a network is reordering packets of transport connections, caused 291 perhaps by ECMP misconfiguration (e.g., described in [RFC2991] and 292 [RFC7690]), the end-points may react as if packet loss is occurring 293 and retransmit packets or reduce forwarding rates. Therefore a 294 network operator desires the ability to diagnose packet reordering. 296 For TCP, packet reordering can be detected by observing TCP sequence 297 numbers per direction. See for example a number of standard packet 298 reordering metrics in [RFC4737] and informational metrics in 299 [RFC5236]. 301 2.4. Throughput and Bottleneck Identification 303 Although throughput to or from an IP address can be measured without 304 transport-layer measurements, the transport layer provides clues 305 about what the end-points were attempting to do. 307 One way of quickly excluding the network as the bottleneck during 308 troubleshooting is to check whether the speed is limited by the 309 endpoints. For example, the connection speed might instead be 310 limited by suboptimal TCP options, the sender's congestion window, 311 the sender temporarily running out of data to send, the sender 312 waiting for the receiver to send another request, or the receiver 313 closing the receive window. 315 This data is also useful for middleboxes used to measure network 316 quality of service. Connections, or portions of connections, that 317 are limited by the endpoints do not provide an accurate measure of 318 network's speed, and can be discounted or completely excluded in such 319 analyses. 321 2.5. Congestion Responsiveness 323 Congestion control mechanisms continue to evolve. Tools exist that 324 can interpret protocol sequence numbers (e.g., from TCP, RTP) to 325 infer the congestion response of a flow. Such tools can be used by 326 operators to help understand the impact of specific transport 327 protocols on other traffic that shares their network. For example, 328 analysing packet sequence numbers can be used to help understand 329 whether an application flow backs-off its load in the face of 330 persistent congestion (as TCP does), and hence to understand whether 331 the behaviour is appropriate for sharing limited network capacity. 333 These tools can also be used to determine whether mechanisms are 334 needed in the network to prevent flows from acquiring excessive 335 network capacity under severe congestion (e.g., by deploying rate- 336 limiters or network transport circuit breakers [RFC8084]). 338 2.6. Attack Detection 340 When an application or network resource is under attack, it is useful 341 to identify this situation from the network perspective, upstream of 342 the attacked resource. 344 Although detection methods tend to be proprietary, attack detection 345 from within the network may comprise: 347 o Identifying uncharacteristic traffic volumes or sources; 349 o Identifying congestion, possibly using techniques in Section 2.1 350 and Section 2.2; 352 o Identifying incomplete connections or transactions, from attacks 353 which attempt to exhaust server resources; 355 o Fingerprinting based on whatever available fields are determined 356 to be useful in discriminating an attack from desirable traffic. 358 Two trends in protocol design will make attack detection more 359 difficult: 361 o the desire to encrypt transport-layer fields; 363 o the desire to avoid statistical fingerprinting by adding entropy 364 in various forms. 366 While improving privacy, those approaches may hinder attack 367 detection. 369 2.7. Packet Corruption 371 One notable source of packet loss is packet corruption. This 372 corruption will generally not be detected until the checksums are 373 validated by the endpoint, and the packet is dropped. This means 374 that detecting the exact location where packets are lost is not 375 sufficient when troubleshooting networks. An operator would like to 376 find out where packets are being corrupted. IP and TCP checksum 377 verification allows a measurement device to correctly distinguish 378 between upstream packet corruption and normal downstream packet loss. 380 Transport protocol designers should consider whether a middlebox will 381 be able to detect corrupted or tampered packets. 383 2.8. Application-Layer Measurements 385 Network health may also be gleaned from application-layer diagnosis. 386 E.g., 388 o DNS response times and retransmissions by correlating answers to 389 queries. 391 o Various protocol-aware voice and video quality analysis. 393 Could this type of information be provided in a transport layer? 395 3. Functions Beyond Measurement: A Few Examples 397 This section describes features provided by in-line devices that go 398 beyond measurement by modifying, discarding, delaying, or 399 prioritizing traffic. 401 3.1. NAT 403 Network Address Translators (NATs) allow multiple devices to share a 404 public address by dividing the transport-layer port space among the 405 devices. 407 NAT behavior recommendations are found for UDP in BCP 127 [RFC4787] 408 and for TCP in BCP 142 [RFC7857]. 410 To support NAT, there must be transport-layer port numbers that can 411 be modified by the network. The application-layer must not assume 412 the port number was left unchanged (e.g., by including it in a 413 checksum or signing it). 415 Address sharing is also used in the context of IPv6 transition. For 416 example, DS-Lite AFTR [RFC6333], NAT64 [RFC6146], or MAP-* are 417 features that are enabled in the network to allow for IPv4 service 418 continuity over an IPv6 network. 420 Further, because of some multi-homing considerations, IPv6 prefix 421 translation may be enabled by some enterprises by means of NPTv6 422 [RFC6296]. 424 3.2. Firewall 426 Firewalls are pervasive and essential components that inspect 427 incoming and outgoing traffic. Firewalls are usually the cornerstone 428 of a security policy that is enforced in end-user premises and other 429 locations to provide strict guarantees about traffic that may be 430 authorized to enter/leave the said premises, as well as end-users who 431 may be assigned different clearance levels regarding which networks 432 and portions of the Internet they may acess. 434 An important aspect of a firewall policy is differentiating 435 internally-initiated from externally-initiated communications. 437 For TCP, this is easily done by tracking the TCP state machine. 438 Furthermore, the ending of a TCP connection is indicated by RST or 439 FIN flags. 441 For UDP, the firewall can be opened if the first packet comes from 442 an internal user, but the closing is generally done by an idle 443 timer of arbitrary duration, which might not match the 444 expectations of the application. 446 Simple IPv6 firewall capabilities for customer premises equipment 447 (both stateless and stateful) are described in [RFC6092]. 449 A firewall functions better when it can observe the protocol state 450 machine, described generally by Transport-Independent Path Layer 451 State Management [I-D.trammell-plus-statefulness]. 453 3.3. DDoS Scrubbing 455 In the context of a distributed denial-of-service (DDoS) attack, the 456 purpose of a scrubber is to discard attack traffic while permitting 457 useful traffic. E.g., such a mitigator is described in 458 [I-D.ietf-dots-architecture]. 460 When attacks occur against constrained resources, some traffic will 461 be discarded before reaching the intended destination. A user 462 receives better experience and a server runs more efficiently if a 463 scrubber can discard attack traffic, leaving room for legitimate 464 traffic. 466 Scrubbing must be provided by an on-path network device because 467 neither end-point of a legitimate connection has any control over the 468 source of the attack traffic. 470 Source-spoofed DDoS attacks can be mitigated at the source using BCP 471 38 ([RFC2827]), but it is more difficult if source address filtering 472 cannot be applied. 474 In contrast to devices in the core of the Internet, middleboxes 475 statefully observing bidirectional transport connections can reject 476 source-spoofed TCP traffic based on the inability to provide sensible 477 acknowledgement numbers to complete the three-way handshake. 479 Obviously this requires middlebox visibility into transport-layer 480 state machine. 482 Middleboxes may also scrub on the basis of statistical 483 classification: testing how likely a given packet is legitimate. As 484 protocol designers add more entropy to headers and lengths, this test 485 becomes less useful and the best scrubbing strategy becomes random 486 drop. 488 3.4. Implicit Identification 490 In order to enhance the end-user's quality of experience, some 491 operators deploy implicit identification features that rely upon the 492 correlation of network-related information to access some local 493 services. For example, service portals operated by some operators 494 may be accessed immediately by end-users without any explicit 495 identification for the sake of improved service availability. This 496 is doable thanks to on-path devices that inject appropriate metadata 497 that can be used by the remote server to enforce per-subscriber 498 policies. The information can be injected at the application layer 499 or at the transport layer (when an address sharing mechanism is in 500 use). 502 An experimental implementation using a TCP option is described in 503 [RFC7974]. 505 For the intended use of implicit identification, it is more secure to 506 have a trusted middlebox mark this traffic than to trust end-user 507 devices. 509 3.5. Performance-Enhancing Proxies 511 Performance-Enhancing Proxies (PEPs) can improve performance in some 512 types of networks by improving packet spacing or generating local 513 acknowledgements, and are most commonly used in satellite and 514 cellular networks. Transport-Layer PEPs are described in section 515 2.1.1 of [RFC3135]. 517 PEPs allow central deployment of congestion control algorithms more 518 suited to the specific network, most commonly use of delay-based 519 congestion control. More advanced TCP PEPs deploy congestion control 520 systems that treat all of a single end-user's TCP connections as a 521 single unit, improving fairness and allowing faster reaction to 522 changing network conditions. 524 Local acknowledgements generated by PEPs speed up TCP slow start by 525 splitting the effective latency, and allow for retransmissions to be 526 done from the PEP rather than from the actual sender, saving downlink 527 bandwidth on retransmissions. Local acknowledgements will also allow 528 a PEP to maintain a local buffer of data appropriate to the actual 529 network conditions, whereas the actual endpoints would often send too 530 much or too little. 532 A PEP function requires transport-layer fields that allow chunks of 533 data to be identified (e.g., TCP sequence numbers), acknowledgements 534 to be identified (e.g., TCP ACK numbers), and acknowledgements to be 535 created from the PEP. 537 Note that PEPs are only useful in some types of networks, and poor 538 design could make performance worse. 540 3.6. Network Coding 542 Network Coding is a technique for improving transmission performance 543 over low-bandwidth, long-latency links such as satellite links. 544 Coding may involve lossless compression and/or adding redundancy to 545 headers and payload. A Network Coding Taxonomy is provided by 546 [I-D.irtf-nwcrg-network-coding-taxonomy]. It is typically deployed 547 with network-coding gateways at each end of those links, with a 548 network-coding tunnel between them via the slow/lossy/long-latency 549 links. 551 Network coding implementations may be specific to TCP, taking 552 advantage of known properties of the protocol. 554 The network coding gateways may employ some techniques of PEPs, such 555 as creating acknowledgements of queued data, removing retransmissions 556 and pacing data rates to reduce queue oscillation. 558 Note: this is not to be confused with transcoding, which performs 559 lossy compression on transmitted media streams, and not in scope for 560 this document. 562 3.7. Network-Assisted Bandwidth Aggregation 564 The Hybrid Access Aggregation Point (HAAP) is a middlebox that allows 565 customers to aggregate the bandwidth of multiple access technologies 566 [I-D.zhang-banana-problem-statement]. 568 One of the approaches uses MPTCP proxies 569 [I-D.nam-mptcp-deployment-considerations] to forward traffic along 570 multiple paths. The MPTCP proxy operates at the transport layer 571 while being located in the operator's network. 573 The support of multipath transport capabilities by communicating 574 hosts remains a privileged target design so that such hosts can 575 directly use the available resources provided by a variety of access 576 networks they can connect to. Nevertheless, network operators do not 577 control end hosts while the support of MPTCP by content servers 578 remains marginal. 580 Network-Assisted MPTCP deployment models are designed to facilitate 581 the adoption of MPTCP for the establishment of multi-path 582 communications without making any assumption about the support of 583 MPTCP capabilities by communicating peers. Network-Assisted MPTCP 584 deployment models rely upon MPTCP Conversion Points (MCPs) that act 585 on behalf of hosts so that they can take advantage of establishing 586 communications over multiple paths [I-D.boucadair-mptcp-plain-mode]. 588 Note there are cases when end-to-end MPTCP cannot be used even though 589 both client and server are MPTCP-compliant. An MPTCP proxy can 590 provide multipath utilization in these cases. Examples of such cases 591 are listed below: 593 1. The use of private IPv4 addresses in some access networks. 594 Typically, additional subflows can not be added to the MPTCP 595 connection without the help of an MCP. 597 2. The assignment of IPv6 prefixes only by some networks. If the 598 server is IPv4-only, IPv6 subflows cannot be added to an MPTCP 599 connection established with that server, by definition. 601 3. Subscription to some service offerings is subject to volume 602 quota. 604 3.8. Prioritization and Differentiated Services 606 Bulk traffic may be served with a higher latency than interactive 607 traffic with no reduction in throughput. This fact allows a 608 middlebox function to improve response times in interactive 609 applications by prioritizing, policing, or remarking interactive 610 transport connections differently from bulk traffic transport 611 connections. E.g., gaming traffic may be prioritized over email or 612 software updates. 614 Middleboxes may identify different classes of traffic by inspecting 615 multiple layers of header and length of payload. 617 3.9. Measurement-Based Shaping 619 Basic traffic shaping functionality requires no transport-layer 620 information. All that is needed is a way of mapping each packet to a 621 traffic shaper quota. For example, there may be a rate limit per 622 5-tuple or per subscriber IP address. However, such fixed traffic 623 shaping rules are wasteful as they end up rate limiting traffic even 624 when the network has free resources available. 626 More advanced traffic shaping devices use transport layer metrics 627 described in Section 2 to detect congestion on either a per-site or 628 per-user level, and use different traffic shaping rules when 629 congestion is detected. This type of device can overcome limitations 630 of down-stream devices that behave poorly (e.g., by excessive 631 buffering or sub-optimally dropping packets). 633 3.10. Fairness to End-User Quota 635 Several service offerings rely upon a volume-based charging model. 636 Operators may assist end-users in conserving their data quota by 637 deploying on-path functions that shape traffic that would otherwise 638 be agressively transferred. 640 For example, a fast download of a video that won't be viewed 641 completely by the subscriber may lead to quick exhaustion of the data 642 quota. Limiting the video download rate conserves quota for the 643 benefit of the end-user. 645 4. Acknowledgements 647 The authors thank Brian Trammell, Brian Carpenter, Mirja Kuehlewind, 648 Kathleen Moriarty, and Gorry Fairhurst for their review and 649 suggestions. 651 5. IANA Considerations 653 This memo includes no request to IANA. 655 6. Security Considerations 657 6.1. Confidentiality 659 This document intentionally excludes middleboxes that observe or 660 manipulate application-layer data. 662 The measurements and functions described in this document can all be 663 implemented without violating confidentiality. However, there is 664 always the question of whether the fields and packet properties used 665 to achieve operational benefits may also be used for harm. 667 In particular, we want to ask what confidentiality is lost by 668 exposing transport-layer fields beyond what can be learned by 669 observing IP-layer fields. 671 Sequence numbers: an observer can learn how much data is transferred. 673 Start/Stop indicators: an observer can count transactions for some 674 applications. 676 Device fingerprinting: an observer may be more easily able to 677 identify a device type when different devices use different default 678 field values or options. 680 6.2. Active Attacks 682 Being able to observe sequence numbers or session identifiers may 683 make it easier to modify or terminate a transport connection. E.g., 684 observing TCP sequence numbers allows generation of a RST packet that 685 terminates the connection. However, signing transport fields 686 mitigates this attack. The attack and solution are described for the 687 TCP authentication option [RFC5925]. 689 6.3. More Information Can Improve Security 691 Proposition: network maintainability and security can be improved by 692 providing firewalls and DDoS mechanisms with some information about 693 transport connections. In contrast, it would be very difficult to 694 secure a network in which every packet appears unique and filled with 695 random bits. 697 For denial-of-service (DoS) attacks on bandwidth, the receiving end- 698 point is usually on the wrong side of the constrained network link. 699 This fact makes it seem reasonable to give some clues to allow a 700 middlebox device to help out before the constrained link. 702 E.g., in a blind attack, an attacker cannot receive data from the 703 target of the attack (section 4.6.3.2 of [RFC3552]). In the case of 704 TCP, the blind attacker cannot complete the three-way handshake. 706 In the balance, some features providing the ability to mitigate/ 707 filter attacks and fix broken networks will improve security vs. the 708 scenario when all packets are completely opaque. 710 7. References 712 7.1. Normative References 714 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 715 Requirement Levels", BCP 14, RFC 2119, 716 DOI 10.17487/RFC2119, March 1997, 717 . 719 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 720 Defeating Denial of Service Attacks which employ IP Source 721 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 722 May 2000, . 724 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 725 Text on Security Considerations", BCP 72, RFC 3552, 726 DOI 10.17487/RFC3552, July 2003, 727 . 729 [RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, 730 S., and J. Perser, "Packet Reordering Metrics", RFC 4737, 731 DOI 10.17487/RFC4737, November 2006, 732 . 734 [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address 735 Translation (NAT) Behavioral Requirements for Unicast 736 UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 737 2007, . 739 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP 740 Authentication Option", RFC 5925, DOI 10.17487/RFC5925, 741 June 2010, . 743 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 744 NAT64: Network Address and Protocol Translation from IPv6 745 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 746 April 2011, . 748 [RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual- 749 Stack Lite Broadband Deployments Following IPv4 750 Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011, 751 . 753 [RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar, 754 S., and K. Naito, "Updates to Network Address Translation 755 (NAT) Behavioral Requirements", BCP 127, RFC 7857, 756 DOI 10.17487/RFC7857, April 2016, 757 . 759 7.2. Informative References 761 [I-D.boucadair-mptcp-plain-mode] 762 Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel, 763 D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R., 764 Vinapamula, S., Seo, S., Cloetens, W., Meyer, U., 765 Contreras, L., and B. Peirens, "Extensions for Network- 766 Assisted MPTCP Deployment Models", draft-boucadair-mptcp- 767 plain-mode-10 (work in progress), March 2017. 769 [I-D.ietf-dots-architecture] 770 Mortensen, A., Andreasen, F., Reddy, T., 771 christopher_gray3@cable.comcast.com, c., Compton, R., and 772 N. Teague, "Distributed-Denial-of-Service Open Threat 773 Signaling (DOTS) Architecture", draft-ietf-dots- 774 architecture-05 (work in progress), October 2017. 776 [I-D.ietf-sfc-use-case-mobility] 777 Haeffner, W., Napper, J., Stiemerling, M., Lopez, D., and 778 J. Uttaro, "Service Function Chaining Use Cases in Mobile 779 Networks", draft-ietf-sfc-use-case-mobility-07 (work in 780 progress), October 2016. 782 [I-D.irtf-nwcrg-network-coding-taxonomy] 783 Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek, 784 F., samah.ghanem@gmail.com, s., Lochin, E., Masucci, A., 785 Montpetit, M., Pedersen, M., Peralta, G., Roca, V., 786 Saxena, P., and S. Sivakumar, "Taxonomy of Coding 787 Techniques for Efficient Network Communications", draft- 788 irtf-nwcrg-network-coding-taxonomy-07 (work in progress), 789 February 2018. 791 [I-D.mm-wg-effect-encrypt] 792 Moriarty, K. and A. Morton, "Effects of Pervasive 793 Encryption on Operators", draft-mm-wg-effect-encrypt-22 794 (work in progress), February 2018. 796 [I-D.nam-mptcp-deployment-considerations] 797 Boucadair, M., Jacquenet, C., Bonaventure, O., Henderickx, 798 W., and R. Skog, "Network-Assisted MPTCP: Use Cases, 799 Deployment Scenarios and Operational Considerations", 800 draft-nam-mptcp-deployment-considerations-01 (work in 801 progress), December 2016. 803 [I-D.trammell-plus-statefulness] 804 Kuehlewind, M., Trammell, B., and J. Hildebrand, 805 "Transport-Independent Path Layer State Management", 806 draft-trammell-plus-statefulness-04 (work in progress), 807 November 2017. 809 [I-D.zhang-banana-problem-statement] 810 Cullen, M., Leymann, N., Heidemann, C., Boucadair, M., 811 Hui, D., Zhang, M., and B. Sarikaya, "Problem Statement: 812 Bandwidth Aggregation for Internet Access", draft-zhang- 813 banana-problem-statement-03 (work in progress), October 814 2016. 816 [RFC2991] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and 817 Multicast Next-Hop Selection", RFC 2991, 818 DOI 10.17487/RFC2991, November 2000, 819 . 821 [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. 822 Shelby, "Performance Enhancing Proxies Intended to 823 Mitigate Link-Related Degradations", RFC 3135, 824 DOI 10.17487/RFC3135, June 2001, 825 . 827 [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and 828 Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002, 829 . 831 [RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R. 832 Whitner, "Improved Packet Reordering Metrics", RFC 5236, 833 DOI 10.17487/RFC5236, June 2008, 834 . 836 [RFC5853] Hautakorpi, J., Ed., Camarillo, G., Penfield, R., 837 Hawrylyshen, A., and M. Bhatia, "Requirements from Session 838 Initiation Protocol (SIP) Session Border Control (SBC) 839 Deployments", RFC 5853, DOI 10.17487/RFC5853, April 2010, 840 . 842 [RFC6092] Woodyatt, J., Ed., "Recommended Simple Security 843 Capabilities in Customer Premises Equipment (CPE) for 844 Providing Residential IPv6 Internet Service", RFC 6092, 845 DOI 10.17487/RFC6092, January 2011, 846 . 848 [RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and 849 P. Roberts, "Issues with IP Address Sharing", RFC 6269, 850 DOI 10.17487/RFC6269, June 2011, 851 . 853 [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix 854 Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011, 855 . 857 [RFC6459] Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen, 858 T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation 859 Partnership Project (3GPP) Evolved Packet System (EPS)", 860 RFC 6459, DOI 10.17487/RFC6459, January 2012, 861 . 863 [RFC7690] Byerly, M., Hite, M., and J. Jaeggli, "Close Encounters of 864 the ICMP Type 2 Kind (Near Misses with ICMPv6 Packet Too 865 Big (PTB))", RFC 7690, DOI 10.17487/RFC7690, January 2016, 866 . 868 [RFC7974] Williams, B., Boucadair, M., and D. Wing, "An Experimental 869 TCP Option for Host Identification", RFC 7974, 870 DOI 10.17487/RFC7974, October 2016, 871 . 873 [RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", 874 BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017, 875 . 877 Authors' Addresses 879 David Dolson 881 Email: ddolson@acm.org 883 Juho Snellman 885 Email: jsnell@iki.fi 887 Mohamed Boucadair 888 Orange 889 4 rue du Clos Courtel 890 Rennes 35000 891 France 893 Email: mohamed.boucadair@orange.com 895 Christian Jacquenet 896 Orange 897 4 rue du Clos Courtel 898 Rennes 35000 899 France 901 Email: christian.jacquenet@orange.com