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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-34) exists of draft-ietf-quic-transport-11 -- Obsolete informational reference (is this intentional?): RFC 793 (Obsoleted by RFC 9293) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group T. Hardie, Ed. 3 Internet-Draft May 16, 2018 4 Intended status: Informational 5 Expires: November 17, 2018 7 Path Signals 8 draft-iab-path-signals-00 10 Abstract 12 This document discusses the nature of signals seen by on-path 13 elements, contrasting implicit and explicit signals. For example, 14 TCP's state mechanics uses a series of well-known messages that are 15 exchanged in the clear. Because these are visible to network 16 elements on the path between the two nodes setting up the transport 17 connection, they are often used as signals by those network elements. 18 In transports that do not exchange these messages in the clear, on- 19 path network elements lack those signals. This document recommends 20 that explict signals be used by transports which encrypt their state 21 mechanics. It also recommends that a signal be exposed to the path 22 only when the signal's originator intends that it be used by the 23 network elements on the path. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at https://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on November 17, 2018. 42 Copyright Notice 44 Copyright (c) 2018 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (https://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 60 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 3. Signals Type Inferred . . . . . . . . . . . . . . . . . . . . 3 62 3.1. Session Establishment . . . . . . . . . . . . . . . . . . 4 63 3.1.1. Session Identity . . . . . . . . . . . . . . . . . . 4 64 3.1.2. Routability and Consent . . . . . . . . . . . . . . . 4 65 3.1.3. Flow Stability . . . . . . . . . . . . . . . . . . . 4 66 3.1.4. Resource Requirements . . . . . . . . . . . . . . . . 4 67 3.2. Network Measurement . . . . . . . . . . . . . . . . . . . 5 68 3.2.1. Path Latency . . . . . . . . . . . . . . . . . . . . 5 69 3.2.2. Path Reliability and Consistency . . . . . . . . . . 5 70 4. Options . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 71 4.1. Do Not Restore These Signals . . . . . . . . . . . . . . 5 72 4.2. Replace These With Network Layer Signals . . . . . . . . 6 73 4.3. Replace These With Per-Transport Signals . . . . . . . . 6 74 4.4. Create a Set of Signals Common to Multiple Transports . . 6 75 5. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 6 76 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 77 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 78 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 79 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 80 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 81 9.2. Informative References . . . . . . . . . . . . . . . . . 8 82 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 84 1. Terminology 86 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 87 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 88 document are to be interpreted as described in RFC 2119 [RFC2119]. 90 2. Introduction 92 This document discusses the nature of signals seen by on-path 93 elements, contrasting implicit and explicit signals. For example, 94 TCP's state mechanics uses a series of well-known messages that are 95 exchanged in the clear. Because these are visible to network 96 elements on the path between the two nodes setting up the transport 97 connection, they are often used as signals by those network elements. 98 In transports that do not exchange these messages in the clear, on- 99 path network elements lack those signals. This document recommends 100 that explict signals be used by transports which encrypt their state 101 mechanics. It also recommends that a signal be exposed to the path 102 only when the signal's originator intends that it be used by the 103 network elements on the path. 105 The interpretation of TCP [RFC0793] by on-path elements is an exmple 106 of implicit signal usage. It uses cleartext handshake messages to 107 establish, maintain, and close connections. While these are 108 primarily intended to create state between two communicating nodes, 109 these handshake messages are visible to network elements along the 110 path between them. It is common for certain network elements to 111 treat the exchanged messages as signals which relate to their own 112 functions. 114 A firewall may, for example, create a rule that allows traffic from a 115 specific host and port to enter its network when the connection was 116 initiated by a host already within the network. It may subsequently 117 remove that rule when the communication has ceased. In the context 118 of TCP handshake, it sets up the pinhole rule on seeing the initial 119 TCP SYN acknowledgement and then removes it upon seeing a RST or FIN 120 & ACK exchange. Note that in this case it does nothing to re-write 121 any portion of the TCP packet; it simply enables a return path that 122 would otherwise have been blocked. 124 When a transport encrypts the fields it uses for state mechanics, 125 these signals are no longer accessible to path elements. The 126 behavior of path elements will then depend on which signal is not 127 available, on the default behavior configured by the path element 128 administrator, and by the security posture of the network as a whole. 130 3. Signals Type Inferred 132 The following list of signals which may be inferred from transport 133 state messages includes those which may be exchanged during sessions 134 establishment and those which derive from the ongoing flow. 136 Some of these signals are derived from the direct examination of 137 packet trains, such as using a sequence number gap pattern to infer 138 network reliability; others are derived from association, such as 139 inferring network latency by timing a flow's packet inter-arrival 140 times. 142 This list is not exhaustive, and it is not the full set of effects 143 due to encrypting data and metadata in flight. Note as well that 144 because these are derived from inference, they do not include any 145 path signals which would not be relevant to the end point state 146 machines; indeed, an inference-based system cannot send such signals. 148 3.1. Session Establishment 150 One of the most basic inferences made by examination of transport 151 state is that a packet will be part of an ongoing flow; that is, an 152 established session will continue until messages are received that 153 terminate it. Path elements may then make subsidiary inferences 154 related to the session. 156 3.1.1. Session Identity 158 Path elements that track session establishment will typically create 159 a session identity for the flow, commonly using a tuple of the 160 visible information in the packet headers. This is then used to 161 associate other information with the flow. 163 3.1.2. Routability and Consent 165 A second common inference that session establishment provides is that 166 the communicating pair of hosts can each reach each other and are 167 interested in continuing communication. The firewall example given 168 above is a consequence of the inference of consent; because the 169 internal host initiates the connection, it is presumed to consent to 170 return traffic. That, in turn justifies the pinhole. 172 Some other on-path elements ( assume that a host which asked to 173 communicate with a remote address consents to establish incoming 174 communications from any other host (Endpoint-Independent Mapping/ 175 Endpoint-Independent Filtering). This is, for example, the default 176 behavior in NAT64. 178 3.1.3. Flow Stability 180 Some on-path devices that are responsible for load-sharing or load- 181 balancing may be instructed to preserve the same path for a given 182 flow, rather than dispatching packets belonging to the some flow on 183 multiple paths as this may cause packets in the flow to be delivered 184 out of order.. 186 3.1.4. Resource Requirements 188 An additional common inference is that network resources will be 189 required for the session. These may be requirements within the 190 network element itself, such as table entry space for a firewall or 191 NAT; they may also be communicated by the network element to other 192 systems. For networks which use resource reservations, this might 193 result in reservation of radio air time, energy, or network capacity. 195 3.2. Network Measurement 197 Some network elements will also observe transport messages to engage 198 in measurement of the paths which are used by flows on their network. 199 The list of measurements below is illustrative, not exhaustive. 201 3.2.1. Path Latency 203 There are several ways in which a network element may measure path 204 latency using transport messages, but two common ones are examining 205 exposed timestamps and associating sequence numbers with a local 206 timer. These measurements are necessarily limited to measuring only 207 the portion of the path between the system which assigned the 208 timestamp or sequence number and the network element. 210 3.2.2. Path Reliability and Consistency 212 A network element may also measure the reliability of a particular 213 path by examining sessions which expose sequence numbers; 214 retransmissions and gaps are then associated with the path segments 215 on which they might have occurred. 217 4. Options 219 The set of options below are alternatives which optimize very 220 different things. Though it comes to a preliminary conclusion, this 221 draft intends to foster a discussion of those tradeoffs and any 222 discussion of them must be understood as preliminary. 224 4.1. Do Not Restore These Signals 226 It is possible, of course, to do nothing. The transport messages 227 were not necessarily intended for consumption by on-path network 228 elements and encrypting them so they are not visible may be taken by 229 some as a benefit. Each network element would then treat packets 230 without these visible elements according to its own defaults. While 231 our experience of that is not extensive, one consequence has been 232 that state tables for flows of this type are generally not kept as 233 long as those for which sessions are identifiable. The result is 234 that heartbeat traffic must be maintained to keep any bindings (e.g. 235 NAT or firewall) from early expiry. When those bindings are not 236 kept, methods like QUIC's connection-id [QUIC] may be necessary to 237 allow load balancers or other systems to continue to maintain a 238 flow's path to the appropriate peer. 240 4.2. Replace These With Network Layer Signals 242 It would be possible to replace these implicit signals with explicit 243 signals at the network layer. Though IPv4 has relatively few 244 facilities for this, IPv6 hop-by-hop headers [RFC7045] might suit 245 this purpose. Further examination of the deployability of these 246 headers may be required. 248 4.3. Replace These With Per-Transport Signals 250 It is possible to replace these implicit signals with signals that 251 are tailored to specific transports, just as the initial signals are 252 derived primarily from TCP. There is a risk here that the first 253 transport which develops these will be reused for many purposes 254 outside its stated purpose, simply because it traverses NATs and 255 firewalls better than other traffic. If done with an explicit intent 256 to re-use the elements of the solution in other transports, the risk 257 of ossification might be slightly lower. 259 4.4. Create a Set of Signals Common to Multiple Transports 261 Several proposals use UDP [RFC0768] as a demux layer, onto which new 262 transport semantics are layered. For those transports, it may be 263 possible to build a common signalling mechanism and set of signals, 264 such as that proposed in "Transport-Independent Path Layer State 265 Management" [PLUS]. 267 This may be taken as a variant of the re-use of common elements 268 mentioned in the section above, but it has a greater chance of 269 avoiding the ossification of the solution into the first moving 270 protocol. 272 5. Recommendation 274 Fundamentally, this paper recommends that implicit signals should be 275 replaced with explicit signals, but that a signal should be exposed 276 to the path only when the signal's originator intends that it be used 277 by the network elements on the path. For many flows, that may result 278 in signal being absent, but it allows them to be present when needed. 280 Discussion of the appropriate mechanism(s) for these signals is 281 continuing but, at minimum, any method should aim to adhere to these 282 basic principles: 284 o The portion of protocol signaling that is intended for end system 285 state machines should be protected by confidentiality and 286 integrity protection such that it is only available to those end 287 systems. 289 o Anything exposed to the path should be done with the intent that 290 it be used by the network elements on the path. This information 291 should be integrity protected. 293 o Signals exposed to the path should be decoupled from signals that 294 drive the protocol state machines in endpoints. This avoids 295 creating opportunities for additional inference. 297 o Intermediate path elements should not add visible signals which 298 identify the user, origin node, or origin network [RFC8164]. 300 6. IANA Considerations 302 This document contains no requests for IANA. 304 7. Security Considerations 306 Path-visible signals allow network elements along the path to act 307 based on the signaled information, whether the signal is implicit or 308 explicit. If the network element is controlled by an attacker, those 309 actions can include dropping, delaying, or mishandling the 310 constituent packets of a flow. It may also characterize the flow or 311 attempt to fingerprint the communicating nodes based on the pattern 312 of signals. 314 Note that actions that do not benefit the flow or the network may be 315 perceived as an attack even if they are conducted by a responsible 316 network element. Designing a system that minimizes the ability to 317 act on signals at all by removing as many signals as possible may 318 reduce this possibility. This approach also comes with risks, 319 principally that the actions will continue to take place on an 320 arbitrary set of flows. 322 Addition of visible signals to the path also increases the 323 information available to an observer and may, when the information 324 can be linked to a node or user, reduce the privacy of the user. 326 When signals from end points to the path are independent from the 327 signals used by endpoints to manage the flow's state mechanics, they 328 may be falsified by an endpoint without affecting the peer's 329 understanding of the flow's state. For encrypted flows, this 330 divergence is not detectable by on-path devices. 332 8. Acknowledgements 334 In addition to the editor listed above, this document incorporates 335 contributions from Brian Trammell, Mirja Kuehlwind, Martin Thomson, 336 Aaron Falk, Mohamed Boucadair and Joe Hildebrand. These ideas were 337 also discussed at the PLUS BoF, sponsored contributions from Brian 338 Trammell, Mirja Kuehlwind, Martin Thomson, Aaron Falk and Joe 339 Hildebrand. These ideas were also discussed at the PLUS BoF, 340 sponsored by Spencer Dawkins. The ideas around the use of IPv6 hop- 341 by-hop headers as a network layer signal benefited from discussions 342 with Tom Herbert. The description of UDP as a demuxing protocol 343 comes from Stuart Cheshire. 345 All errors are those of the editor. 347 9. References 349 9.1. Normative References 351 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 352 Requirement Levels", BCP 14, RFC 2119, 353 DOI 10.17487/RFC2119, March 1997, 354 . 356 9.2. Informative References 358 [PLUS] Kuehlewind, M., Trammell, B., and J. Hildebrand, 359 "Transport-Independent Path Layer State Management", 360 draft-trammell-plus-statefulness-04 (work in progress), 361 November 2017. 363 [QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 364 and Secure Transport", draft-ietf-quic-transport-11 (work 365 in progress), April 2018. 367 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 368 DOI 10.17487/RFC0768, August 1980, 369 . 371 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 372 RFC 793, DOI 10.17487/RFC0793, September 1981, 373 . 375 [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing 376 of IPv6 Extension Headers", RFC 7045, 377 DOI 10.17487/RFC7045, December 2013, 378 . 380 [RFC8164] Nottingham, M. and M. Thomson, "Opportunistic Security for 381 HTTP/2", RFC 8164, DOI 10.17487/RFC8164, May 2017, 382 . 384 Author's Address 386 Ted Hardie (editor) 388 Email: ted.ietf@gmail.com