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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-08) exists of draft-hardie-privsec-metadata-insertion-03 == Outdated reference: A later version (-04) exists of draft-trammell-plus-statefulness-00 -- Obsolete informational reference (is this intentional?): RFC 793 (Obsoleted by RFC 9293) Summary: 0 errors (**), 0 flaws (~~), 4 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 October 28, 2016 4 Intended status: Informational 5 Expires: May 1, 2017 7 Path signals 8 draft-hardie-path-signals-00 10 Abstract 12 TCP's state mechanics uses a series of well-known messages that are 13 exchanged in the clear. Because these are visible to network 14 elements on the path between the two nodes setting up the transport 15 connection, they are often used as signals by those network elements. 16 In transports that do not exchange these messages in the clear, on- 17 path network elements lack those signals. This document discusses 18 the nature of the signals as they are seen by on-path elements and 19 reflects on best practices for transports which encrypt their state 20 mechanics. 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 May 1, 2017. 39 Copyright Notice 41 Copyright (c) 2016 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 3. Signals Type Inferred . . . . . . . . . . . . . . . . . . . . 3 59 3.1. Session establishment . . . . . . . . . . . . . . . . . . 3 60 3.1.1. Session identity . . . . . . . . . . . . . . . . . . 3 61 3.1.2. Routability and Consent . . . . . . . . . . . . . . . 3 62 3.1.3. Resource Requirements . . . . . . . . . . . . . . . . 4 63 3.2. Network Measurement . . . . . . . . . . . . . . . . . . . 4 64 3.2.1. Path Latency . . . . . . . . . . . . . . . . . . . . 4 65 3.2.2. Path reliability and consistency . . . . . . . . . . 4 66 4. Options . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 4.1. Do not restore these signals . . . . . . . . . . . . . . 4 68 4.2. Replace these with network layer signals . . . . . . . . 5 69 4.3. Replace these with per-transport signals . . . . . . . . 5 70 4.4. Create a set of signals common to multiple transports . . 5 71 5. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5 72 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 73 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 74 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 75 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 76 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 77 9.2. Informative References . . . . . . . . . . . . . . . . . 7 78 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 80 1. Terminology 82 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 83 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 84 document are to be interpreted as described in RFC 2119 [RFC2119]. 86 2. Introduction 88 TCP [RFC0793] uses handshake messages to establish, maintain, and 89 close connections. While these are primarily intended to create 90 state between two communicating nodes, these handshake messages are 91 visible to network elements along the path between them. It has been 92 common over time for certain network elements to treat the exchanged 93 messages as signals which related to their own functions. 95 A firewall may, for example, create a rule that allows traffic from a 96 specific host and port to enter its network when the connection was 97 initiated by a host already within the network. It may subsequently 98 remove that rule when the communication has ceased. In the context 99 of TCP handshake, it sets up the pinhole rule on seeing the initial 100 TCP SYN acknowledged and then removes it upon seeing a RST or FIN & 101 ACK exchange. Note that in this case it does nothing to re-write any 102 portion of the TCP packet; it simply enables a return path that would 103 otherwise have been blocked. 105 When a transport encrypts the headers it uses for state mechanics, 106 the signal path elements inferred from examination is no longer 107 available. Their behavior in its absence will depend on which signal 108 is not available, on the default behavior configured by the path 109 element administrator, and by the security posture of the network as 110 a whole. 112 3. Signals Type Inferred 114 The following list of signals which may be inferred from transport 115 state messages includes those which may be exchanged during sessions 116 establishment and those which derive from the ongoing flow. Some of 117 these signals are derived from the direct examination of packet 118 trains, such as using a sequence number gap pattern to infer network 119 reliability; others are derived from association, such as inferring 120 network latency by timing a flow's packet inter-arrival times. This 121 list is not exhaustive, and it is not the full set of effects due to 122 encrypting data and metadata in flight. 124 3.1. Session establishment 126 One of the most basic inferences made by examination of transport 127 state is that a packet will be part of an ongoing flow; that is, an 128 established session will continue until messages are received that 129 terminate it. Path elements may then make subsidiary inferences 130 related to the session. 132 3.1.1. Session identity 134 Path elements that track session establishment will typically create 135 a session identify for the flow, commonly using a tuple of the 136 visible information in the packet headers. This is then used to 137 associate other information with the 139 3.1.2. Routability and Consent 141 A second common inference is that the session establishment provides 142 is that the communicating pair of hosts can each reach each other and 143 are interested in continuing communication. The firewall example 144 given above is a consequence of the inference of consent; because the 145 internal host initiates the connection, it is presumed to consent to 146 return traffic. That, in turn justifies the pinhole. 148 3.1.3. Resource Requirements 150 An additional common inference is that network resources will be 151 required for the session. These may be requirements within the 152 network element itself, such as table entry space for a firewall or 153 NAT; they may also be communicated by the network element to other 154 systems. For networks which use resource reservations, this might 155 result in reservation of radio air time, energy, or network capacity. 157 3.2. Network Measurement 159 Some network elements will also use transport messages to engage in 160 measurement of the paths which are used by flows on their network. 161 The list of measurements below is illustrative, not exhaustive. 163 3.2.1. Path Latency 165 There are several ways in which a network element may measure path 166 latency using transport messages, but two common ones are examining 167 exposed timestamps and associating sequence numbers with a local 168 timer. These measurements are necessarily limited to measuring only 169 the portion of the path between the system which assigned the 170 timestamp or sequence number and the network element. 172 3.2.2. Path reliability and consistency 174 A network element may also measure the reliability of a particular 175 path by examining sessions which expose sequence numbers; 176 retransmissions and gaps are then associated with the path segments 177 on which they might have occurred. 179 4. Options 181 The set of options below are alternatives which optimize very 182 different things. Though it comes to a preliminary conclusion, this 183 draft intends to foster a discussion of those tradeoffs and any 184 discussion of them must be understood as preliminary. 186 4.1. Do not restore these signals 188 It is possible, of course, to do nothing. The transport messages 189 were not necessarily intended for consumption by on-path network 190 elements and encrypting so they are not visible may be taken by some 191 as a benefit. Each network element would then treat packets without 192 these visible elements according to its own defaults. While our 193 experience of that is not extensive, one consequence has been that 194 state tables for flows of this type are generally not kept as long as 195 those for which sessions are identifiable. The result is that 196 heartbeat traffic must be maintained to keep any bindings (e.g. NAT 197 or firewall) from early expiry. 199 4.2. Replace these with network layer signals 201 It would be possible to replace these implicit signals with explicit 202 signals at the network layer. Though IPv4 has relatively few 203 facilities for this, IPv6 hop-by-hop headers [RFC7045] might suit 204 this purpose. Further examination of the deployability of these 205 headers may be required. 207 4.3. Replace these with per-transport signals 209 It is possible to replace these implicit signals with signals that 210 are tailored to specific transports, just as the initial signals are 211 derived primarily from TCP. There is a risk here that the first 212 transport which develops these will be reused for many purposes 213 outside its stated purpose, simply because it traverses NATs and 214 firewalls better than other traffic. If done with an explicit intent 215 to re-use the elements of the solution in other transports, the risk 216 of ossification might be slightly lower. 218 4.4. Create a set of signals common to multiple transports 220 Several proposals use UDP[RFC0768] as a demux layer, onto which new 221 transport semantics are layered. For those transports, it may be 222 possible to build a common signalling mechanism and set of signals, 223 such as that proposed in "Transport-Independent Path Layer State 224 Management" [I-D.trammell-plus-statefulness]. 226 This may be taken as a variant of the re-use of common elements 227 mentioned in the section above, but it has a greater chance of 228 avoiding the ossification of the solution into the first moving 229 protocol. 231 5. Recommendation 233 Fundamentally, this paper recommends that implicit signals should be 234 replaced with explicit signals, but that a signal should exposed to 235 the path only when the signal's originator intends that it be used by 236 the network elements on the path. For many flows, that may result in 237 signal being present, but it allows them to be present when needed. 239 Discussion of the appropriate mechanism(s) for these signals is 240 continuing but, at minimum, any method should meet the principles set 241 out in the security considerations below. 243 6. IANA Considerations 245 This document contains no requests for IANA. 247 7. Security Considerations 249 Addition of visible signals to the path allows network elements along 250 the path to act. If the network element is controlled by an 251 attacker, those actions can include dropping, delaying, or 252 mishandling the constituent packets of a flow. 254 Note that actions that do not benefit the flow or the network may be 255 perceived as an attack even if they are conducted by a responsible 256 network element. Designing a system that minimizes the ability to 257 act on signals at all by removing as many signals as possible may 258 reduce this possibility. This approach also comes with risks, 259 principally that the actions will continue to take place on an 260 arbitrary set of flows. 262 Addition of visible signals to the path also increases the 263 information available to an observer and may, when the information 264 can be linked to a node or user, reduce the privacy of the user. 266 This document recommends three basic principles: 268 o Cryptographic contexts should be available on any flow, derived 269 from ubiquitous end-system cryptographic capabilities. 271 o Anything exposed to the path should be done with the intent that 272 it be used by the network elements on the path. 274 o Intermediate path elements should not add visible signals which 275 identify the user, origin node, or origin network 276 [I-D.hardie-privsec-metadata-insertion]. 278 8. Acknowledgements 280 In addition to the editor listed above, this document incorporates 281 contributions from Brian Trammel, Mirja Kuehlwind, and Joe 282 Hildebrand. These ideas were also discussed at the PLUS BoF, 283 sponsored by Spencer Dawkins. The ideas around the use of IPv6 hop- 284 by-hop headers as a network layer signal benefited from discussions 285 with Tom Herbert. The description of UDP as a demuxing protocol 286 comes from Stuart Cheshire. 288 All errors are those of the editor. 290 9. References 292 9.1. Normative References 294 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 295 Requirement Levels", BCP 14, RFC 2119, 296 DOI 10.17487/RFC2119, March 1997, 297 . 299 9.2. Informative References 301 [I-D.hardie-privsec-metadata-insertion] 302 Hardie, T., "Design considerations for Metadata 303 Insertion", draft-hardie-privsec-metadata-insertion-03 304 (work in progress), October 2016. 306 [I-D.trammell-plus-statefulness] 307 Kuehlewind, M., Trammell, B., and J. Hildebrand, 308 "Transport-Independent Path Layer State Management", 309 draft-trammell-plus-statefulness-00 (work in progress), 310 October 2016. 312 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 313 DOI 10.17487/RFC0768, August 1980, 314 . 316 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 317 RFC 793, DOI 10.17487/RFC0793, September 1981, 318 . 320 [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing 321 of IPv6 Extension Headers", RFC 7045, 322 DOI 10.17487/RFC7045, December 2013, 323 . 325 Author's Address 327 Ted Hardie (editor) 329 Email: ted.ietf@gmail.com