Movatterモバイル変換

Internet Architecture Board (IAB)                            B. TrammellRequest for Comments: 8546                                 M. KuehlewindCategory: Informational                                       April 2019ISSN: 2070-1721The Wire Image of a Network ProtocolAbstract   This document defines the wire image, an abstraction of the   information available to an on-path non-participant in a networking   protocol.  This abstraction is intended to shed light on the   implications that increased encryption has for network functions that   use the wire image.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Architecture Board (IAB)   and represents information that the IAB has deemed valuable to   provide for permanent record.  It represents the consensus of the   Internet Architecture Board (IAB).  Documents approved for   publication by the IAB are not candidates for any level of Internet   Standard; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8546.Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.Trammell & Kuehlewind         Informational                     [Page 1]

RFC 8546                       Wire Image                     April 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Definition  . . . . . . . . . . . . . . . . . . . . . . . . .33.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .33.1.  The Extent of the Wire Image  . . . . . . . . . . . . . .43.2.  Obscuring Timing and Sizing Information . . . . . . . . .53.3.  Integrity Protection of the Wire Image  . . . . . . . . .54.  Engineering the Wire Image  . . . . . . . . . . . . . . . . .64.1.  Declaring Protocol Invariants . . . . . . . . . . . . . .74.2.  Trustworthiness of Engineered Signals . . . . . . . . . .75.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .86.  Security Considerations . . . . . . . . . . . . . . . . . . .87.  Informative References  . . . . . . . . . . . . . . . . . . .8   IAB Members at the Time of Approval . . . . . . . . . . . . . . .9   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .9   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .101.  Introduction   A protocol specification defines a set of behaviors for each   participant in the protocol: which lower-layer protocols are used for   which services, how messages are formatted and protected, which   participant sends which message when, how each participant should   respond to each message, and so on.   Implicit in a protocol specification is the information the protocol   radiates toward nonparticipant observers of the messages sent among   participants, often including participants in lower-layer protocols.   Any information that has a clear definition in the protocol's message   format(s), or is implied by that definition, and is not   cryptographically confidentiality protected can be unambiguously   interpreted by those observers.  This information comprises the   protocol's wire image, which we define and discuss in this document.   The wire image, not the protocol's specification, determines how   third parties on the network paths among protocol participants will   interact with that protocol.   The increasing deployment of transport-layer security [RFC8446] to   protect application-layer headers and payload, as well as the   definition and deployment of transport protocols with encrypted   control information such as QUIC [QUIC], brings new relevance to the   question of how third parties on the network paths will interact with   a protocol.  QUIC is, in effect, the first IETF-defined transport   protocol to take care of the minimization of its own wire image to   prevent ossification and improve end-to-end privacy by reducing   information radiation.Trammell & Kuehlewind         Informational                     [Page 2]

RFC 8546                       Wire Image                     April 2019   The flip side of this trend is the impact of a less visible wire   image on various functions driven by third-party observation of the   wire image.  In contrast to ongoing discussions about this tussle,   this document treats the wire image as a pure abstraction, with the   hope that it can shed some light on these discussions.2.  Definition   The wire image of the set of protocols in use for a given   communication is the view of that set of protocols as observed by an   entity not participating in the communication.  It is the sequence of   packets sent by each participant in the communication, including the   content of those packets and metadata about the observation itself:   the time at which each packet is observed and the vantage point of   the observer.3.  Discussion   This definition illustrates some important properties of the wire   image.   It is key that the wire image is not limited to merely "the   unencrypted bits in the header".  In particular, the metadata, such   as sequences of interpacket timing and packet sizes, can be used to   infer other parameters of the behavior of the protocols in use or to   fingerprint protocols and/or specific implementations of those   protocols; seeSection 3.2.   An important implication of this property is that a protocol that   uses confidentiality protection for the headers it needs to operate   can be deliberately designed to have a specified wire image that is   separate from that machinery; seeSection 4.  Note that this is a   capability unique to encrypted protocols.  Parts of a wire image may   also be made visible to devices on path, but immutable through end-   to-end integrity protection; seeSection 3.3.   Portions of the wire image of a protocol stack that are neither   confidentiality protected nor integrity protected are writable by   devices on the path(s) between the endpoints using the protocols.  A   protocol with a wire image that is largely writable operating over a   path with devices that understand the semantics of the protocol's   wire image can modify it in order to induce behaviors at the   protocol's participants.  TCP is one such protocol in the current   Internet.   The term "wire image" can be applied in different scopes: the wire   image of a single packet refers to the information derivable from   observing that one packet in isolation, and the wire image of aTrammell & Kuehlewind         Informational                     [Page 3]

RFC 8546                       Wire Image                     April 2019   single protocol refers to the information derivable from observing   only the headers belonging to that protocol on a sequence of packets   in isolation from other protocols in use for a communication.  SeeSection 3.1 for more.   For a given packet observed at a given point in the network, the wire   image contains information from the entire stack of protocols in use   at that observation point.  This implies that the wire image depends   on the observer as well: each observer may see a slightly different   image of the same communication.   In this document, we assume that only information at the transport   layer and above is delivered end-to-end, and we focus on the   "Internet" wire image: that portion of the wire image at the network   layer and above.  While confidentiality and integrity protection may   be added at multiple layers in the stack, protection below the   network layer does not prevent modification either by the devices   terminating those security associations or by devices on different   segments of the path.3.1.  The Extent of the Wire Image   While we begin this definition as the properties of a sequence of   packets in isolation, this is not how wire images are typically used   by passive observers.  A passive observer will generally consider the   union of all the information in the wire image in all the packets   generated by a given conversation.   Similarly, the wire image of a single protocol is rarely seen in   isolation.  The dynamics of the application and network stacks on   each endpoint use multiple protocols for any higher-level task.  Most   protocols involving user content, for example, are often seen on the   wire together with DNS traffic; the information from the wire image   from each protocol in use for a given communication can be correlated   to infer information about the dynamics of the overlying application.   Information from protocol wire images is also not generally used on   its own but is rather additionally correlated with other context   information available to the observer, e.g., information about other   communications engaged in by each endpoint, information about the   implementations of the protocols at each endpoint, information about   the network and internetwork topology near those endpoints, and so   on.  This context can be used together with information from the wire   image to reach more detailed inferences about endpoint and end-user   behavior.Trammell & Kuehlewind         Informational                     [Page 4]

RFC 8546                       Wire Image                     April 2019   Note also that the wire image is multidimensional.  This implies that   the name "image" is not merely metaphorical and that general image   recognition techniques may be applicable to extracting patterns and   information from it.3.2.  Obscuring Timing and Sizing Information   Cryptography can protect the confidentiality of a protocol's headers   to the extent that forwarding devices do not need the   confidentiality-protected information for basic forwarding   operations.  Ciphersuites and other transmission techniques designed   to prevent timing analysis can also be applied at the sender to   reduce the information content of the metadata portion of the wire   image.  However, there are limits to these techniques.  Packets   cannot be made smaller than their information content, be sent faster   than processing time requirements at the sender allow, or be   transmitted through the network faster than the speed of light.   Since these techniques operate at the expense of bandwidth efficiency   and latency, they are also limited to the application's tolerance for   latency and bandwidth inefficiency.3.3.  Integrity Protection of the Wire Image   Adding end-to-end integrity protection to portions of the wire image   makes it impossible for on-path devices to modify them without   detection by the endpoints, which can then take action in response to   those modifications, making these portions of the wire image   effectively immutable.  However, they can still be observed by   devices on path.  This allows the creation of signals intended by the   endpoints solely for the consumption of these on-path devices.   Integrity protection can only practically be applied to the sequence   of bits in each packet, which implies that a protocol's visible wire   image cannot be made completely immutable in a packet-switched   network.  Interarrival timings, for instance, cannot be easily   protected, as the observable delay sequence is modified as packets   move through the network and experience different delays on different   links.  Message sequences are also not practically protectable,   because packets may be dropped or reordered at any point in the   network as a consequence of the network's operation.  Intermediate   systems with knowledge of the protocol semantics in the readable   portion of the wire image can also purposely delay or drop packets in   order to affect the protocol's operation.Trammell & Kuehlewind         Informational                     [Page 5]

RFC 8546                       Wire Image                     April 20194.  Engineering the Wire Image   Understanding the nature of a protocol's wire image allows it to be   engineered.  The general principle at work here, observed through   experience with deployability and non-deployability of protocols at   the network and transport layers in the Internet, is that all   observable parts of a protocol's wire image will eventually be used   by devices on path.  Consequently, changes or future extensions that   affect the observable part of the wire image become difficult or   impossible to deploy.   A network function that serves a purpose useful to its deployer will   use the information it needs from the wire image and will tend to get   that information from the wire image in the simplest way possible.   For example, consider the case of the ubiquitous TCP [RFC793]   transport protocol.  As described in [RFC8558], several key   in-network functions have evolved to take advantage of implicit   signals in TCP's wire image, which, as TCP provides neither integrity   or confidentiality protection for its headers, is inseparable from   its internal operation.  Some of these include:   o  Determining return routability and consent: For example, TCP's      wire image contains both an implicit indication that the sender of      a packet is at least on the path toward its source address (in the      acknowledgement number during the handshake), as well as an      implicit indication that a receiving device consents to continue      communication.  These are used by stateful network firewalls.   o  Measuring loss and latency: For example, examining the sequence of      TCP's sequence and acknowledgement numbers, as well as the ECN      [RFC3168] control bits, allows the inference of congestion, loss,      and retransmission along the path.  The sequence and      acknowledgement numbers together with the timestamp option      [RFC7323] allow the measurement of application-experienced      latency.   During the design of a protocol, the utility of features like these   should be considered.  The protocol's wire image can be designed to   explicitly expose information to those network functions deemed   important by the designers.  The wire image should expose as little   other information as possible.   However, even when information is explicitly provided to the network,   any information that is exposed by the wire image, even information   not intended to be consumed by an observer, must be designed   carefully, as deployed network functions using that information may   render it immutable for future versions of the protocol.  ForTrammell & Kuehlewind         Informational                     [Page 6]

RFC 8546                       Wire Image                     April 2019   example, information needed to support decryption by the receiving   endpoint (cryptographic handshakes, sequence numbers, and so on) may   be used by devices along the path for their own purposes.4.1.  Declaring Protocol Invariants   One potential approach to reduce the extent of the wire image that   will be used by devices on the path is to define a set of invariants   for a protocol during its development.  Declaring a protocol's   invariants represents a promise made by the protocol's developers   that certain bits in the wire image, and behaviors observable in the   wire image, will be preserved through the specification of all future   versions of the protocol.  QUIC's invariants [QUIC-INVARIANTS] are an   initial attempt to apply this approach to QUIC.   While static aspects of the wire image (bits with simple semantics at   fixed positions in protocol headers) can easily be made invariant,   different aspects of the wire image may be more or less appropriate   to define as invariants.  For a protocol with a version and/or   extension negotiation mechanism, the bits in the header and the   behaviors tied to those bits, which implement version negotiation,   should be made invariant.  More fluid aspects of the wire image and   behaviors that are not necessary for interoperability are not   appropriate as invariants.   Parts of a protocol's wire image not declared invariant but intended   to be visible to devices on path should be protected against   "accidental invariance": the deployment of on-path devices over time   that make simplifying assumptions about the behavior of those parts   of the wire image, making new behaviors not meeting those assumptions   difficult to deploy.  Integrity protection of the wire image may   itself help protect against accidental invariance, because read-only   wire images invite less meddling than path-writable wire images.  The   techniques discussed in [USE-IT] may also be useful in further   preventing accidental invariance and ossification.   Likewise, parts of a protocol's wire image not declared invariant and   not intended to be visible to the path should be encrypted to protect   their confidentiality.  When confidentiality protection is either not   possible or not practical, then, as above, the approaches discussed   in [USE-IT] may be useful in ossification prevention.4.2.  Trustworthiness of Engineered Signals   Since signals in the wire image that are engineered to be exposed are   separate from the signals that drive an encrypted protocol's   mechanisms, the accuracy of these signals intended for consumption by   the path may not be verifiable by on-path devices; see [RFC8558].Trammell & Kuehlewind         Informational                     [Page 7]

RFC 8546                       Wire Image                     April 2019   Indeed, any two endpoints with a secret channel between them (in this   case, the encrypted protocol itself) may collude to change the   semantics and information content of these signals.  This is an   unavoidable consequence of the separation of the wire image from the   protocol's operation afforded by confidentiality protection of the   protocol's headers.5.  IANA Considerations   This document has no IANA actions.6.  Security Considerations   This document explores the information exposed by the wire image that   may be relevant to end-to-end communication privacy and security.   When designing the wire image of a network protocol, care must be   taken to expose only that information to the network deemed necessary   in the protocol's design, and careful design is necessary to reduce   the risk that information not explicitly included in the wire image   is derivable from its observation.7.  Informative References   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed              and Secure Transport", Work in Progress,draft-ietf-quic-transport-19, March 2019.   [QUIC-INVARIANTS]              Thomson, M.,"Version-Independent Properties of QUIC",              Work in Progress,draft-ietf-quic-invariants-04, April              2019.   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7,RFC 793, DOI 10.17487/RFC0793, September 1981,              <https://www.rfc-editor.org/info/rfc793>.   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC 3168, DOI 10.17487/RFC3168, September 2001,              <https://www.rfc-editor.org/info/rfc3168>.   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.              Scheffenegger, Ed., "TCP Extensions for High Performance",RFC 7323, DOI 10.17487/RFC7323, September 2014,              <https://www.rfc-editor.org/info/rfc7323>.Trammell & Kuehlewind         Informational                     [Page 8]

RFC 8546                       Wire Image                     April 2019   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol              Version 1.3",RFC 8446, DOI 10.17487/RFC8446, August 2018,              <https://www.rfc-editor.org/info/rfc8446>.   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",RFC 8558, DOI 10.17487/RFC8558, April 2019,              <https://www.rfc-editor.org/info/rfc8558>.   [USE-IT]   Thomson, M., "Long-term Viability of Protocol Extension              Mechanisms", Work in Progress,draft-thomson-use-it-or-lose-it-03, January 2019.IAB Members at the Time of Approval      Jari Arkko      Alissa Cooper      Ted Hardie      Christian Huitema      Gabriel Montenegro      Erik Nordmark      Mark Nottingham      Melinda Shore      Robert Sparks      Jeff Tantsura      Martin Thomson      Brian Trammell      Suzanne WoolfAcknowledgments   Thanks to Martin Thomson, Stephen Farrell, Thomas Fossati, Ted   Hardie, Mark Nottingham, Tommy Pauly, and the membership of the IAB   Stack Evolution Program for text, feedback, and discussions that have   improved this document.   This work is partially supported by the European Commission under   Horizon 2020 grant agreement No. 688421, Measurement and Architecture   for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat   for Education, Research, and Innovation under contract No. 15.0268.   This support does not imply endorsement.Trammell & Kuehlewind         Informational                     [Page 9]

RFC 8546                       Wire Image                     April 2019Authors' Addresses   Brian Trammell   ETH Zurich   Gloriastrasse 35   8092 Zurich   Switzerland   Email: ietf@trammell.ch   Mirja Kuehlewind   ETH Zurich   Gloriastrasse 35   8092 Zurich   Switzerland   Email: ietf@kuehlewind.netTrammell & Kuehlewind         Informational                    [Page 10]