    Internet Architecture Board (IAB)                            B. Trammell
    Request for Comments: 8546                                 M. Kuehlewind
    Category: Informational                                       April 2019
    ISSN: 2070-1721

                      The Wire Image of a Network Protocol

    Abstract

       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; see Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://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 to BCP 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.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Definition  . . . . . . . . . . . . . . . . . . . . . . . . .   3
       3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   3
         3.1.  The Extent of the Wire Image  . . . . . . . . . . . . . .   4
         3.2.  Obscuring Timing and Sizing Information . . . . . . . . .   5
         3.3.  Integrity Protection of the Wire Image  . . . . . . . . .   5
       4.  Engineering the Wire Image  . . . . . . . . . . . . . . . . .   6
         4.1.  Declaring Protocol Invariants . . . . . . . . . . . . . .   7
         4.2.  Trustworthiness of Engineered Signals . . . . . . . . . .   7
       5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
       6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
       7.  Informative References  . . . . . . . . . . . . . . . . . . .   8
       IAB Members at the Time of Approval . . . . . . . . . . . . . . .   9
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

    1.  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.

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       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; see Section 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; see Section 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; see Section 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 a

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       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.  See
       Section 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.

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       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.

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    4.  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.  For

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       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].

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       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>.

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       [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 Woolf

    Acknowledgments

       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.

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    Authors' 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.net

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