The Wire Image of a Network Protocol
RFC 8546

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: [email protected]

   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: [email protected]

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