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The Wire Image of a Network Protocol

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8546.
Authors Brian Trammell , Mirja Kühlewind
Last updated 2018-10-10
RFC stream Internet Architecture Board (IAB)
Stream IAB state (None)
Consensus boilerplate Unknown
IAB shepherd (None)
Network Working Group                                        B. Trammell
Internet-Draft                                             M. Kuehlewind
Intended status: Informational                                ETH Zurich
Expires: April 13, 2019                                 October 10, 2018

                  The Wire Image of a Network Protocol


   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 on increased encryption has for network functions that
   use the wire image.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 13, 2019.

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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.  It is the wire image, not the
   protocol's specification, that determines how third parties on the
   network paths among protocol participants will interact with that

   The increasing deployment of transport-layer security [RFC8226] to
   protect application-layer headers and payload, as well as the
   definition and deployment of QUIC [I-D.ietf-quic-transport], a
   transport protocol which encrypts most of its own control
   information, bring new relevance to this question.  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.

   The flipside of this trend is the impact of a less visible wire image
   on various functions driven by third-party observation of the wire
   image.  [RFC8404] examines this issue from a network operator's
   viewpoint, and [I-D.ietf-tsvwg-transport-encrypt] focuses on
   transport-layer implications of increasing encryption.
   [I-D.ietf-quic-manageability] is, in part, a third-party user's guide
   to the QUIC wire image.  In contrast to those documents, this draft
   treats the wire image as a pure abstraction, with the hope that it
   can shed some light on these discussions.

2.  Definition

   More formally, the wire image of the set of protocols in use for a
   communication observed at a given point in the network consists of
   the sequence of packets sent by each participant in the
   communication, each expressed as a sequence of bits with the
   associated arbitrary-precision time at which the packet was observed.

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3.  Discussion

   This definition appears at first glance to be so impractically formal
   as to be difficult to apply to protocol analysis, but it does
   illustrate some important properties of the wire image.

   Key is that the wire image is not limited to merely "the unencrypted
   bits in the header".  In particular, the sequences of interpacket
   timing and packet sizes can also 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 which
   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.  This is the case with TCP in the current

   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; the wire image of a 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.  In
   general, it refers to everything observable about a communication at
   a given vantage point; 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.  Confidentiality and integrity protection
   may be added at multiple layers in the stack.  However, information
   at the transport layer and above is presumed to be delivered end-to-
   end in the the Internet architecture.  For example, MAC-layer
   integrity and confidentiality protection do not prevent modification
   by the devices terminating those security associations, or by devices
   on different segments of the path.  This document therefore does not

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   concern itself directly with portions of the wire image below the
   network layer.

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

   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.  However, it cannot be applied to protecting non-header
   information in the wire image.  Of particular interest is the
   sequence of packet sizes and the sequence of packet times.  These are
   characteristic of the operation of the protocol.  While packets
   cannot be made smaller than their information content, nor sent
   faster than processing time requirements at the sender allow, a
   sender may use padding to increase the size of packets, and add delay
   to transmission scheduling in order to increase interpacket delay.
   However, it does this as the expense of bandwidth efficiency and

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   latency, so this technique is 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, as
   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.

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 which 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

   For example, consider the case of the ubiquitous TCP [RFC0793]
   transport protocol.  As described in [PATH-SIGNALS], 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:

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   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

   During the design of a protocol, the utility of features such as
   these shoud be considered, and the protocol's wire image should
   therefore be designed to explicitly expose information to those
   network functions deemed important by the designers in an obvious
   way.  The wire image should expose as little other information as

   However, even when information is explicitly provided to the network,
   any information that is exposed by the wire image, even that
   information not intended to be consumed by an observer, must be
   designed carefully as it might ossify, making it immutable for future
   versions of the protocol.  For 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
   behaviors tied to those bits which implement version negotiation

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   should be made invariant.  More fluid aspects of the wire image and
   behaviors which 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 they are separate from the signals that drive an encrypted
   protocol's mechanisms, the accuracy of integrity-protected signals in
   an engineered wire image intended for consumption by the path may not
   be verifiable by on-path devices; see [PATH-SIGNALS].  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

5.  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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6.  Informative References

              Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", draft-ietf-quic-manageability-02
              (work in progress), July 2018.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-15 (work
              in progress), October 2018.

              Fairhurst, G. and C. Perkins, "The Impact of Transport
              Header Confidentiality on Network Operation and Evolution
              of the Internet", draft-ietf-tsvwg-transport-encrypt-00
              (work in progress), September 2018.

              Hardie, T., "Path Signals", draft-hardie-path-signals-03
              (work in progress), April 2018.

              Thomson, M., "Version-Independent Properties of QUIC",
              draft-ietf-quic-invariants-03 (work in progress), October

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [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,

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

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   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [USE-IT]   Thomson, M., "Long-term Viability of Protocol Extension
              Mechanisms", draft-thomson-use-it-or-lose-it-02 (work in
              progress), June 2018.

Authors' Addresses

   Brian Trammell
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


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