The Wire Image of a Network Protocol

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Authors Brian Trammell  , Mirja K├╝hlewind 
Last updated 2018-04-03
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Network Working Group                                        B. Trammell
Internet-Draft                                             M. Kuehlewind
Intended status: Informational                                ETH Zurich
Expires: October 5, 2018                                  April 03, 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

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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.  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

   Several documents currently under discussion in IETF working groups
   and the IETF in general, for example [QUIC-MANAGEABILITY],
   [EFFECT-ENCRYPT], and [TRANSPORT-ENCRYPT], discuss in part impacts on
   the third-party use of wire images caused by a migration from
   protocols whose wire images are largely not confidentiality protected
   (e.g.  HTTP over TCP) to protocols whose wire images are
   confidentiality protected (e.g.  H2 over QUIC).

   This document presents the wire image abstraction with the hope that
   it can shed some light on these discussions.

2.  Definition

   More formally, the wire image of a protocol consists of the sequence
   of messages sent by each participant in the protocol, each expressed
   as a sequence of bits with an associated arbitrary-precision time at
   which it was sent.

3.  Discussion

   This definition is so vague 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, interpacket timing, packet size,
   and message sequence information can be used to infer other

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   parameters of the behavior of the protocol, or to fingerprint
   protocols and/or specific implementations of the protocol; see
   Section 3.1.

   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 3.3.  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 3.2.

   Portions of the wire image of a protocol that are neither
   confidentiality-protected nor integrity-protected are writable by
   devices on the path(s) between the endpoints using the protocol.  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

   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.

   From the point of view of a passive observer, 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 these two wire images can be correlated
   to infer information about the dynamics of the overlying application.

3.1.  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.

3.2.  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.

3.3.  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.

3.3.1.  Invariants

   One 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.  Invariants are, in essence, 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.

3.3.2.  Trustworthiness of Engineered Signals

   Since they are separate from the signals that drive an encrypted
   protocol's mechanisms, the veracity 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

4.  Acknowledgments

   Thanks to Martin Thomson, Thomas Fossati, Ted Hardie, and the
   membership of the IAB Stack Evolution Program, for 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.

5.  Informative References

              Moriarty, K. and A. Morton, "Effects of Pervasive
              Encryption on Operators", draft-mm-wg-effect-encrypt-25
              (work in progress), March 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-01 (work in progress), March

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              Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", draft-ietf-quic-manageability-01
              (work in progress), October 2017.

   [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,

              Fairhurst, G. and C. Perkins, "The Impact of Transport
              Header Confidentiality on Network Operation and Evolution
              of the Internet", draft-fairhurst-tsvwg-transport-
              encrypt-06 (work in progress), February 2018.

Authors' Addresses

   Brian Trammell
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


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