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.

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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 protocol.

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.

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 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; 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 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 behavior.

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 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 possible.

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:

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 possible.

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 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 headers.

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.

6. Informative References

[I-D.ietf-quic-manageability] Kuehlewind, M. and B. Trammell, "Manageability of the QUIC Transport Protocol", Internet-Draft draft-ietf-quic-manageability-02, July 2018.
[I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-15, October 2018.
[I-D.ietf-tsvwg-transport-encrypt] Fairhurst, G. and C. Perkins, "The Impact of Transport Header Confidentiality on Network Operation and Evolution of the Internet", Internet-Draft draft-ietf-tsvwg-transport-encrypt-00, September 2018.
[PATH-SIGNALS] Hardie, T., "Path Signals", Internet-Draft draft-hardie-path-signals-03, April 2018.
[QUIC-INVARIANTS] Thomson, M., "Version-Independent Properties of QUIC", Internet-Draft draft-ietf-quic-invariants-03, October 2018.
[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, "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.
[RFC8404] Moriarty, K. and A. Morton, "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", Internet-Draft draft-thomson-use-it-or-lose-it-02, June 2018.

Authors' Addresses

Brian Trammell ETH Zurich Gloriastrasse 35 8092 Zurich, Switzerland EMail:
Mirja Kuehlewind ETH Zurich Gloriastrasse 35 8092 Zurich, Switzerland EMail:

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