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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