Network Working Group B. Trammell
Internet-Draft M. Kuehlewind
Intended status: Informational ETH Zurich
Expires: October 12, 2018 April 10, 2018
The Wire Image of a Network Protocol
draft-trammell-wire-image-04
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 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, 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.
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,
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and message sequence information can be used to infer other
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
Internet.
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.
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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.
3.2. 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
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
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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 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.
3.3.1. Declaring Protocol 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. 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
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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.
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
headers.
4. Acknowledgments
Thanks to Martin Thomson, Thomas Fossati, Ted Hardie, Mark
Nottingham, 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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5. Informative References
[EFFECT-ENCRYPT]
Moriarty, K. and A. Morton, "Effects of Pervasive
Encryption on Operators", draft-mm-wg-effect-encrypt-25
(work in progress), March 2018.
[PATH-SIGNALS]
Hardie, T., "Path Signals", draft-hardie-path-signals-03
(work in progress), April 2018.
[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
draft-ietf-quic-invariants-01 (work in progress), March
2018.
[QUIC-MANAGEABILITY]
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,
<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>.
[TRANSPORT-ENCRYPT]
Fairhurst, G. and C. Perkins, "The Impact of Transport
Header Confidentiality on Network Operation and Evolution
of the Internet", draft-fairhurst-tsvwg-transport-
encrypt-07 (work in progress), April 2018.
[USE-IT] Thomson, M., "Long-term Viability of Protocol Extension
Mechanisms", draft-thomson-use-it-or-lose-it-01 (work in
progress), March 2018.
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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: mirja.kuehlewind@tik.ee.ethz.ch
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