Network Working Group B. Trammell
Internet-Draft M. Kuehlewind
Intended status: Informational ETH Zurich
Expires: May 20, 2018 November 16, 2017
The Wire Image of a Network Protocol
draft-trammell-wire-image-00
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 current
discussions within the IETF 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. 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:
o The wire image is not limited to merely "the unencrypted bits in
the header". In particular, timing, size, and sequence
information can be used to infer other parameters of the behavior
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of the protocol, or to fingerprint protocols and/or specific
implmentations of the protocol.
o The wire image is multidimensional. This implies that the name
"image" is not merely metaphorical, and that general image
recognition techniques can be applied to extracting paterns and
information from it.
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. A sender may use
padding to increase the size of packets, and inject delay into
sending in order to increase delay components, however it does this
as the expense of bandwidth efficiency and latency. This technique
is therefore limited to the tolerance for inefficiency and latency of
the application.
3.2. Integrity Protection of the Wire Image
Portions of the wire image of a protocol that are neither
confidentiality-protected nor integrity-protected are writable by
devices on a path. 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 modern Internet.
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.
Note that a protocol's wire image cannot be made completely immutable
in a packet-switched network. The observed delay sequence is
modified as packets move through the network and experience different
delays on different links, and packets may be dropped at any time, 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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3.3. Engineering the Wire Image
We note that 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
ossify, and become difficult or impossible to change in future
extensions or revisions of the protocol.
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. A protocol's wire image should therefore be designed to
explicitly expose information to those network functions in an
obvious way, and to 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
informaiton 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 the path for its own
purposes.
4. Acknowledgments
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
[EFFECT-ENCRYPT]
Moriarty, K. and A. Morton, "Effect of Pervasive
Encryption on Operators", draft-mm-wg-effect-encrypt-13
(work in progress), October 2017.
[QUIC-MANAGEABILITY]
Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
Transport Protocol", draft-ietf-quic-manageability-01
(work in progress), October 2017.
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[TRANSPORT-ENCRYPT]
Fairhurst, G. and C. Perkins, "The Impact of Transport
Header Encryption on Operation and Evolution of the
Internet", draft-fairhurst-tsvwg-transport-encrypt-04
(work in progress), September 2017.
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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