[Spud] updated PLUS charter

[Brian Trammell <[e-mail]>]

Greetings, all,

Consolidating edits from yesterday, here's the present state of the charter (working copy at https://github.com/ietf-plus/charter).

Thanks, cheers,

Brian

Path Layer UDP Substrate (PLUS)
===============================

The PLUS working group's goal is to enable the deployment of new, encrypted
transport protocols, while providing a transport-independent method to signal
flow semantics under transport and application control. The approach from
which the working group will start is a shim layer based on the User Datagram
Protocol (UDP), which provides compatibility with existing middleboxes in the
Internet as well as ubiquitous support in endpoints, and provides for
userspace implementation.

The current Internet protocol stack does not provide explicit, transport-
independent signaling of flow characteristics to on-path network devices. This
has led to the deployment of devices which perform implicit discovery of these
characteristics, through inspection of protocol headers all they way up to the
application layer, a practice made possible because these headers are sent in
the clear. Inspection and modification of unencrypted protocol headers is a
key cause of the ossification that makes it hard or impossible to deploy new
protocols.

In order to support more ubiquitous deployment of encryption, and the
encryption of transport headers to allow deployment of new transport
protocols, explicit signaling must be added to the stack, and it must be
transport protocol independent. While IP would seem to be the natural home for
this facility, both IPv4 and IPv6 options and extensions have deployment
problems on their own, which makes it hard to include any additional
information in these protocols. Additionally, a feedback channel that provides
information from on-path devices back to endpoints and applications, e.g. for
error handling, is essential for the deployment and success of an explicit
cooperation approach.

The PLUS working group will specify a new protocol as a Path Layer
User Substrate (PLUS), to support experimental deployment of
explicit cooperation between endpoints and devices on path, with the following goals:

- enable ubiquitous deployment of encrypted higher layer protocols
  by providing exposure of basic TCP-like semantics (e.g. SYN, FIN,
  RST flags) to devices on path (e.g. NATs and firewalls).

- allow applications and transport protocols to explicitly provide
  limited information in cleartext to devices on path

- allow devices on path to provide feedback and information about
  the path in cleartext to sending endpoints, under sending endpoint
  control

- allow devices on path to provide information about the path in cleartext
  to receiving endpoints, with feedback to the sending endpoint, under
  sending endpoint control

Note that this approach explicitly gives the control of information
exposure back the application and/or transport layer protocol on
the end host. It is the goal of PLUS to minimize the information
exposed at the level of detail that is useful for the network,
while encrypting everything else. This is important to avoid future
implicit treatment and the resulting ossification, as well as to
leverage the principle of least exposure to minimize privacy risks
presented by explicit cooperation.

Given that the primary goal of PLUS is to enable the deployment of arbitrary
transport protocols, encrypting transport protocol headers, we assume that
the higher-layer protocol can provide an encryption context that can be used by
PLUS to provide authentication, integrity, and encryption where needed. The
primary threat model to defend against will be modification or deletion of
exposed information by middleboxes and other devices on path, by allowing a
remote endpoint to detect modifications.

The working group will start with an initial set of use cases (see
draft-kuehlewind-spud-use-cases) and requirements (see
draft-trammell-spud-req), taken from experience with the Substrate
Protocol for User Datagrams (SPUD) prototype.
The working group's main output will be an experimental protocol
specification, together with an initial registry of types of
information that can be exposed using PLUS, clearly aligned to
these use cases and requirements. The working group will close if
it is not able to come to consensus on a protocol design to meet
these requirements.

The working group will additionally aim to identify other working
groups that could or should address parts of these requirements
within existing protocols, e.g. by specifying new protocol
extensions or as input for on-going standardization work. It will
aim to work with working groups defining encryption protocols (e.g.
TLS) which could be used for encryption of transport protocols
running over PLUS.