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Versions: 00 01 02 03 draft-iab-path-signals

Network Working Group                                     T. Hardie, Ed.
Internet-Draft                                          October 28, 2016
Intended status: Informational
Expires: May 1, 2017


                              Path signals
                      draft-hardie-path-signals-00

Abstract

   TCP's state mechanics uses a series of well-known messages that are
   exchanged in the clear.  Because these are visible to network
   elements on the path between the two nodes setting up the transport
   connection, they are often used as signals by those network elements.
   In transports that do not exchange these messages in the clear, on-
   path network elements lack those signals.  This document discusses
   the nature of the signals as they are seen by on-path elements and
   reflects on best practices for transports which encrypt their state
   mechanics.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 1, 2017.

Copyright Notice

   Copyright (c) 2016 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Signals Type Inferred . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Session establishment . . . . . . . . . . . . . . . . . .   3
       3.1.1.  Session identity  . . . . . . . . . . . . . . . . . .   3
       3.1.2.  Routability and Consent . . . . . . . . . . . . . . .   3
       3.1.3.  Resource Requirements . . . . . . . . . . . . . . . .   4
     3.2.  Network Measurement . . . . . . . . . . . . . . . . . . .   4
       3.2.1.  Path Latency  . . . . . . . . . . . . . . . . . . . .   4
       3.2.2.  Path reliability and consistency  . . . . . . . . . .   4
   4.  Options . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Do not restore these signals  . . . . . . . . . . . . . .   4
     4.2.  Replace these with network layer signals  . . . . . . . .   5
     4.3.  Replace these with per-transport signals  . . . . . . . .   5
     4.4.  Create a set of signals common to multiple transports . .   5
   5.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Introduction

   TCP [RFC0793] uses handshake messages to establish, maintain, and
   close connections.  While these are primarily intended to create
   state between two communicating nodes, these handshake messages are
   visible to network elements along the path between them.  It has been
   common over time for certain network elements to treat the exchanged
   messages as signals which related to their own functions.

   A firewall may, for example, create a rule that allows traffic from a
   specific host and port to enter its network when the connection was



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   initiated by a host already within the network.  It may subsequently
   remove that rule when the communication has ceased.  In the context
   of TCP handshake, it sets up the pinhole rule on seeing the initial
   TCP SYN acknowledged and then removes it upon seeing a RST or FIN &
   ACK exchange.  Note that in this case it does nothing to re-write any
   portion of the TCP packet; it simply enables a return path that would
   otherwise have been blocked.

   When a transport encrypts the headers it uses for state mechanics,
   the signal path elements inferred from examination is no longer
   available.  Their behavior in its absence will depend on which signal
   is not available, on the default behavior configured by the path
   element administrator, and by the security posture of the network as
   a whole.

3.  Signals Type Inferred

   The following list of signals which may be inferred from transport
   state messages includes those which may be exchanged during sessions
   establishment and those which derive from the ongoing flow.  Some of
   these signals are derived from the direct examination of packet
   trains, such as using a sequence number gap pattern to infer network
   reliability; others are derived from association, such as inferring
   network latency by timing a flow's packet inter-arrival times.  This
   list is not exhaustive, and it is not the full set of effects due to
   encrypting data and metadata in flight.

3.1.  Session establishment

   One of the most basic inferences made by examination of transport
   state is that a packet will be part of an ongoing flow; that is, an
   established session will continue until messages are received that
   terminate it.  Path elements may then make subsidiary inferences
   related to the session.

3.1.1.  Session identity

   Path elements that track session establishment will typically create
   a session identify for the flow, commonly using a tuple of the
   visible information in the packet headers.  This is then used to
   associate other information with the

3.1.2.  Routability and Consent

   A second common inference is that the session establishment provides
   is that the communicating pair of hosts can each reach each other and
   are interested in continuing communication.  The firewall example
   given above is a consequence of the inference of consent; because the



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   internal host initiates the connection, it is presumed to consent to
   return traffic.  That, in turn justifies the pinhole.

3.1.3.  Resource Requirements

   An additional common inference is that network resources will be
   required for the session.  These may be requirements within the
   network element itself, such as table entry space for a firewall or
   NAT; they may also be communicated by the network element to other
   systems.  For networks which use resource reservations, this might
   result in reservation of radio air time, energy, or network capacity.

3.2.  Network Measurement

   Some network elements will also use transport messages to engage in
   measurement of the paths which are used by flows on their network.
   The list of measurements below is illustrative, not exhaustive.

3.2.1.  Path Latency

   There are several ways in which a network element may measure path
   latency using transport messages, but two common ones are examining
   exposed timestamps and associating sequence numbers with a local
   timer.  These measurements are necessarily limited to measuring only
   the portion of the path between the system which assigned the
   timestamp or sequence number and the network element.

3.2.2.  Path reliability and consistency

   A network element may also measure the reliability of a particular
   path by examining sessions which expose sequence numbers;
   retransmissions and gaps are then associated with the path segments
   on which they might have occurred.

4.  Options

   The set of options below are alternatives which optimize very
   different things.  Though it comes to a preliminary conclusion, this
   draft intends to foster a discussion of those tradeoffs and any
   discussion of them must be understood as preliminary.

4.1.  Do not restore these signals

   It is possible, of course, to do nothing.  The transport messages
   were not necessarily intended for consumption by on-path network
   elements and encrypting so they are not visible may be taken by some
   as a benefit.  Each network element would then treat packets without
   these visible elements according to its own defaults.  While our



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   experience of that is not extensive, one consequence has been that
   state tables for flows of this type are generally not kept as long as
   those for which sessions are identifiable.  The result is that
   heartbeat traffic must be maintained to keep any bindings (e.g.  NAT
   or firewall) from early expiry.

4.2.  Replace these with network layer signals

   It would be possible to replace these implicit signals with explicit
   signals at the network layer.  Though IPv4 has relatively few
   facilities for this, IPv6 hop-by-hop headers [RFC7045] might suit
   this purpose.  Further examination of the deployability of these
   headers may be required.

4.3.  Replace these with per-transport signals

   It is possible to replace these implicit signals with signals that
   are tailored to specific transports, just as the initial signals are
   derived primarily from TCP.  There is a risk here that the first
   transport which develops these will be reused for many purposes
   outside its stated purpose, simply because it traverses NATs and
   firewalls better than other traffic.  If done with an explicit intent
   to re-use the elements of the solution in other transports, the risk
   of ossification might be slightly lower.

4.4.  Create a set of signals common to multiple transports

   Several proposals use UDP[RFC0768] as a demux layer, onto which new
   transport semantics are layered.  For those transports, it may be
   possible to build a common signalling mechanism and set of signals,
   such as that proposed in "Transport-Independent Path Layer State
   Management" [I-D.trammell-plus-statefulness].

   This may be taken as a variant of the re-use of common elements
   mentioned in the section above, but it has a greater chance of
   avoiding the ossification of the solution into the first moving
   protocol.

5.  Recommendation

   Fundamentally, this paper recommends that implicit signals should be
   replaced with explicit signals, but that a signal should exposed to
   the path only when the signal's originator intends that it be used by
   the network elements on the path.  For many flows, that may result in
   signal being present, but it allows them to be present when needed.






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   Discussion of the appropriate mechanism(s) for these signals is
   continuing but, at minimum, any method should meet the principles set
   out in the security considerations below.

6.  IANA Considerations

   This document contains no requests for IANA.

7.  Security Considerations

   Addition of visible signals to the path allows network elements along
   the path to act.  If the network element is controlled by an
   attacker, those actions can include dropping, delaying, or
   mishandling the constituent packets of a flow.

   Note that actions that do not benefit the flow or the network may be
   perceived as an attack even if they are conducted by a responsible
   network element.  Designing a system that minimizes the ability to
   act on signals at all by removing as many signals as possible may
   reduce this possibility.  This approach also comes with risks,
   principally that the actions will continue to take place on an
   arbitrary set of flows.

   Addition of visible signals to the path also increases the
   information available to an observer and may, when the information
   can be linked to a node or user, reduce the privacy of the user.

   This document recommends three basic principles:

   o  Cryptographic contexts should be available on any flow, derived
      from ubiquitous end-system cryptographic capabilities.

   o  Anything exposed to the path should be done with the intent that
      it be used by the network elements on the path.

   o  Intermediate path elements should not add visible signals which
      identify the user, origin node, or origin network
      [I-D.hardie-privsec-metadata-insertion].

8.  Acknowledgements

   In addition to the editor listed above, this document incorporates
   contributions from Brian Trammel, Mirja Kuehlwind, and Joe
   Hildebrand.  These ideas were also discussed at the PLUS BoF,
   sponsored by Spencer Dawkins.  The ideas around the use of IPv6 hop-
   by-hop headers as a network layer signal benefited from discussions
   with Tom Herbert.  The description of UDP as a demuxing protocol
   comes from Stuart Cheshire.



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   All errors are those of the editor.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

9.2.  Informative References

   [I-D.hardie-privsec-metadata-insertion]
              Hardie, T., "Design considerations for Metadata
              Insertion", draft-hardie-privsec-metadata-insertion-03
              (work in progress), October 2016.

   [I-D.trammell-plus-statefulness]
              Kuehlewind, M., Trammell, B., and J. Hildebrand,
              "Transport-Independent Path Layer State Management",
              draft-trammell-plus-statefulness-00 (work in progress),
              October 2016.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <http://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <http://www.rfc-editor.org/info/rfc7045>.

Author's Address

   Ted Hardie (editor)

   Email: ted.ietf@gmail.com









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