Transport Protocol Path Signals
RFC 8558

Internet Architecture Board (IAB)                         T. Hardie, Ed.
Request for Comments: 8558                                    April 2019
Category: Informational
ISSN: 2070-1721

                    Transport Protocol Path Signals

Abstract

   This document discusses the nature of signals seen by on-path
   elements examining transport protocols, contrasting implicit and
   explicit signals.  For example, TCP's state machine 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.
   Often, the removal of those signals is intended by those moving the
   messages to confidential channels.  Where the endpoints desire that
   network elements along the path receive these signals, this document
   recommends explicit signals be used.

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/rfc8558.

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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.

Table of Contents

   1. Introduction ....................................................3
   2. Signal Types Inferred ...........................................4
      2.1. Session Establishment ......................................4
           2.1.1. Session Identity ....................................4
           2.1.2. Routability and Intent ..............................4
           2.1.3. Flow Stability ......................................5
           2.1.4. Resource Requirements ...............................5
      2.2. Network Measurement ........................................5
           2.2.1. Path Latency ........................................5
           2.2.2. Path Reliability and Consistency ....................5
   3. Options .........................................................5
      3.1. Do Not Restore These Signals ...............................6
      3.2. Replace These with Network-Layer Signals ...................6
      3.3. Replace These with Per-Transport Signals ...................6
      3.4. Create a Set of Signals Common to Multiple Transports ......6
   4. Recommendation ..................................................7
   5. IANA Considerations .............................................8
   6. Security Considerations .........................................8
   7. Informative References ..........................................9
   IAB Members at the Time of Approval ...............................10
   Acknowledgements ..................................................10
   Author's Address ..................................................10

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1.  Introduction

   This document discusses the nature of signals seen by on-path
   elements examining transport protocols, contrasting implicit and
   explicit signals.  For example, TCP's state machine 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.  While the architecture of the Internet
   may be best realized by end-to-end protocols [RFC1958], there are
   cases such as the use of Network Address Translators [RFC3234] where
   some functions are commonly provided by on-path network elements.  In
   transports that do not exchange these messages in the clear, on-path
   network elements lack those signals.  Often, the removal of those
   signals is intended by those moving the messages to confidential
   channels.  Where the endpoints desire that network elements along the
   path receive these signals, this document recommends explicit signals
   be used.

   The interpretation of TCP [RFC0793] by on-path elements is an example
   of implicit signal usage.  It uses cleartext 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 is common for certain network elements to
   treat the exchanged messages as signals that relate 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
   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 acknowledgement and then removes it upon seeing a RST or FIN
   and ACK exchange.  Note that in this case, it does nothing to rewrite
   any portion of the TCP packet; it simply enables a return path that
   would otherwise have been blocked.

   When a transport encrypts the fields it uses for state mechanics,
   these signals are no longer accessible to path elements.  The
   behavior of path elements will then 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.

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2.  Signal Types Inferred

   The following list of signals that may be inferred from transport
   state messages includes those that may be exchanged during session
   establishment and those that derive from the ongoing flow.

   Some of these signals are derived from the direct examination of
   packet sequences, 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.  Note as well that
   because these are derived from inference, they do not include any
   path signals that would not be relevant to the endpoint state
   machines; indeed, an inference-based system cannot send such signals.

2.1.  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.

2.1.1.  Session Identity

   Path elements that track session establishment will typically create
   a session identity 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 flow.

2.1.2.  Routability and Intent

   A second common inference that 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 that inference; because the internal host
   initiates the connection, it is presumed to want to receive return
   traffic.  That, in turn, justifies the pinhole.

   Some other on-path elements assume that a host that asked to
   communicate with a remote address has authorized receiving incoming
   communications from any other host (e.g., Endpoint-Independent
   Mapping or Endpoint-Independent Filtering [RFC7857]).  This is, for
   example, the default behavior in Network Address and Protocol
   Translation from IPv6 Clients to IPv4 Servers (NAT64).

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2.1.3.  Flow Stability

   Some on-path devices that are responsible for load-sharing or load-
   balancing may be instructed to preserve the same path for a given
   flow rather than dispatching packets belonging to the same flow on
   multiple paths as this may cause packets in the flow to be delivered
   out of order.

2.1.4.  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 that use resource reservations, this might
   result in reservation of radio air time, energy, or network capacity.

2.2.  Network Measurement

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

2.2.1.  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 that assigned the
   timestamp or sequence number and the network element.

2.2.2.  Path Reliability and Consistency

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

3.  Options

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

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3.1.  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 them 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 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.  When those bindings are not
   kept, methods like a QUIC connection-id [QUIC] may be necessary to
   allow load balancers or other systems to continue to maintain a
   flow's path to the appropriate peer.

3.2.  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.

3.3.  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 that 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 reuse the elements of the solution in other transports, the risk
   of ossification might be slightly lower.

3.4.  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 signaling mechanism and set of signals,
   such as that proposed in "Transport-Independent Path Layer State
   Management" [PLUS].

   This may be taken as a variant of the reuse 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.

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4.  Recommendation

   The IAB urges protocol designers to design for confidential operation
   by default.  We strongly encourage developers to include encryption
   in their implementations and to make them encrypted by default.  We
   similarly encourage network and service operators to deploy
   encryption where it is not yet deployed, and we urge firewall policy
   administrators to permit encrypted traffic.  One of the consequences
   of the change will be the loss of implicit signals.

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

   Discussion of the appropriate mechanism(s) for these signals is
   continuing, but at a minimum, any method should aim to adhere to
   these basic principles:

   o  The portion of protocol signaling that is intended for end-system
      state machines should be protected by confidentiality and
      integrity protection such that it is only available to those end
      systems.

   o  Anything exposed to the path should be done with the intent that
      it be used by the network elements on the path.  This information
      should be integrity protected, so that end systems can detect if
      path elements have made changes in flight.

   o  Signals exposed to the path should be decoupled from signals that
      drive the protocol state machines in endpoints.  This avoids
      creating opportunities for additional inference.

   o  Intermediate path elements should not add visible signals that
      identify the user, origin node, or origin network [RFC8164].  Note
      that if integrity protection is provided as suggested above, any
      signals added by intermediate path elements will be clearly
      distinguishable from those added by endpoints, as they will not be
      within the integrity-protected portion of the packet.

   The IAB notes that methods for allowing on-path actors to verify
   integrity protection are not available unless those actors have
   shared keys with the end systems or share a common set of trust
   points.  As a result, integrity protection can generally be reliably
   applied by and verified only by endpoints.

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   Verifying the authenticity of signals generated by on-path actors is
   similarly difficult.  Endpoints that consume signals generated by
   on-path actors, particularly where those signals are unauthenticated,
   need to fully consider the implications of doing so.  Managing the
   authentication of on-path signals is an area of active research, and
   defining or recommending methods for it is outside the scope of this
   document.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   Path-visible signals allow network elements along the path to act
   based on the signaled information, whether the signal is implicit or
   explicit.  If the network element is controlled by an attacker, those
   actions can include dropping, delaying, or mishandling the
   constituent packets of a flow.  An attacker may also characterize the
   flow or attempt to fingerprint the communicating nodes based on the
   pattern of signals.

   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.

   When signals from endpoints to the path are independent from the
   signals used by endpoints to manage the flow's state mechanics, they
   may be falsified by an endpoint without affecting the peer's
   understanding of the flow's state.  For encrypted flows, this
   divergence is not detectable by on-path devices.  The intent of this
   practice may be to garner improved treatment from the network or to
   avoid strictures.  Protocol designers should be cautious when
   introducing explicit signals to consider how falsified signals would
   impact protocol operation and deployment.  Similarly, operators
   should be cautious in deployments to be sure that default operation
   without these signals does not encourage gaming the system by
   providing false signals.

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7.  Informative References

   [PLUS]     Kuehlewind, M., Trammell, B., and J. Hildebrand,
              "Transport-Independent Path Layer State Management", Work
              in Progress, draft-trammell-plus-statefulness-04, November
              2017.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", Work in Progress,
              draft-ietf-quic-transport-19, March 2019.

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

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

   [RFC1958]  Carpenter, B., Ed., "Architectural Principles of the
              Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
              <https://www.rfc-editor.org/info/rfc1958>.

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
              <https://www.rfc-editor.org/info/rfc3234>.

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

   [RFC7857]  Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
              S., and K. Naito, "Updates to Network Address Translation
              (NAT) Behavioral Requirements", BCP 127, RFC 7857,
              DOI 10.17487/RFC7857, April 2016,
              <https://www.rfc-editor.org/info/rfc7857>.

   [RFC8164]  Nottingham, M. and M. Thomson, "Opportunistic Security for
              HTTP/2", RFC 8164, DOI 10.17487/RFC8164, May 2017,
              <https://www.rfc-editor.org/info/rfc8164>.

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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

Acknowledgements

   In addition to the editor listed in the header, this document
   incorporates contributions from Brian Trammell, Mirja Kuehlewind,
   Martin Thomson, Aaron Falk, Mohamed Boucadair, 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.  Mark Smith, Kazuho Oku, Stephen Farrell, and Eliot Lear
   provided valuable comments on earlier draft versions of this
   document.

   All errors are those of the editor.

Author's Address

   Ted Hardie (editor)

   Email: ted.ietf@gmail.com

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