Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-09

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TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                C. Perkins
Expires: May 6, 2020                               University of Glasgow
                                                        November 3, 2019

    Considerations around Transport Header Confidentiality, Network
     Operations, and the Evolution of Internet Transport Protocols
                 draft-ietf-tsvwg-transport-encrypt-09

Abstract

   To protect user data and privacy, Internet transport protocols have
   supported payload encryption and authentication for some time.  Such
   encryption and authentication is now also starting to be applied to
   the transport protocol headers.  This helps avoid transport protocol
   ossification by middleboxes, while also protecting metadata about the
   communication.  Current operational practice in some networks inspect
   transport header information within the network, but this is no
   longer possible when those transport headers are encrypted.  This
   document discusses the possible impact when network traffic uses a
   protocol with an encrypted transport header.  It suggests issues to
   consider when designing new transport protocols, to account for
   network operations, prevent network ossification, and enable
   transport evolution, while still respecting user privacy.

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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2020.

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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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Context and Rationale . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Use of Transport Header Information in the Network  . . .   5
     2.2.  Authentication of Transport Header Information  . . . . .   6
     2.3.  Observable Transport Header Fields  . . . . . . . . . . .   7
   3.  Current uses of Transport Headers within the Network  . . . .  10
     3.1.  Observing Transport Information in the Network  . . . . .  11
     3.2.  Transport Measurement . . . . . . . . . . . . . . . . . .  17
     3.3.  Use for Network Diagnostics and Troubleshooting . . . . .  21
     3.4.  Header Compression  . . . . . . . . . . . . . . . . . . .  22
   4.  Encryption and Authentication of Transport Headers  . . . . .  23
   5.  Addition of Transport Information to Network-Layer Headers  .  26
     5.1.  Use of OAM within a Maintenance Domain  . . . . . . . . .  26
     5.2.  Use of OAM across Multiple Maintenance Domains  . . . . .  26
   6.  Implications of Protecting the Transport Headers  . . . . . .  27
     6.1.  Independent Measurement . . . . . . . . . . . . . . . . .  27
     6.2.  Characterising "Unknown" Network Traffic  . . . . . . . .  29
     6.3.  Accountability and Internet Transport Protocols . . . . .  30
     6.4.  Impact on Operational Cost  . . . . . . . . . . . . . . .  30
     6.5.  Impact on Research, Development and Deployment  . . . . .  31
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  32
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  37
   11. Informative References  . . . . . . . . . . . . . . . . . . .  38
   Appendix A.  Revision information . . . . . . . . . . . . . . . .  45
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

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1.  Introduction

   Transport protocols have supported end-to-end encryption of payload
   data for many years.  Examples include Transport Layer Security (TLS)
   over TCP [RFC8446], Datagram TLS (DTLS) over UDP [RFC6347], and their
   corresponding usage guidelines [RFC7525], Secure RTP [RFC3711], and
   TCPcrypt [RFC8548] which permits opportunistic encryption of the TCP
   transport payload.  Some of these also provide integrity protection
   of all or part of the transport header.

   This end-to-end transport payload encryption brings many benefits in
   terms of providing confidentiality and protecting user privacy.  Such
   benefits have been widely discussed [RFC7258] [RFC7624].  This
   document strongly supports and encourages increased use of end-to-end
   payload encryption in transport protocols.  The implications of
   protecting the transport payload data are therefore not further
   discussed in this document.

   A further level of protection can be achieved by encrypting the
   entire network layer payload, including both the transport layer
   headers and the payload.  This method provides confidentiality for
   the entire transport packet.  It therefore does not expose any
   transport information to devices in the network, and prevents
   modification along a network path.  An example of encryption at the
   network layer is the IPsec Encapsulating Security Payload (ESP)
   [RFC4303] in tunnel mode.  Some Virtual Private Network (VPN) methods
   also encrypt these headers.  This form of encryption is not further
   discussed in this document.

   There is also a middle ground, comprising transport protocols that
   encrypt some, or all, of their transport layer header information, in
   addition to the payload.  An example of such a protocol, that is
   seeing widespread interest and deployment, is the QUIC transport
   protocol [I-D.ietf-quic-transport].  Encryption and authentication of
   the transport header information can prevent unwanted modification of
   transport headers by middleboxes.  It also reduces the amount of
   metadata about the progress of the transport connection that is
   visible to the network.

   As discussed in [RFC7258], Pervasive Monitoring (PM) nis a technical
   attack that needs to be mitigated in the design of IETF protocols.
   This document supports that conclusion and the use of transport
   header encryption to protect against pervasive monitoring.  RFC 7258
   also notes, though, that "Making networks unmanageable to mitigate PM
   is not an acceptable outcome, but ignoring PM would go against the
   consensus documented here.  An appropriate balance will emerge over
   time as real instances of this tension are considered".

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   The following sections further considers some of the costs and
   changes to network management and research that are implied by
   widespread use of transport protocols that encrypt the transport
   header information.  It reviews the implications of developing
   transport protocols that use end-to-end encryption to provide
   confidentiality of their transport layer headers, and considers the
   effect of such changes on transport protocol design and network
   operations.  It also considers some anticipated implications on
   transport and application evolution.  That is, it considers the
   issues in designing transport protocols that both protect their
   header information and respect user privacy.

2.  Context and Rationale

   The transport layer provides end-to-end interactions between
   endpoints (processes) using an Internet path.  Transport protocols
   layer directly over the network-layer service, and are sent in the
   payload of network-layer packets.  They support end-to-end
   communication between applications, using higher-layer protocols
   running on the end systems (transport endpoints).

   This simple architectural view hides one of the core functions of the
   transport: to discover and adapt to the Internet path that is
   currently being used.  The design of Internet transport protocols is
   as much about trying to avoid the unwanted side effects of congestion
   on a flow and other capacity-sharing flows, avoiding congestion
   collapse, adapting to changes in the path characteristics, etc., as
   it is about end-to-end feature negotiation, flow control, and
   optimising for performance of a specific application.

   To achieve stable Internet operations, the IETF transport community
   has to date relied heavily on the results of measurements and the
   insights of the network operations community to understand the trade-
   offs, and to inform selection of appropriate mechanisms to ensure a
   safe, reliable, and robust Internet (e.g., [RFC1273]).  In turn, the
   network operator and access provider communities have relied on being
   able to understand the pattern and requirements of traffic passing
   over the Internet, both in aggregate and at the flow level.  The
   widespread use of transport header encryption could change this.

   Encryption is expected to form a core part of future transport
   protocol designs.  This can be in the form of encrypted transport
   protocols (i.e., transport protocols that use encryption to provide
   confidentiality of some or all of the transport-layer header
   information), and/or the encryption of transport payloads (i.e.,
   confidentiality of the payload data).  There are many motivations for
   deploying such transports.  Increasing public concerns about
   interference with Internet traffic [RFC7624] have led to a rapidly

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   expanding deployment of encrypted transport protocols such as QUIC
   [I-D.ietf-quic-transport].

   Using encryption to provide confidentiality of the transport layer
   therefore brings some well-known privacy and security benefits.

2.1.  Use of Transport Header Information in the Network

   In-network measurement of transport flow characteristics can be used
   to enhance performance, and control cost and service reliability.  To
   support network operations and enhance performance, some operators
   have deployed functionality that utilises on-path observations of the
   transport headers of packets passing through their network.  These
   devices can rely on the presence and semantics of specific header
   information, which leads to ossification where an endpoint has to
   supply a specific header to receive the network service that it
   desires.

   In some cases, network-layer use of transport header information can
   be benign or advantageous to the protocol (e.g., recognising the
   start of a TCP connection, providing header compression for a Secure
   RTP flow, or explicitly using exposed protocol information to provide
   consistent decisions by on-path devices).  However, in other cases,
   ossification can frustrate the evolution of the transport protocol.
   A mechanism implemented in a network device, such as a firewall, that
   requires a header field to have only a specific known set of values
   (i.e., that regards the field as invariant) can prevent the device
   from forwarding packets using a different version of a protocol that
   introduces a feature that changes the value of the observed field.

   An example of such ossification was observed in the development of
   Transport Layer Security (TLS) 1.3 [RFC8446].  This necessitated a
   design that recognised that deployed middleboxes relied on the
   presence of certain header fields exposed in TLS 1.2, and failed if
   those headers were changed.

   The design of MPTCP also had to be revised to account for middleboxes
   (known as "TCP Normalizers") that monitor the evolution of the window
   advertised in the TCP header and reset connections when the window
   does not grow as expected.  Similarly, Issues have been reported with
   TCP Fast Open using middleboxes that modify the transport header of
   packets by removing unknown TCP options, that drop segments with
   unknown TCP options, drop segments that contain data and have the SYN
   bit set, drop packets with SYN/ACK that acknowledge data, or that
   disrupt connections that send data before the three-way handshake
   completes.  Other examples of ossification have included middleboxes
   that rewrite TCP sequence and acknowledgement numbers, but are
   unaware of the (newer) TCP selective acknowledgement (SACK) Option

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   and therefore fail to correctly rewrite the selective acknowledgement
   header information to match the changes that were made to the fixed
   TCP header.

   In all these cases, the issue was caused by middleboxes that had a
   hard-coded understanding of transport behaviour, and that interacted
   poorly with transport protocols when the transport behaviour changed.
   Many protocol specifications had also failed to clearly indicate the
   invariant parts of the transport header and were designed without
   thought for how header information could be used within the network.

   Transport header encryption can help reduce such ossification of the
   transport layer.  A protocol design that uses header encryption with
   secure key distribution can provide confidentiality for some, or all,
   of the protocol header information.  This prevents an on-path device
   from observing the transport headers, and stops mechanisms being
   built that directly rely on transport header information, or that
   seek to infer semantics of exposed header fields.  This encryption is
   normally combined with authentication of the protected information.
   RFC 8546 summarises this, stating that it is "The wire image, not the
   protocol's specification, determines how third parties on the network
   paths among protocol participants will interact with that protocol"
   [RFC8546].

   While encryption can hide transport header information and therefore
   help to reduce ossification of the transport protocol, it does not
   prevent ossification of the network service.  People seeking to
   understand network traffic could come to rely on pattern inferences
   and other heuristics as the basis for network decision and to derive
   measurement data.  This can create new dependencies on the transport
   protocol, or the patterns of traffic it can generate.  This use of
   machine-learning methods usually demands large data sets, presenting
   it own requirements for collecting and distributing the data.

2.2.  Authentication of Transport Header Information

   The design of a transport protocol needs to determine whether to
   encrypt all or a part of the transport information.  It is possible
   that on-path devices could develop mechanisms that rely on the
   presence of any non-encrypted field, or a known value in the field.
   Section 4 of RFC8558 goes further, to state: "Anything exposed to the
   path should be done with the intent that it be used by the network
   elements on the path" [RFC8558].  In this context, specification of a
   non-encrypted transport header field explicitly allows protocol
   designers to make the certain header information observable by the
   network.  This supports use of this information by on-path devices,
   but at the same time this can lead to ossification of the exposed
   part of a transport header.  That is, network forwarding could evolve

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   to depend on the presence and/or value of these fields (even if the
   header is not modified by the in-network device).

   New protocol designs will make use of authentication to provide a
   cryptographic integrity check for the transport header fields.
   Transport header information that is authenticated, but not
   encrypted, permits inspection of the non-encrypted header fields by
   devices on the path, but does prevent undetected manipulation by
   network devices.

   Sometimes a protocol design employs a header field that is not
   encrypted, but it is desired to avoid unwanted inspection restricting
   the choice of usable values in the field (and the resulting potential
   for undesirable ossification).  In this case, the protocol designers
   can choose to intentionally vary the format and/or value of exposed
   header fields to reduce the chance of ossification (see Section 4 and
   [I-D.ietf-tls-grease]).

2.3.  Observable Transport Header Fields

   Transport headers have end-to-end meaning, but are often observed by
   equipment within the network.  The decision about which transport
   headers fields are made observable offers trade-offs around header
   confidentiality versus header observability (including non-encrypted
   but authenticated header fields) for network operations and
   management, and the implications for ossification and user privacy.
   The impact differs depending on the activity, as discussed below and
   developed in the remainder of this document:

   Network Operations: Observable transport headers enable explicit
                       measurement and analysis of protocol performance,
                       network anomalies, and failure pathologies at any
                       point along the Internet path.  In many cases, it
                       is important to relate observations to specific
                       equipment/configurations or a specific network
                       segment.

                       Concealing transport header information makes
                       performance/behaviour unavailable to passive
                       observers along the path.  Operators will then be
                       unable to use this information directly and could
                       turn to more ambitious ways to collect, estimate,
                       or infer that data.  (Operational practices aimed
                       at guessing transport parameters are out of scope
                       for this document, and are only mentioned here to
                       recognize that encryption does not stop operators
                       from attempting to apply practices that have been
                       used with unencrypted transport headers.)

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                       See also Sections 3, 5, and 6.4.

   Traffic Analysis:   Observable transport headers can be used to
                       determine which transport protocols and features
                       are being used across a network segment, and to
                       measure trends in the pattern of usage.  For some
                       use cases, end-to-end measurements/traces are
                       sufficient and can assist in developing and
                       debugging new transports and analysing their
                       deployment.  In other uses, it is important to
                       relate observations to specific equipment/
                       configurations or particular network segments.

                       Concealing transport header information can make
                       analysis harder or impossible.  This could impact
                       the ability to anticipate the need for network
                       upgrades and roll-out, or affect on-going traffic
                       engineering activities performed by operators
                       such as determining which parts of the path
                       contribute delay, jitter, or loss.  While this
                       impact could, in many cases, be small, there are
                       scenarios where operators will actively monitor
                       and support particular services, e.g., to explore
                       issues relating to Quality of Service (QoS), to
                       perform fast re-routing of critical traffic, to
                       mitigate the characteristics of specific radio
                       links, and so on.

                       See also Sections 3.1-3.2, and 5.

   Troubleshooting:    Observable transport headers can be utilised by
                       operators for network troubleshooting and
                       diagnostics.  Effective troubleshooting often
                       requires visibility into the transport layer
                       behaviour.  Flows experiencing packet loss or
                       jitter are hard to distinguish from unaffected
                       flows when only observing network layer headers.

                       Concealing transport header information reduces
                       the incentive for operators to troubleshoot,
                       since they cannot interpret the data.  This can
                       limit understanding of transport dynamics, such
                       as the impact of packet loss or latency on the
                       flows, or make it harder to localise the network
                       segment introducing the packet loss or latency.
                       Additional mechanisms will be needed to help
                       reconstruct or replace transport-level metrics
                       for troubleshooting and diagnostics.  These can

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                       add complexity and operational costs (e.g., in
                       deploying additional functions in equipment or
                       adding traffic overhead).

                       See also Section 3.3 and 5.

   Network Protection: Observable transport headers currently provide
                       useful input to classify and detect anomalous
                       events, such as changes in application behaviour
                       or distributed denial of service attacks.  An
                       operator needs to uniquely disambiguate unwanted
                       traffic.

                       Concealing transport header information would
                       prevent disambiguation based on transport
                       information.  This could result in less-efficient
                       identification of unwanted traffic, the use of
                       heuristics to identify anomalous flows, or the
                       introduction of rate limits for uncharacterised
                       traffic.

                       See also Sections 6.2 and 6.3.

   SLA Compliance:     Observable transport headers coupled with
                       published transport specifications allow
                       operators and regulators to explore teh
                       compliance with Service Level Agreements (SLAs).
                       Independently verifiable performance metrics can
                       also be utilised to demonstrate regulatory
                       compliance in some jurisdictions, and as a basis
                       for informing design decisions.  This can bring
                       assurance to those operating networks, often
                       avoiding the need to deploy complex techniques
                       that routinely monitor and manage Internet
                       traffic flows (e.g., avoiding the capital and
                       operational costs of deploying flow rate-limiting
                       and network circuit-breaker methods [RFC8084]).

                       When transport header information is concealed,
                       it is not possible to observe transport header
                       information.  Methods are still needed to confirm
                       that the traffic produced conforms to the
                       expectations of the operator or developer.

                       See also Sections 5 and 6.1-6.3.

   Verifiable Data:    Observable transport headers can provide open and
                       verifiable measurements to support operations,

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                       research, and protocol development.  The ability
                       of other stake holders to review transport header
                       traces helps develop insight into performance and
                       traffic contribution of specific variants of a
                       protocol.  Independently observed data is
                       important to help ensure the health of the
                       research and development communities.

                       Concealing transport header information can
                       reduce the range of actors that can observe
                       useful data.  This limits the information sources
                       available to the Internet community to understand
                       the operation of new transport protocols,
                       reducing information to inform design decisions
                       and standardisation of the new protocols and
                       related operational practices

                       See also Section 6.

   There are architectural challenges and considerations in the way
   transport protocols are designed, and the ability to characterise and
   compare different transport solutions [Measure].  Different parties
   will view the relative importance of these differently.  For some,
   the benefits of encrypting the transport headers could outweigh the
   impact of doing so; others might make a different trade-off.

3.  Current uses of Transport Headers within the Network

   In response to pervasive monitoring [RFC7624] revelations and the
   IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
   efforts are underway to increase encryption of Internet traffic.
   Applying confidentiality to transport header fields affects how
   protocol information is used [RFC8404], requiring consideration of
   the trade-offs discussed in Section 2.3.  To understand the
   implications, it is necessary to understand how transport layer
   headers are currently observed and/or modified by middleboxes within
   the network.

   This section reviews some current usage.  This review does not
   consider the intentional modification of transport headers by
   middleboxes (such as in Network Address Translation, NAT, or
   Firewalls).  Common issues concerning IP address sharing are
   described in [RFC6269].

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3.1.  Observing Transport Information in the Network

   If in-network observation of transport protocol headers is needed,
   this requires knowledge of the format of the transport header:

   o  Flows need to be identified at the level needed to perform the
      observation;

   o  The protocol and version of the header need to be visible, e.g.,
      by defining the wire image [RFC8546].  As protocols evolve over
      time and there could be a need to introduce new transport headers.
      This could require interpretation of protocol version information
      or connection setup information;

   o  The location and syntax of any observed transport headers need to
      be known.  IETF transport protocols can specify this information.

   The following subsections describe various ways that observable
   transport information has been utilised.

3.1.1.  Flow Identification Using Transport Layer Headers

   Flow/Session identification [RFC8558] is a common function.  For
   example, performed by measurement activities, QoS classification,
   firewalls, Denial of Service, DOS, prevention.

   Observable transport header information, together with information in
   the network header, has been used to identify flows and their
   connection state, together with the protocol options being used.
   Transport protocols, such as TCP and the Stream Control Transport
   Protocol (SCTP), specify a standard base header that includes
   sequence number information and other data.  They also have the
   possibility to negotiate additional headers at connection setup,
   identified by an option number in the transport header.

   In some uses, a low-numbered (well-known) transport port number can
   identify the protocol.  However, port information alone is not
   sufficient to guarantee identification when applications can use
   arbitrary ports, multiple sessions can be multiplexed on a single
   port, and ports can be re-used by subsequent sessions.  UDP-based
   protocols often do not use well-known port numbers.  Some flows can
   instead be identified by observing signalling protocol data (e.g.,
   [RFC3261], [I-D.ietf-rtcweb-overview]) or through the use of magic
   numbers placed in the first byte(s) of the datagram payload
   [RFC7983].

   Concealing transport header information can remove information used
   to classify flows by passive observers along the path, so operators

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   will be unable to use this information directly.  Operators could
   turn to more ambitious ways to collect, estimate, or infer that data,
   including heuristics based on the analysis of traffic patterns.  For
   example, an operator that cannot access the Session Description
   Protocol (SDP) session descriptions to classify a flow as audio
   traffic, might instead use (possibly less-reliable) heuristics to
   infer that short UDP packets with regular spacing carry audio
   traffic.  Operational practices aimed at inferring transport
   parameters are out of scope for this document, and are only mentioned
   here to recognize that encryption does not prevent operators from
   attempting to apply practices that were used with unencrypted
   transport headers.

3.1.2.  Metrics derived from Transport Layer Headers

   Observable transport headers enable explicit measurement and analysis
   of protocol performance, network anomalies, and failure pathologies
   at any point along the Internet path.  Some operators use passive
   monitoring to manage their portion of the Internet by characterizing
   the performance of link/network segments.  Inferences from transport
   headers are used to derive performance metrics.  A variety of open
   source and commercial tools have been deployed that utilise transport
   header information in this way to derive the following metrics:

   Traffic Rate and Volume:  Protocol sequence number and packet size
      can be used to derive volume measures per-application, to
      characterise the traffic that uses a network segment or the
      pattern of network usage.  Measurements can be per endpoint or for
      an endpoint aggregate (e.g., to assess subscriber usage).
      Measurments can also be used to trigger traffic shaping, and to
      associate QoS support within the network and lower layers.  Volume
      measures can also be valuable for capacity planning and providing
      detail of trends in usage.

   Loss Rate and Loss Pattern:  Flow loss rate can be derived (e.g.,
      from transport sequence numbers) and has been used as a metric for
      performance assessment and to characterise transport behaviour.
      Understanding the location and root cause of loss can help an
      operator determine whether this requires corrective action.
      Network operators have used the variation in patterns of loss as a
      key performance metric, utilising this to detect changes in the
      offered service.

      There are various causes of loss, including: corruption of link
      frames (e.g., due to interference on a radio link), buffering loss
      (e.g., overflow due to congestion, Active Queue Management, AQM
      [RFC7567], or inadequate provision following traffic pre-emption),
      and policing (traffic management).  Understanding flow loss rates

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      requires either observing sequence numbers in transport headers,
      or maintaining per-flow packet counters (flow identification often
      requires transport header information).  Per-hop loss can also
      sometimes be monitored at the interface level by devices in the
      network.  It is often valuable to understand the conditions under
      which packet loss occurs, which usually requires relating loss to
      the traffic flowing on the network node/segment at the time of
      loss.

      Observation of transport feedback information (e.g., RTP Control
      Protocol (RTCP) reception reports [RFC3550], TCP SACK blocks) can
      increase understanding of the impact of loss and help identify
      cases where loss could have been wrongly identified, or where the
      transport did not require transmission of the lost packet.  It is
      sometimes more helpful to understand the pattern of loss, than the
      loss rate, because losses can often occur as bursts, rather than
      randomly-timed events.

   Throughput and Goodput:  Throughput is the amount of data sent by a
      flow per time interval.  Goodput [RFC7928] is a measure of useful
      data exchanged (the ratio of useful data to total volume of
      traffic sent by a flow).  The throughput of a flow can be
      determined even when transport header information is concealed,
      providing the individual flow can be identified.  Goodput requires
      ability to differentiate loss and retransmission of packets, for
      example by observing packet sequence numbers in the TCP or the
      Real-time Transport Protocol (RTP) headers [RFC3550].

   Latency:  Latency is a key performance metric that impacts
      application and user-perceived response times.  It often
      indirectly impacts throughput and flow completion time.  This
      determines the reaction time of the transport protocol itself,
      impacting flow setup, congestion control, loss recovery, and other
      transport mechanisms.  The observed latency can have many
      components [Latency].  Of these, unnecessary/unwanted queuing in
      network buffers has often been observed as a significant factor
      [bufferbloat].  Once the cause of unwanted latency has been
      identified, this can often be eliminated.

      To measure latency across a part of a path, an observation point
      [RFC7799] can measure the experienced round trip time (RTT) using
      packet sequence numbers, and acknowledgements, or by observing
      header timestamp information.  Such information allows an
      observation point in the network to determine not only the path
      RTT, but also allows measurement of the upstream and downstream
      contribution to the RTT.  This could be used to locate a source of
      latency, e.g., by observing cases where the median RTT is much
      greater than the minimum RTT for a part of a path.

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      The service offered by network operators can benefit from latency
      information to understand the impact of configuration changes and
      to tune deployed services.  Latency metrics are key to evaluating
      and deploying AQM [RFC7567], DiffServ [RFC2474], and Explicit
      Congestion Notification (ECN) [RFC3168] [RFC8087].  Measurements
      could identify excessively large buffers, indicating where to
      deploy or configure AQM.  An AQM method is often deployed in
      combination with other techniques, such as scheduling [RFC7567]
      [RFC8290] and although parameter-less methods are desired
      [RFC7567], current methods often require tuning [RFC8290]
      [RFC8289] [RFC8033] because they cannot scale across all possible
      deployment scenarios.

   Variation in delay:  Some network applications are sensitive to
      (small) changes in packet timing (jitter).  Short and long-term
      delay variation can impact on the latency of a flow and hence the
      perceived quality of applications using the network.  For example,
      jitter metrics are often cited when characterising paths
      supporting real-time traffic.  To assess the performance of such
      applications, it can be necessary to measure the variation in
      delay observed along a portion of the path [RFC3393] [RFC5481].
      The requirements for observable transport headers resemble those
      for the measurement of latency.

   Flow Reordering:  Significant packet reordering within a flow can
      impact time-critical applications and can be interpreted as loss
      by reliable transports.  Many transport protocol techniques are
      impacted by reordering (e.g., triggering TCP retransmission or re-
      buffering of real-time applications).  Packet reordering can occur
      for many reasons, from equipment design to misconfiguration of
      forwarding rules.  Since this impacts transport performance,
      network tools are needed to detect and measure unwanted/excessive
      reordering.

      There have been initiatives in the IETF transport area to reduce
      the impact of reordering within a transport flow, possibly leading
      to a reduction in the requirements for preserving ordering.  These
      have potential to simplify network equipment design as well as the
      potential to improve robustness of the transport service.
      Measurements of reordering can help understand the present level
      of reordering within deployed infrastructure, and inform decisions
      about how to progress such mechanisms.  Key performance indicators
      are retransmission rate, packet drop rate, sector utilisation
      level, a measure of reordering, peak rate, the ECN congestion
      experienced (CE) marking rate, etc.

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      Metrics have been defined that evaluate whether a network has
      maintained packet order on a packet-by-packet basis [RFC4737]
      [RFC5236].

      Techniques for measuring reordering typically observe packet
      sequence numbers.  Some protocols provide in-built monitoring and
      reporting functions.  Transport fields in the RTP header [RFC3550]
      [RFC4585] can be observed to derive traffic volume measurements
      and provide information on the progress and quality of a session
      using RTP.  As with other measurement, metadata is often needed to
      understand the context under which the data was collected,
      including the time, observation point [RFC7799], and way in which
      metrics were accumulated.  The RTCP protocol directly reports some
      of this information in a form that can be directly visible in the
      network.  A user of summary measurement data needs to trust the
      source of this data and the method used to generate the summary
      information.

   This information can support network operations, inform capacity
   planning, and assist in determining the need for equipment and/or
   configuration changes by network operators.  It can also inform
   Internet engineering activities by informing the development of new
   protocols, methodologies, and procedures.

3.1.3.  Transport use of Network Layer Header Fields

   Information from the transport protocol can be used by a multi-field
   classifier as a part of policy framework.  Policies are commonly used
   for management of the QoS or Quality of Experience (QoE) in resource-
   constrained networks, and by firewalls to implement access rules (see
   also section 2.2.2 of [RFC8404]).  Network-layer classification
   methods that rely on a multi-field classifier (e.g., inferring QoS
   from the 5-tuple or choice of application protocol) are incompatible
   with transport protocols that encrypt the transport information.
   Traffic that cannot be classified will typically receive a default
   treatment.

   Transport information can also be explicitly set in network-layer
   header fields that are not encrypted, serving as a replacement/
   addition to the exposed transport information [RFC8558].  This can
   provide information to enable a different forwarding treatment by the
   network, even when a transport employs encryption to protect other
   header information.

   The user of a transport that multiplexes multiple sub-flows might
   want to hide the presence and characteristics of these sub-flows.  On
   the other hand, an encrypted transport could set the network-layer
   information to indicate the presence of sub-flows, and to reflect the

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   network needs of individual sub-flows.  There are several ways this
   could be done:

   IP Address:  Applications normally expose the addresses used by
      endpoints, and this is used in the forwarding decisions in network
      devices.  Address and other protocol information can be used by a
      Multi-Field (MF) classifier to determine how traffic is treated
      [RFC2475], and hence the quality of experience for a flow.

   Using the IPv6 Network-Layer Flow Label:  A number of Standards Track
      and Best Current Practice RFCs (e.g., [RFC8085], [RFC6437],
      [RFC6438]) encourage endpoints to set the IPv6 Flow label field of
      the network-layer header.  IPv6 "source nodes SHOULD assign each
      unrelated transport connection and application data stream to a
      new flow" [RFC6437].  A multiplexing transport could choose to use
      multiple Flow labels to allow the network to independently forward
      subflows.  RFC6437 provides further guidance on choosing a flow
      label value, stating these "should be chosen such that their bits
      exhibit a high degree of variability", and chosen so that "third
      parties should be unlikely to be able to guess the next value that
      a source of flow labels will choose".

      Once set, a flow label can provide information that can help
      inform network-layer queuing and forwarding [RFC6438], for example
      with Equal Cost Multi-Path routing and Link Aggregation [RFC6294].
      Considerations when using IPsec are further described in
      [RFC6438].

      The choice of how to assign a Flow Label needs to avoid
      introducing linkability that a network device could observe.
      Inappropriate use by the transport can have privacy implications
      (e.g., assigning the same label to two independent flows that
      ought not to be classified the same).

   Using the Network-Layer Differentiated Services Code Point:
      Applications can expose their delivery expectations to the network
      by setting the Differentiated Services Code Point (DSCP) field of
      IPv4 and IPv6 packets [RFC2474].  For example, WebRTC applications
      identify different forwarding treatments for individual sub-flows
      (audio vs. video) based on the value of the DSCP field
      [I-D.ietf-tsvwg-rtcweb-qos]).  This provides explicit information
      to inform network-layer queuing and forwarding, rather than an
      operator inferring traffic requirements from transport and
      application headers via a multi-field classifier.  Inappropriate
      use can have privacy implications (e.g., assigning the same label
      to two independent flows that ought not to be classified the
      same).  Inappropriate use by the transport can have privacy
      implications (e.g., assigning a different DSCP to a subflow could

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      assist in a network device discovering the traffic pattern used by
      an application).  The field is mutable, i.e., some network devices
      can be expected to change this field (use of each DSCP value is
      defined by an RFC).

      Since the DSCP value can impact the quality of experience for a
      flow, observations of service performance need to consider this
      field when a network path has support for differentiated service
      treatment.

   Using Explicit Congestion Marking:  ECN [RFC3168] is a transport
      mechanism that utilises the ECN field in the network-layer header.
      Use of ECN explicitly informs the network-layer that a transport
      is ECN-capable, and requests ECN treatment of the flow.  An ECN-
      capable transport can offer benefits when used over a path with
      equipment that implements an AQM method with CE marking of IP
      packets [RFC8087], since it can react to congestion without also
      having to recover from lost packets.

      ECN exposes the presence of congestion.  The reception of CE-
      marked packets can be used to estimate the level of incipient
      congestion on the upstream portion of the path from the point of
      observation (Section 2.5 of [RFC8087]).  Interpreting the marking
      behaviour (i.e., assessing congestion and diagnosing faults)
      requires context from the transport layer, such as path RTT.

      AQM and ECN offer a range of algorithms and configuration options.
      Tools therefore need to be available to network operators and
      researchers to understand the implication of configuration choices
      and transport behaviour as the use of ECN increases and new
      methods emerge [RFC7567].

   When transport headers are concealed, operators will be unable to use
   this information directly.  Careful use of the network layer features
   can help address provide similar information in the case where the
   network is unable to inspect transport protocol headers.
   Section Section 5 describes use of network extension headers.

3.2.  Transport Measurement

   The common language between network operators and application/content
   providers/users is packet transfer performance at a layer that all
   can view and analyse.  For most packets, this has been the transport
   layer, until the emergence of transport protocols performing header
   encryption, with the obvious exception of VPNs and IPsec.

   When encryption conceals more layers in each packet, people seeking
   understanding of the network operation rely more on pattern inference

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   and other heuristics.  It remains to be seen whether more complex
   inferences can be mastered to produce the same monitoring accuracy
   (see section 2.1.1 of [RFC8404]).

   When measurement datasets are made available by servers or client
   endpoints, additional metadata, such as the state of the network, is
   often necessary to interpret this data to answer questions about
   network performance or understand a pathology.  Collecting and
   coordinating such metadata is more difficult when the observation
   point is at a different location to the bottleneck/device under
   evaluation [RFC7799].

   Packet sampling techniques are used to scale the processing involved
   in observing packets on high rate links.  This exports only the
   packet header information of (randomly) selected packets.  The
   utility of these measurements depends on the type of bearer and
   number of mechanisms used by network devices.  Simple routers are
   relatively easy to manage, a device with more complexity demands
   understanding of the choice of many system parameters.  This level of
   complexity exists when several network methods are combined.

   This section discusses topics concerning observation of transport
   flows, with a focus on transport measurement.

3.2.1.  Point of Observation

   On-path measurements are particularly useful for locating the source
   of problems, or to assess the performance of a network segment or a
   particular device configuration.  Often issues can only be understood
   in the context of the other flows that share a particular path,
   common network device, interface port, etc.  A simple example is
   monitoring of a network device that uses a scheduler or active queue
   management technique [RFC7567], where it could be desirable to
   understand whether the algorithms are correctly controlling latency,
   or if overload protection is working.  This understanding implies
   knowledge of how traffic is assigned to any sub-queues used for flow
   scheduling, but can also require information about how the traffic
   dynamics impact active queue management, starvation prevention
   mechanisms, and circuit-breakers.

   Sometimes multiple on-path observation points are needed.  By
   correlating observations of headers at multiple points along the path
   (e.g., at the ingress and egress of a network segment), an observer
   can determine the contribution of a portion of the path to an
   observed metric, to locate a source of delay, jitter, loss,
   reordering, congestion marking, etc.

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3.2.2.  Use by Operators to Plan and Provision Networks

   Traffic measurements are used by operators to help plan deployment of
   new equipment and configuration in their networks.  Data is also
   valuable to equipment vendors who want to understand traffic trends
   and patterns of usage as inputs to decisions about planning products
   and provisioning for new deployments.  This measurement information
   can also be correlated with billing information when this is also
   collected by an operator.

   A network operator supporting traffic that uses transport header
   encryption might not have access to per-flow measurement data.
   Trends in aggregate traffic can be observed and can be related to the
   endpoint addresses being used, but it might be impossible to
   correlate patterns in measurements with changes in transport
   protocols (e.g., the impact of changes in introducing a new transport
   protocol mechanism).  This increases the dependency on other indirect
   sources of information to inform planning and provisioning.

3.2.3.  Service Performance Measurement

   Traffic measurements (e.g., traffic volume, loss, latency) can be
   used by various actors to help analyse the performance offered to the
   users of a network segment, and to inform operational practice.

   While active measurements (see section 3.4 of [RFC7799]) could be
   used within a network, passive measurements (see section 3.6 of
   [RFC7799]) can have advantages in terms of eliminating unproductive
   test traffic, reducing the influence of test traffic on the overall
   traffic mix, and the ability to choose the point of observation (see
   Section 3.2.1).  Passive measurements can rely on observing transport
   headers, which is not possible if those headers are encrypted, but
   could utilise information about traffic volumes or patterns of
   interaction to deduce metrics.

3.2.4.  Measuring Transport to Support Network Operations

   Information provided by tools observing transport headers can help
   determine whether mechanisms are needed in the network to prevent
   flows from acquiring excessive network capacity.  Operators can
   implement operational practices to manage traffic flows (e.g., under
   severe congestion) by deploying rate-limiters, traffic shaping or
   network transport circuit breakers [RFC8084].

   Congestion Control Compliance of Traffic:  Congestion control is a
      key transport function [RFC2914].  Many network operators
      implicitly accept that TCP traffic complies with a behaviour that
      is acceptable for use in the shared Internet.  TCP algorithms have

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      been continuously improved over decades and they have reached a
      level of efficiency and correctness that custom application-layer
      mechanisms will struggle to easily duplicate [RFC8085].

      A standards-compliant TCP stack provides congestion control that
      is judged safe for use across the Internet.  Applications
      developed on top of well-designed transports can be expected to
      appropriately control their network usage, reacting when the
      network experiences congestion, by back-off and reduce the load
      placed on the network.  This is the normal expected behaviour for
      IETF-specified transports (e.g., TCP and SCTP).

      However, when anomalies are detected, tools can interpret the
      transport protocol header information to help understand the
      impact of specific transport protocols (or protocol mechanisms) on
      the other traffic that shares a network.  An observation in the
      network can gain an understanding of the dynamics of a flow and
      its congestion control behaviour.  Analysing observed flows can
      help to build confidence that an application flow backs-off its
      share of the network load in the face of persistent congestion,
      and hence to understand whether the behaviour is appropriate for
      sharing limited network capacity.  For example, it is common to
      visualise plots of TCP sequence numbers versus time for a flow to
      understand how a flow shares available capacity, deduce its
      dynamics in response to congestion, etc.

      The ability to identify sources that contribute to persistent
      congestion is important to the safe operation of network
      infrastructure, and can inform configuration of network devices to
      complement the endpoint congestion avoidance mechanisms [RFC7567]
      [RFC8084] to avoid a portion of the network being driven into
      congestion collapse [RFC2914].

   Congestion Control Compliance for UDP traffic:  UDP provides a
      minimal message-passing datagram transport that has no inherent
      congestion control mechanisms.  Because congestion control is
      critical to the stable operation of the Internet, applications and
      other protocols that choose to use UDP as a transport need to
      employ mechanisms to prevent collapse, avoid unacceptable
      contributions to jitter/latency, and to establish an acceptable
      share of capacity with concurrent traffic [RFC8085].

      A network operator needs tools to understand if datagram flows
      (e.g., using UDP) comply with congestion control expectations and
      therefore whether there is a need to deploy methods such as rate-
      limiters, transport circuit breakers, or other methods to enforce
      acceptable usage for the offered service.

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      UDP flows that expose a well-known header by specifying the format
      of header fields can allow information to be observed to gain
      understanding of the dynamics of a flow and its congestion control
      behaviour.  For example, tools exist to monitor various aspects of
      RTP and RTCP header information for real-time flows (see
      Section 3.1.2).  The Secure RTP extensions [RFC3711] were
      explicitly designed to expose some header information to enable
      such observation, while protecting the payload data.

3.3.  Use for Network Diagnostics and Troubleshooting

   Transport header information can be useful for a variety of
   operational tasks [RFC8404]: to diagnose network problems, assess
   network provider performance, evaluate equipment/protocol
   performance, capacity planning, management of security threats
   (including denial of service), and responding to user performance
   questions.  Section 3.1.2 and Section 5 of [RFC8404] provide further
   examples.  These tasks seldom involve the need to determine the
   contents of the transport payload, or other application details.  The
   use of payload encryption has the desirable effect of preventing
   unintended observation of the user data.

   A network operator supporting traffic that uses transport header
   encryption can see only encrypted transport headers.  This prevents
   deployment of performance measurement tools that rely on transport
   protocol information.  Choosing to encrypt all the information
   reduces the ability of an operator to observe transport performance
   and could limit the ability of network operators to trace problems,
   make appropriate QoS decisions, or response to other queries about
   the network service.  For some this will be blessing, for others it
   might be a curse.  For example, operational performance data about
   encrypted flows needs to be determined by traffic pattern analysis,
   rather than relying on traditional tools.  This can impact the
   ability of the operator to respond to faults, it could require
   reliance on endpoint diagnostic tools or user involvement in
   diagnosing and troubleshooting unusual use cases or non-trivial
   problems.  A key need here is for tools to provide useful information
   during network anomalies (e.g., significant reordering, high or
   intermittent loss).

   Measurements can be used to monitor the health of a portion of the
   Internet, to provide early warning of the need to take action.  They
   can assist in setting buffer sizes, debugging and diagnosing the root
   causes of faults that concern a particular user's traffic.  They can
   also be used to support post-mortem investigation after an anomaly to
   determine the root cause of a problem.

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   In some cases, measurements could involve active injection of test
   traffic to perform a measurement.  However, most operators do not
   have access to user equipment, therefore the point of test is
   normally different from the transport endpoint.  Injection of test
   traffic can incur an additional cost in running such tests (e.g., the
   implications of capacity tests in a mobile network are obvious).
   Some active measurements [RFC7799] (e.g., response under load or
   particular workloads) perturb other traffic, and could require
   dedicated access to the network segment.  An alternative approach is
   to use in-network techniques that observe transport packet headers
   added while traffic traverses an operational network to make the
   measurements.  These measurements do not require the cooperation of
   an endpoint.

   In other cases, measurement involves dissecting network traffic
   flows.  The observed transport layer information can help identify
   whether the link/network tuning is effective and alert to potential
   problems that can be hard to derive from link or device measurements
   alone.  The design trade-offs for radio networks are often very
   different from those of wired networks.  A radio-based network (e.g.,
   cellular mobile, enterprise WiFi, satellite access/back-haul, point-
   to-point radio) has the complexity of a subsystem that performs radio
   resource management, with direct impact on the available capacity,
   and potentially loss/reordering of packets.  The impact of the
   pattern of loss and congestion, differs for different traffic types,
   correlation with propagation and interference can all have
   significant impact on the cost and performance of a provided service.
   The need for this type of information is expected to increase as
   operators bring together heterogeneous types of network equipment and
   seek to deploy opportunistic methods to access radio spectrum.

   A flow that conceals its transport header information could imply
   "don't touch" to some operators.  This could limit a trouble-shooting
   response to "can't help, no trouble found".

3.4.  Header Compression

   Header compression saves link capacity by compressing network and
   transport protocol headers on a per-hop basis.  It was widely used
   with low bandwidth dial-up access links, and still finds application
   on wireless links that are subject to capacity constraints.  Header
   compression has been specified for use with TCP/IP and RTP/UDP/IP
   flows [RFC2507], [RFC2508], [RFC4995].

   While it is possible to compress only the network layer headers,
   significant savings can be made if both the network and transport
   layer headers are compressed together as a single unit.  The Secure
   RTP extensions [RFC3711] were explicitly designed to leave the

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   transport protocol headers unencrypted, but authenticated, since
   support for header compression was considered important.  Encrypting
   the transport protocol headers does not break such header
   compression, but does cause a fall back to compressing only the
   network layer headers, with a significant reduction in efficiency.

4.  Encryption and Authentication of Transport Headers

   End-to-end encryption can be applied at various protocol layers.  It
   can be applied above the transport to encrypt the transport payload
   (e.g., using TLS).  This can hide information from an eavesdropper in
   the network.  It can also help protect the privacy of a user, by
   hiding data relating to user/device identity or location.

   There are several motivations for encryption:

   o  One motive to use encryption is a response to perceptions that the
      network has become ossified by over-reliance on middleboxes that
      prevent new protocols and mechanisms from being deployed.  This
      has lead to a perception that there is too much "manipulation" of
      protocol headers within the network, and that designing to deploy
      in such networks is preventing transport evolution.  In the light
      of this, a method that authenticates transport headers could help
      improve the pace of transport development, by eliminating the need
      to always consider deployed middleboxes
      [I-D.trammell-plus-abstract-mech], or potentially to only
      explicitly enable use by middleboxes for particular paths with
      particular middleboxes that are deliberately deployed to realise a
      useful function for the network and/or users[RFC3135].

   o  Another motivation stems from increased concerns about privacy and
      surveillance.  Some Internet users have valued the ability to
      protect identity, user location, and defend against traffic
      analysis, and have used methods such as IPsec Encapsulated
      Security Payload (ESP), VPNs and other encrypted tunnel
      technologies.  Revelations about the use of pervasive surveillance
      [RFC7624] have, to some extent, eroded trust in the service
      offered by network operators, and following the Snowden
      revelations in the USA in 2013 has led to an increased desire for
      people to employ encryption to avoid unwanted "eavesdropping" on
      their communications.  Concerns have also been voiced about the
      addition of information to packets by third parties to provide
      analytics, customization, advertising, cross-site tracking of
      users, to bill the customer, or to selectively allow or block
      content.  Whatever the reasons, the IETF is designing new
      protocols that include transport header encryption (e.g., QUIC
      [I-D.ietf-quic-transport]) to supplement the already widespread
      payload encryption.

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   o  Any header information that has a clear definition in the protocol
      message format(s), or is implied by that definition, and is not
      cryptographically confidentiality-protected can be unambiguously
      interpreted by on-path observers [RFC8546].

   Encryption methods do not prevent traffic analysis, and usage needs
   to reflect that profiling of users, identification of location, and
   fingerprinting of behaviour can take place even on encrypted traffic
   flows.  The use of transport layer authentication and encryption
   exposes a tussle between middlebox vendors, operators, applications
   developers and users:

   o  On the one hand, future Internet protocols that enable large-scale
      encryption assist in the restoration of the end-to-end nature of
      the Internet by returning complex processing to the endpoints,
      since middleboxes cannot modify what they cannot see.

   o  On the other hand, encryption of transport layer header
      information has implications for people who are responsible for
      operating networks and researchers and analysts seeking to
      understand the dynamics of protocols and traffic patterns.

   Whatever the motives, a decision to use pervasive transport header
   encryption will have implications on the way in which design and
   evaluation is performed.  This can, in turn, impact the direction of
   evolution of the transport protocol stack.  While the IETF can
   specify protocols, the success in actual deployment is often
   determined by many factors [RFC5218] that are not always clear at the
   time when protocols are being defined.

   The following briefly reviews some security design options for
   transport protocols.  A Survey of Transport Security Protocols
   [I-D.ietf-taps-transport-security] provides more details concerning
   commonly used encryption methods at the transport layer.

   Authenticating the Transport Protocol Header:  Transport layer header
      information can be authenticated.  An integrity check that
      protects the immutable transport header fields, but can still
      expose the transport protocol header information in the clear,
      allows in-network devices to observe these fields.  An integrity
      check is not able to prevent in-network modification, but can
      prevent a receiving from accepting changes and avoid impact on the
      transport protocol operation.

      An example transport authentication mechanism is TCP-
      Authentication (TCP-AO) [RFC5925].  This TCP option authenticates
      the IP pseudo header, TCP header, and TCP data.  TCP-AO protects
      the transport layer, preventing attacks from disabling the TCP

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      connection itself and provides replay protection.  TCP-AO might
      interact with middleboxes, depending on their behaviour [RFC3234].

      The IPsec Authentication Header (AH) [RFC4302] was designed to
      work at the network layer and authenticate the IP payload.  This
      approach authenticates all transport headers, and verifies their
      integrity at the receiver, preventing in-network modification.
      Secure RTP [RFC3711] is another example of a transport protocol
      that allows header authentication.

   Greasing:  Protocols often provide extensibility features, reserving
      fields or values for use by future versions of a specification.
      The specification of receivers has traditionally ignored
      unspecified values, however in-network devices have emerged that
      ossify to require a certain value in a field, or re-use a field
      for another purpose.  When the specification is later updated, it
      is impossible to deploy the new use of the field, and forwarding
      of the protocol could even become conditional on a specific header
      field value.

      A protocol can intentionally vary the value, format, and/or
      presence of observable transport header fields.  This behaviour,
      known as GREASE (Generate Random Extensions And Sustain
      Extensibility) is designed to avoid a network device ossifying the
      use of a specific observable field.  Greasing seeks to ease
      deployment of new methods.  It can also prevent in-network devices
      utilising the information in a transport header, or can make an
      observation robust to a set of changing values, rather than a
      specific set of values

   Selectively Encrypting Transport Headers and Payload:  A transport
      protocol design can encrypt selected header fields, while also
      choosing to authenticate the entire transport header.  This allows
      specific transport header fields to be made observable by network
      devices.  End-to end integrity checks can prevent an endpoint from
      undetected modification of the immutable transport headers.

      Mutable fields in the transport header provide opportunities for
      middleboxes to modify the transport behaviour (e.g., the extended
      headers described in [I-D.trammell-plus-abstract-mech]).  This
      considers only immutable fields in the transport headers, that is,
      fields that can be authenticated End-to-End across a path.

      An example of a method that encrypts some, but not all, transport
      information is GRE-in-UDP [RFC8086] when used with GRE encryption.

   Optional Encryption of Header Information:  There are implications to
      the use of optional header encryption in the design of a transport

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      protocol, where support of optional mechanisms can increase the
      complexity of the protocol and its implementation, and in the
      management decisions that are needed to use variable format
      fields.  Instead, fields of a specific type ought to always be
      sent with the same level of confidentiality or integrity
      protection.

   As seen, different transports use encryption to protect their header
   information to varying degrees.  There is, however, a trend towards
   increased protection with newer transport protocols.

5.  Addition of Transport Information to Network-Layer Headers

   An on-path device can make measurements by utilising additional
   protocol headers carrying operations, administration and management
   (OAM) information in an additional packet header.  Using network-
   layer approaches to reveal information has the potential that the
   same method (and hence same observation and analysis tools) can be
   consistently used by multiple transport protocols [RFC8558].  There
   could also be less desirable implications of separating the operation
   of the transport protocol from the measurement framework.

5.1.  Use of OAM within a Maintenance Domain

   OAM information can be added at the ingress to a maintenance domain
   (e.g., an Ethernet protocol header with timestamps and sequence
   number information using a method such as 802.11ag or in-situ OAM
   [I-D.ietf-ippm-ioam-data], or as a part of encapsulation protocol).
   The additional header information is typically removed the at the
   egress of the maintenance domain.

   Although some types of measurements are supported, this approach does
   not cover the entire range of measurements described in this
   document.  In some cases, it can be difficult to position measurement
   tools at the appropriate segments/nodes and there can be challenges
   in correlating the downstream/upstream information when in-band OAM
   data is inserted by an on-path device.

5.2.  Use of OAM across Multiple Maintenance Domains

   OAM information can also be added at the network layer as an IPv6
   extension header or an IPv4 option.  This information can be used
   across multiple network segments, or between the transport endpoints.

   One example is the IPv6 Performance and Diagnostic Metrics (PDM)
   Destination Option [RFC8250].  This allows a sender to optionally
   include a destination option that caries header fields that can be
   used to observe timestamps and packet sequence numbers.  This

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   information could be authenticated by receiving transport endpoints
   when the information is added at the sender and visible at the
   receiving endpoint, although methods to do this have not currently
   been proposed.  This method needs to be explicitly enabled at the
   sender.

   Current measurement results suggest that it could currently be
   undesirable to rely on methods requiring end to end support of
   network options or extension headers across the Internet.  IPv4
   network options are often not supported (or are carried on a slower
   processing path) and some IPv6 networks have been observed to drop
   packets that set an IPv6 header extension (e.g., results from 2016 in
   [RFC7872]).

   Another potential issue is that protocols that separately expose
   header information do not necessarily have an incentive to expose the
   actual information that is utilised by the protocol itself and could
   therefore manipulate the exposed header information to gain an
   advantage from the network.  Where the information is provided by an
   endpoint, the incentive to reflect actual transport information needs
   to be considered when proposing a method.

6.  Implications of Protecting the Transport Headers

   The choice of which fields to expose and which to encrypt is a design
   choice for the transport protocol.  Any selective encryption method
   requires trading two conflicting goals for a transport protocol
   designer to decide which header fields to encrypt.  Security work
   typically employs a design technique that seeks to expose only what
   is needed.  This approach provides incentives to not reveal any
   information that is not necessary for the end-to-end communication.
   However, there can be performance and operational benefits in
   exposing selected information to network tools.

   This section explores key implications of working with encrypted
   transport protocols.

6.1.  Independent Measurement

   Independent observation by multiple actors is important if the
   transport community is to maintain an accurate understanding of the
   network.  Encrypting transport header encryption changes the ability
   to collect and independently analyse data.  Internet transport
   protocols employ a set of mechanisms.  Some of these need to work in
   cooperation with the network layer for loss detection and recovery,
   congestion detection and control.  Others need to work only end-to-
   end (e.g., parameter negotiation, flow-control).

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   The majority of present Internet applications use two well-known
   transport protocols, TCP and UDP.  Although TCP represents the
   majority of current traffic, many real-time applications use UDP, and
   much of this traffic utilises RTP format headers in the payload of
   the UDP datagram.  Since these protocol headers have been fixed for
   decades, a range of tools and analysis methods have became common and
   well-understood.

   Protocols that expose the state information used by the transport
   protocol in their header information (e.g., timestamps used to
   calculate the RTT, packet numbers used to asses congestion and
   requests for retransmission) provide an incentive for the sending
   endpoint to provide correct information, since the protocol will not
   work otherwise.  This increases confidence that the observer
   understands the transport interaction with the network.  For example,
   when TCP is used over an unencrypted network path (i.e., one that
   does not use IPsec or other encryption below the transport), it
   implicitly exposes header information that can be used for
   measurement at any point along the path.  This information is
   necessary for the protocol's correct operation, therefore there is no
   incentive for a TCP or RTP implementation to put incorrect
   information in this transport header.  A network device can have
   confidence that the well-known (and ossified) transport information
   represents the actual state of the endpoints.

   When encryption is used to conceal some or all of the transport
   headers, the transport protocol chooses which information to reveal
   to the network about its internal state, what information to leave
   encrypted, and what fields to grease to protect against future
   ossification.  Such a transport could be designed, for example, to
   provide summary data regarding its performance, congestion control
   state, etc., or to make an explicit measurement signal available.
   For example, a QUIC endpoint can optionally set the spin bit to
   reflect to explicitly reveal the RTT of an encrypted transport
   session to the on-path network devices [I-D.ietf-quic-transport]).

   When providing or using such information, it becomes important to
   consider the privacy of the user and their incentive for providing
   accurate and detailed information.  Protocols that selectively reveal
   some transport state or measurement signals are choosing to establish
   a trust relationship with the network operators.  There is no
   protocol mechanism that can guarantee that the information provided
   represents the actual transport state of the endpoints, since those
   endpoints can always send additional information in the encrypted
   part of the header, to update or replace whatever they reveal.  This
   reduces the ability to independently measure and verify that a
   protocol is behaving as expected.  Some operational uses need the
   information to contain sufficient detail to understand, and possibly

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   reconstruct, the network traffic pattern for further testing; such
   operators need to gain the trust of transport protocol implementers
   if they are to correctly reveal such information.

   Operations, Administration, and Maintenance (OAM) data records
   [I-D.ietf-ippm-ioam-data] could be embedded into a variety of
   encapsulation methods at different layers to support the goals of a
   specific operational domain.  OAM-related metadata can support
   functions such as performance evaluation, path-tracing, path
   verification information, classification and a diversity of other
   uses.  When encryption is used to conceal some or all of the
   transport headers, analysis will require coordination between actors
   at different layers to successfully characterise flows and correlate
   the performance or behaviour of a specific mechanism with the
   configuration and traffic using operational equipment (e.g.,
   combining transport and network measurements to explore congestion
   control dynamics, the implications of designs for active queue
   management or circuit breakers).

   Some measurements could be completed by utilising a standardised
   endpoint-based logging format (e.g., based on Quic-Trace
   [Quic-Trace]).  Such information will have a diversity of uses,
   including developers wishing to debug/understand the transport/
   application protocols with which they work, researchers seeking to
   spot trends and anomalies, and to characterise variants of protocols.
   Logs collected at endpoints could be shared (after appropriate
   annoymisation) to help understand performance and pathologies.
   Measurements based on logging will need to establish the validity and
   provenance of the logged information to establish how and when traces
   were captured.

   However, endpoint logs do not provide equivalent information to in-
   network measurements.  In particular, endpoint logs contain only a
   part of the information needed to understand the operation of network
   devices and identify issues such as link performance or capacity
   sharing between multiple flows.  Additional information is needed to
   determine which equipment/links are used and the configuration of
   equipment along the network paths being measured.

6.2.  Characterising "Unknown" Network Traffic

   The patterns and types of traffic that share Internet capacity change
   over time as networked applications, usage patterns and protocols
   continue to evolve.

   If "unknown" or "uncharacterised" traffic patterns form a small part
   of the traffic aggregate passing through a network device or segment
   of the network the path, the dynamics of the uncharacterised traffic

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   might not have a significant collateral impact on the performance of
   other traffic that shares this network segment.  Once the proportion
   of this traffic increases, the need to monitor the traffic and
   determine if appropriate safety measures need to be put in place.

   Tracking the impact of new mechanisms and protocols requires traffic
   volume to be measured and new transport behaviours to be identified.
   This is especially true of protocols operating over a UDP substrate.
   The level and style of encryption needs to be considered in
   determining how this activity is performed.  On a shorter timescale,
   information could also need to be collected to manage denial of
   service attacks against the infrastructure.

6.3.  Accountability and Internet Transport Protocols

   Information provided by tools observing transport headers can be used
   to classify traffic, and to limit the network capacity used by
   certain flows, as discussed in Section 3.2.4).  Equally, operators
   could use analysis of transport headers and transport flow state to
   demonstrate that they are not providing differential treatment to
   certain flows.  Obfuscating or hiding this information using
   encryption could lead operators and maintainers of middleboxes
   (firewalls, etc.) to seek other methods to classify, and potentially
   other mechanisms to condition, network traffic.

   A lack of data that reduces the level of precision with which flows
   can be classified also reduces the design space for conditioning
   mechanisms (e.g., rate limiting, circuit breaker techniques
   [RFC8084], or blocking of uncharacterised traffic), and this needs to
   be considered when evaluating the impact of designs for transport
   encryption [RFC5218].

6.4.  Impact on Operational Cost

   Many network operators currently utilise observed transport
   information as a part of their operational practice, and have
   developed tools and operational practices based around currently
   deployed transports and their applications.  Encryption of the
   transport information prevents tools from directly observing this
   information.  A variety of open source and commercial tools have been
   deployed that utilise this information for a variety of short and
   long term measurements.

   The network will not break just because transport headers are
   encrypted, although alternative diagnostic and troubleshooting tools
   would need to be developed and deployed.  Introducing a new protocol
   or application can require these tool chains and practice to be
   updated, and could in turn impact operational mechanisms, and

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   policies.  Each change can introduce associated costs, including the
   cost of collecting data, and the tooling needed to handle multiple
   formats (possibly as these co-exist in the network, when measurements
   need to span time periods during which changes are deployed, or to
   compare with historical data).  These costs are incurred by an
   operator to manage the service and debug network issues.

   At the time of writing, the additional operational cost of using
   encrypted transports is not yet well understood.  Design trade-offs
   could mitigate these costs by explicitly choosing to expose selected
   information (e.g., header invariants and the spin-bit in QUIC
   [I-D.ietf-quic-transport]), the specification of common log formats,
   and development of alternative approaches.

6.5.  Impact on Research, Development and Deployment

   Evolution and the ability to understand (measure) the impact need to
   proceed hand-in-hand.  Observable transport headers can provide open
   and verifiable measurement data.  Observation of pathologies has a
   critical role in the design of transport protocol mechanisms and
   development of new mechanisms and protocols.  This helps
   understanding the interactions between cooperating protocols and
   network mechanism, the implications of sharing capacity with other
   traffic and the impact of different patterns of usage.  The ability
   of other stake holders to review transport header traces helps
   develop insight into performance and traffic contribution of specific
   variants of a protocol.

   In development of new transport protocol mechanisms, attention needs
   to be paid to the expected scale of deployment.  Whatever the
   mechanism, experience has shown that it is often difficult to
   correctly implement combinations of mechanisms [RFC8085].  Mechanisms
   often evolve as a protocol matures, or in response to changes in
   network conditions, changes in network traffic, or changes to
   application usage.  Analysis is especially valuable when based on the
   behaviour experienced across a range of topologies, vendor equipment,
   and traffic patterns.

   New transport protocol formats are expected to facilitate an
   increased pace of transport evolution, and with it the possibility to
   experiment with and deploy a wide range of protocol mechanisms.
   There has been recent interest in a wide range of new transport
   methods, e.g., Larger Initial Window, Proportional Rate Reduction
   (PRR), congestion control methods based on measuring bottleneck
   bandwidth and round-trip propagation time, the introduction of AQM
   techniques and new forms of ECN response (e.g., Data Centre TCP,
   DCTP, and methods proposed for L4S).The growth and diversity of
   applications and protocols using the Internet also continues to

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   expand.  For each new method or application it is desirable to build
   a body of data reflecting its behaviour under a wide range of
   deployment scenarios, traffic load, and interactions with other
   deployed/candidate methods.

   Concealing transport header information could reduce the range of
   actors that can observe useful data.  This would limit the
   information sources available to the Internet community to understand
   the operation of new transport protocols, reducing information to
   inform design decisions and standardisation of the new protocols and
   related operational practices.  The cooperating dependence of
   network, application, and host to provide communication performance
   on the Internet is uncertain when only endpoints (i.e., at user
   devices and within service platforms) can observe performance, and
   when performance cannot be independently verified by all parties.

   Independently observed data is also important to ensure the health of
   the research and development communities and can help promote
   acceptance of proposed specifications by the wider community (e.g.,
   as a method to judge the safety for Internet deployment) and provides
   valuable input during standardisation.  Open standards motivate a
   desire to include independent observation and evaluation of
   performance data, which in turn demands control over where and when
   measurement samples are collected.  This requires consideration of
   the methods used to observe data and the appropriate balance between
   encrypting all and no transport information.

7.  Conclusions

   Header encryption and strong integrity checks are being incorporated
   into new transport protocols and have important benefits.  The pace
   of development of transports using the WebRTC data channel, and the
   rapid deployment of the QUIC transport protocol, can both be
   attributed to using the combination of UDP as a substrate while
   providing confidentiality and authentication of the encapsulated
   transport headers and payload.

   To achieve stable Internet operations, the IETF transport community
   has, to date, relied heavily on measurement and insights of the
   network operations community to understand the trade-offs, and to
   inform selection of appropriate mechanisms, to ensure a safe,
   reliable, and robust Internet (e.g., [RFC1273],[RFC2914]).

   The traffic that can be observed by on-path network devices (the
   "wire image") is a function of transport protocol design/options,
   network use, applications, and user characteristics.  In general,
   when only a small proportion of the traffic has a specific
   (different) characteristic, such traffic seldom leads to operational

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   concern, although the ability to measure and monitor it is less.  The
   desire to understand the traffic and protocol interactions typically
   grows as the proportion of traffic increases in volume.  The
   challenges increase when multiple instances of an evolving protocol
   contribute to the traffic that share network capacity.

   An increased pace of evolution therefore needs to be accompanied by
   methods that can be successfully deployed and used across operational
   networks.  This leads to a need for network operators at various
   levels (ISPs, enterprises, firewall maintainer, etc.) to identify
   appropriate operational support functions and procedures.  Protocols
   that change their transport header format (wire image) or their
   behaviour (e.g., algorithms that are needed to classify and
   characterise the protocol), will require new network tooling to be
   developed to catch-up with each change.  If a protocol changes so
   that the currently deployed tools and methods are no longer relevant,
   then these tools can not be used to measure performance.  This can
   increase the response-time after faults, and can impact the ability
   to manage the network resulting in traffic causing traffic to be
   treated inappropriately (e.g., rate-limiting as a result of incorrect
   classification or monitoring).

   There are benefits in exposing consistent information to the network
   that avoids traffic being inappropriately classified and then
   receiving a default treatment by the network.  The flow label and
   DSCP fields provide examples of how transport information can be made
   available for network-layer decisions.  Extension headers could also
   be used to carry transport information that can inform network-layer
   decisions.  Other information might also be useful to various
   stakeholders, however this document does not make recommendations
   about what information ought to be exposed, to whom it ought to be
   observable, or how this will be achieved.

   There are trade-offs and implications of increased use of transport
   header encryption when designing a protocol.  Transport protocol
   designers have often ignored the implications of whether the
   information in transport header fields can or will be used by in-
   network devices, and the implications this places on protocol
   evolution.  This motivates a design that provides confidentiality of
   header information.  This lack of visibility of transport header
   information can be expected to impact the ways that protocols are
   deployed, standardised, and their operational support.  The impact of
   hiding transport headers therefore needs to be considered in the
   specification and development of protocols and standards.  This has a
   potential impact on the way in which the IRTF and IETF develop new
   protocols, specifications, and guidelines:

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   o  Coexistence of Transport Protocols and Configurations: TCP is
      currently the predominant transport protocol used over Internet
      paths.  Its many variants have broadly consistent approaches to
      avoiding congestion collapse, and to ensuring the stability of the
      Internet.  Increased use of transport layer encryption can
      overcome ossification, allowing deployment of new transports and
      different types of congestion control.  This flexibility can be
      beneficial, but it could come at the cost of fragmenting the
      ecosystem.  There is little doubt that developers will try to
      produce high quality transports for their intended target uses,
      but it is not yet clear there are sufficient incentives to ensure
      good practice that benefits the wide diversity of requirements for
      the Internet community as a whole.

   o  Supporting Common Specifications: Common open specifications can
      stimulate engagement by developers, users, and researchers.
      Increased diversity, and the ability to innovate without public
      scrutiny, risks point solutions that optimise for specific needs,
      but accidentally disrupt operations of/in different parts of the
      network.  The social contract that maintains the stability of the
      Internet relies on accepting common interworking specifications,
      and on it being possible to detect violations.

   o  Benchmarking and Understanding Feature Interactions: An
      appropriate vantage point for observation, coupled with timing
      information about traffic flows, provides a valuable tool for
      benchmarking network devices, endpoint stacks, functions, and/or
      configurations.  This can also help with understanding complex
      feature interactions.  An inability to observe transport layer
      header information can make it harder to diagnose and explore
      interactions between features at different protocol layers, a
      side-effect of not allowing a choice of vantage point from which
      this information is observed.  New approaches will need to be
      developed.

   o  Operational Practice: The network operations community relies on
      being able to understand the pattern and requirements of traffic
      passing over the Internet, both in aggregate and at the flow
      level.  These operational practices have developed based on the
      information available from unencrypted transport headers.  The
      IETF supports this activity by developing operations and
      management specifications, interface specifications, and
      associated Best Current Practice (BCP) specifications.  Concealing
      transport header information impacts current practice and demand
      new specifications.

   o  Research and Development: Concealing transport information can
      impede independent research into new mechanisms, measurement of

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      behaviour, and development initiatives.  Experience shows that
      transport protocols are complicated to design and complex to
      deploy, and that individual mechanisms need to be evaluated while
      considering other mechanisms, across a broad range of network
      topologies and with attention to the impact on traffic sharing the
      capacity.  If increased use of transport header encryption results
      in reduced availability of open data, it could eliminate the
      independent self-checks to the standardisation process that have
      previously been in place from research and academic contributors
      (e.g., the role of the IRTF Internet Congestion Control Research
      Group (ICCRG) and research publications in reviewing new transport
      mechanisms and assessing the impact of their experimental
      deployment).

   The design of future transport protocols needs to consider encryption
   of their transport headers to satisfy security and privacy concerns.
   This choice to encrypt all, or part, of the transport layer protocol
   headers needs to also take into account the impact on operations,
   standards, and research.  As [RFC7258] notes, "Making networks
   unmanageable to mitigate (pervasive monitoring) is not an acceptable
   outcome, but ignoring (pervasive monitoring) would go against the
   consensus documented here."

   As part of a protocol's design, the community therefore needs to
   weigh the benefits of ossifying common headers versus the potential
   demerits of exposing specific information that could be observed
   along the network path, to ensure network operators, researchers and
   other stakeholders have appropriate tools to manage their networks
   and enable stable operation of the Internet as new protocols are
   deployed.  An appropriate balance will emerge over time as real
   instances of this tension are analysed [RFC7258].  This balance
   between information exposed and information concealed ought to be
   carefully considered when specifying new transport protocols.

8.  Security Considerations

   This document is about design and deployment considerations for
   transport protocols.  Issues relating to security are discussed
   throughout this document.

   Authentication, confidentiality protection, and integrity protection
   are identified as Transport Features by [RFC8095].  As currently
   deployed in the Internet, these features are generally provided by a
   protocol or layer on top of the transport protocol
   [I-D.ietf-taps-transport-security].

   Confidentiality and strong integrity checks have properties that can
   also be incorporated into the design of a transport protocol.

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   Integrity checks can protect an endpoint from undetected modification
   of protocol fields by network devices, whereas encryption and
   obfuscation or greasing can further prevent these headers being
   utilised by network devices.  Hiding headers can therefore provide
   the opportunity for greater freedom to update the protocols and can
   ease experimentation with new techniques and their final deployment
   in endpoints.  A protocol specification needs to weigh the costs of
   ossifying common headers, versus the potential benefits of exposing
   specific information that could be observed along the network path to
   provide tools to manage new variants of protocols.

   A protocol design that uses header encryption can provide
   confidentiality of some or all of the protocol header information.
   This prevents an on-path device from knowledge of the header field.
   It therefore prevents mechanisms being built that directly rely on
   the information or seeks to infer semantics of an exposed header
   field.  Hiding headers reduces visibility into transport metadata,
   and can limit the ability to measure and characterise traffic.  It
   can also provide privacy benefits in some cases.

   Exposed transport headers are sometimes utilised as a part of the
   information to detect anomalies in network traffic.  This can be used
   as the first line of defence to identify potential threats from DOS
   or malware and redirect suspect traffic to dedicated nodes
   responsible for DOS analysis, malware detection, or to perform packet
   "scrubbing" (the normalization of packets so that there are no
   ambiguities in interpretation by the ultimate destination of the
   packet).  These techniques are currently used by some operators to
   also defend from distributed DOS attacks.

   Exposed transport header fields are sometimes also utilised as a part
   of the information used by the receiver of a transport protocol to
   protect the transport layer from data injection by an attacker.  In
   evaluating this use of exposed header information, it is important to
   consider whether it introduces a significant DOS threat.  For
   example, an attacker could construct a DOS attack by sending packets
   with a sequence number that falls within the currently accepted range
   of sequence numbers at the receiving endpoint, this would then
   introduce additional work at the receiving endpoint, even though the
   data in the attacking packet might not finally be delivered by the
   transport layer.  This is sometimes known as a "shadowing attack".
   An attack can, for example, disrupt receiver processing, trigger loss
   and retransmission, or make a receiving endpoint perform unproductive
   decryption of packets that cannot be successfully decrypted (forcing
   a receiver to commit decryption resources, or to update and then
   restore protocol state).

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   One mitigation to off-path attack is to deny knowledge of what header
   information is accepted by a receiver or obfuscate the accepted
   header information, e.g., setting a non-predictable initial value for
   a sequence number during a protocol handshake, as in [RFC3550] and
   [RFC6056], or a port value that can not be predicted (see section 5.1
   of [RFC8085]).  A receiver could also require additional information
   to be used as a part of a validation check before accepting packets
   at the transport layer (e.g., utilising a part of the sequence number
   space that is encrypted; or by verifying an encrypted token not
   visible to an attacker).  This would also mitigate against on-path
   attacks.  An additional processing cost can be incurred when
   decryption needs to be attempted before a receiver is able to discard
   injected packets.

   Open standards motivate a desire for this evaluation to include
   independent observation and evaluation of performance data, which in
   turn suggests control over where and when measurement samples are
   collected.  This requires consideration of the appropriate balance
   between encrypting all and no transport information.  Open data, and
   accessibility to tools that can help understand trends in application
   deployment, network traffic and usage patterns can all contribute to
   understanding security challenges.

   The Security and Privacy Considerations in the Framework for Large-
   Scale Measurement of Broadband Performance (LMAP) [RFC7594] contain
   considerations for Active and Passive measurement techniques and
   supporting material on measurement context.

9.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.

10.  Acknowledgements

   The authors would like to thank Mohamed Boucadair, Spencer Dawkins,
   Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen
   Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris
   Wood, Thomas Fossati, and other members of the TSVWG for their
   comments and feedback.

   This work has received funding from the European Union's Horizon 2020
   research and innovation programme under grant agreement No 688421,
   and the EU Stand ICT Call 4.  The opinions expressed and arguments
   employed reflect only the authors' view.  The European Commission is
   not responsible for any use that might be made of that information.

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   This work has received funding from the UK Engineering and Physical
   Sciences Research Council under grant EP/R04144X/1.

11.  Informative References

   [bufferbloat]
              Gettys, J. and K. Nichols, "Bufferbloat: dark buffers in
              the Internet. Communications of the ACM, 55(1):57-65",
              January 2012.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
              Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
              P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
              "Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
              data-06 (work in progress), July 2019.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-22 (work
              in progress), July 2019.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-19
              (work in progress), November 2017.

   [I-D.ietf-taps-transport-security]
              Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K.
              Rose, "A Survey of Transport Security Protocols", draft-
              ietf-taps-transport-security-08 (work in progress), August
              2019.

   [I-D.ietf-tls-grease]
              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-04 (work in progress), August 2019.

   [I-D.ietf-tsvwg-rtcweb-qos]
              Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
              Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
              qos-18 (work in progress), August 2016.

   [I-D.trammell-plus-abstract-mech]
              Trammell, B., "Abstract Mechanisms for a Cooperative Path
              Layer under Endpoint Control", draft-trammell-plus-
              abstract-mech-00 (work in progress), September 2016.

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   [Latency]  Briscoe, B., "Reducing Internet Latency: A Survey of
              Techniques and Their Merits, IEEE Comm. Surveys &
              Tutorials. 26;18(3) p2149-2196", November 2014.

   [Measure]  Fairhurst, G., Kuehlewind, M., and D. Lopez, "Measurement-
              based Protocol Design, Eur. Conf. on Networks and
              Communications, Oulu, Finland.", June 2017.

   [Quic-Trace]
              "https:QUIC trace utilities //github.com/google/quic-
              trace".

   [RFC1273]  Schwartz, M., "Measurement Study of Changes in Service-
              Level Reachability in the Global TCP/IP Internet: Goals,
              Experimental Design, Implementation, and Policy
              Considerations", RFC 1273, DOI 10.17487/RFC1273, November
              1991, <https://www.rfc-editor.org/info/rfc1273>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC2507]  Degermark, M., Nordgren, B., and S. Pink, "IP Header
              Compression", RFC 2507, DOI 10.17487/RFC2507, February
              1999, <https://www.rfc-editor.org/info/rfc2507>.

   [RFC2508]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
              Headers for Low-Speed Serial Links", RFC 2508,
              DOI 10.17487/RFC2508, February 1999,
              <https://www.rfc-editor.org/info/rfc2508>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,
              <https://www.rfc-editor.org/info/rfc3135>.

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   [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>.

   [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>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,
              <https://www.rfc-editor.org/info/rfc3393>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
              S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
              DOI 10.17487/RFC4737, November 2006,
              <https://www.rfc-editor.org/info/rfc4737>.

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   [RFC4995]  Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
              Header Compression (ROHC) Framework", RFC 4995,
              DOI 10.17487/RFC4995, July 2007,
              <https://www.rfc-editor.org/info/rfc4995>.

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
              <https://www.rfc-editor.org/info/rfc5218>.

   [RFC5236]  Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R.
              Whitner, "Improved Packet Reordering Metrics", RFC 5236,
              DOI 10.17487/RFC5236, June 2008,
              <https://www.rfc-editor.org/info/rfc5236>.

   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
              March 2009, <https://www.rfc-editor.org/info/rfc5481>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <https://www.rfc-editor.org/info/rfc6056>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,
              <https://www.rfc-editor.org/info/rfc6269>.

   [RFC6294]  Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
              the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
              2011, <https://www.rfc-editor.org/info/rfc6294>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

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   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC7594]  Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
              Aitken, P., and A. Akhter, "A Framework for Large-Scale
              Measurement of Broadband Performance (LMAP)", RFC 7594,
              DOI 10.17487/RFC7594, September 2015,
              <https://www.rfc-editor.org/info/rfc7594>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,
              <https://www.rfc-editor.org/info/rfc7872>.

   [RFC7928]  Kuhn, N., Ed., Natarajan, P., Ed., Khademi, N., Ed., and
              D. Ros, "Characterization Guidelines for Active Queue
              Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July
              2016, <https://www.rfc-editor.org/info/rfc7928>.

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   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,
              <https://www.rfc-editor.org/info/rfc7983>.

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
              <https://www.rfc-editor.org/info/rfc8033>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
              <https://www.rfc-editor.org/info/rfc8084>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,
              <https://www.rfc-editor.org/info/rfc8087>.

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095,
              DOI 10.17487/RFC8095, March 2017,
              <https://www.rfc-editor.org/info/rfc8095>.

   [RFC8250]  Elkins, N., Hamilton, R., and M. Ackermann, "IPv6
              Performance and Diagnostic Metrics (PDM) Destination
              Option", RFC 8250, DOI 10.17487/RFC8250, September 2017,
              <https://www.rfc-editor.org/info/rfc8250>.

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,
              <https://www.rfc-editor.org/info/rfc8289>.

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   [RFC8290]  Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
              J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
              and Active Queue Management Algorithm", RFC 8290,
              DOI 10.17487/RFC8290, January 2018,
              <https://www.rfc-editor.org/info/rfc8290>.

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,
              <https://www.rfc-editor.org/info/rfc8404>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <https://www.rfc-editor.org/info/rfc8546>.

   [RFC8548]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic Protection of TCP Streams
              (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
              <https://www.rfc-editor.org/info/rfc8548>.

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,
              <https://www.rfc-editor.org/info/rfc8558>.

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Appendix A.  Revision information

   -00 This is an individual draft for the IETF community.

   -01 This draft was a result of walking away from the text for a few
   days and then reorganising the content.

   -02 This draft fixes textual errors.

   -03 This draft follows feedback from people reading this draft.

   -04 This adds an additional contributor and includes significant
   reworking to ready this for review by the wider IETF community Colin
   Perkins joined the author list.

   Comments from the community are welcome on the text and
   recommendations.

   -05 Corrections received and helpful inputs from Mohamed Boucadair.

   -06 Updated following comments from Stephen Farrell, and feedback via
   email.  Added a draft conclusion section to sketch some strawman
   scenarios that could emerge.

   -07 Updated following comments from Al Morton, Chris Seal, and other
   feedback via email.

   -08 Updated to address comments sent to the TSVWG mailing list by
   Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on
   11/05/2018, and Spencer Dawkins.

   -09 Updated security considerations.

   -10 Updated references, split the Introduction, and added a paragraph
   giving some examples of why ossification has been an issue.

   -01 This resolved some reference issues.  Updated section on
   observation by devices on the path.

   -02 Comments received from Kyle Rose, Spencer Dawkins and Tom
   Herbert.  The network-layer information has also been re-organised
   after comments at IETF-103.

   -03 Added a section on header compression and rewriting of sections
   referring to RTP transport.  This version contains author editorial
   work and removed duplicate section.

   -04 Revised following SecDir Review

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   o  Added some text on TLS story (additional input sought on relevant
      considerations).

   o  Section 2, paragraph 8 - changed to be clearer, in particular,
      added "Encryption with secure key distribution prevents"

   o  Flow label description rewritten based on PS/BCP RFCs.

   o  Clarify requirements from RFCs concerning the IPv6 flow label and
      highlight ways it can be used with encryption. (section 3.1.3)

   o  Add text on the explicit spin-bit work in the QUIC DT.  Added
      greasing of spin-bit.  (Section 6.1)

   o  Updated section 6 and added more explanation of impact on
      operators.

   o  Other comments addressed.

   -05 Editorial pass and minor corrections noted on TSVWG list.

   -06 Updated conclusions and minor corrections.  Responded to request
   to add OAM discussion to Section 6.1.

   -07 Addressed feedback from Ruediger and Thomas.

   Section 2 deserved some work to make it easier to read and avoid
   repetition.  This edit finally gets to this, and eliminates some
   duplication.  This also moves some of the material from section 2 to
   reform a clearer conclusion.  The scope remains focussed on the usage
   of transport headers and the implications of encryption - not on
   proposals for new techniques/specifications to be developed.

   -08 Addressed feedback and completed editorial work, including
   updating the text referring to RFC7872, in preparation for a WGLC.

   -09 Updated following WGLC.  In particular, thanks to Joe Touch
   (specific comments and commentry on style and tone); Dimitri Tikonov
   (editorial); Christian Huitema (various) David Black (various).
   Ammended privacy considerations based on SECDIR review.  Emile
   Stephan (inputs on operations measurement); Various others.

   Added summary text and refs to key sections.  Note to editors: The
   section numbers are hard-linked.

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Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   Department of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   Scotland

   EMail: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk/

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ
   Scotland

   EMail: csp@csperkins.org
   URI:   https://csperkins.org/

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