The Impact of Transport Header Confidentiality on Network Operation and Evolution of the Internet
draft-ietf-tsvwg-transport-encrypt-07

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TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                C. Perkins
Expires: January 5, 2020                           University of Glasgow
                                                           July 04, 2019

The Impact of Transport Header Confidentiality on Network Operation and
                       Evolution of the Internet
                 draft-ietf-tsvwg-transport-encrypt-07

Abstract

   This document describes implications of applying end-to-end
   encryption at the transport layer.  It identifies in-network uses of
   transport layer header information.  It then reviews the implications
   of developing end-to-end transport protocols that use authentication
   to protect the integrity of transport information or encryption to
   provide confidentiality of the transport protocol header and expected
   implications of transport protocol design and network operation.
   Since transport measurement and analysis of the impact of network
   characteristics have been important to the design of current
   transport protocols, it also considers the impact on transport and
   application evolution.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 5, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Context and Rationale . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Use of Transport Header Information in the Network  . . .   4
     2.2.  Encryption of Transport Header Information  . . . . . . .   5
     2.3.  Encryption tradeoffs  . . . . . . . . . . . . . . . . . .   6
   3.  Current uses of Transport Headers within the Network  . . . .   8
     3.1.  Observing Transport Information in the Network  . . . . .   9
     3.2.  Transport Measurement . . . . . . . . . . . . . . . . . .  15
     3.3.  Use for Network Diagnostics and Troubleshooting . . . . .  19
     3.4.  Header Compression  . . . . . . . . . . . . . . . . . . .  20
   4.  Encryption and Authentication of Transport Headers  . . . . .  21
   5.  Addition of Transport Information to Network-Layer Protocol
       Headers . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   6.  Implications of Protecting the Transport Headers  . . . . . .  25
     6.1.  Independent Measurement . . . . . . . . . . . . . . . . .  26
     6.2.  Characterising "Unknown" Network Traffic  . . . . . . . .  28
     6.3.  Accountability and Internet Transport Protocols . . . . .  28
     6.4.  Impact on Operational Cost  . . . . . . . . . . . . . . .  28
     6.5.  Impact on Research, Development and Deployment  . . . . .  29
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  30
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  35
   11. Informative References  . . . . . . . . . . . . . . . . . . .  36
   Appendix A.  Revision information . . . . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   There is increased interest in, and deployment of, protocols that
   employ end-to-end encryption at the transport layer, including the
   transport layer headers.  An example of such a transport is the QUIC
   transport protocol [I-D.ietf-quic-transport], currently being
   standardised in the IETF.  Encryption of transport layer headers and
   payload data has many benefits in terms of protecting user privacy.
   These benefits have been widely discussed [RFC7258], [RFC7624], and

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   this document strongly supports the increased use of encryption in
   transport protocols.  Encryption can also be used to prevent unwanted
   modification of transport header information by middleboxes.  There
   are also, however, some costs, in that the widespread use of
   transport encryption requires changes to network operations, and
   complicates network measurement for research, operational, and
   standardisation purposes.  The direction in which the use of
   transport header confidentiality evolves could have significant
   implications on the way the Internet architecture develops, and
   therefore needs to be considered as a part of protocol design.

   The remainder of this document discusses some consequences of
   applying end-to-end encryption at the transport layer.  It reviews
   the implications of developing end-to-end transport protocols that
   use encryption to provide confidentiality of the transport protocol
   header, and considers the effect of such changes on transport
   protocol design and network operations.  It also considers
   anticipated implications on transport and application evolution.

   Transports are also increasingly encrypting and authenticating the
   payload (i.e., the application data carried within the transport
   connection) end-to-end.  Such protection is encouraged, and the
   implications of protecting the payload are not further discussed in
   this memo.

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, supported by 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 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 measurement 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) community has relied on being

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   able to understand the pattern and requirements of traffic passing
   over the Internet, both in aggregate and at the flow level.

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.
   Some operators have deployed functionality in middleboxes to both
   support network operations can be deployed to enhance performance.  A
   reliance on the presence and semantics of specific header information
   leads to ossification, where an endpoint could be required to supply
   a specific header to receive the network service that it desires.  In
   some case this could be benign or advantageous to the protocol (e.g.,
   recognising the start of a connection, or explicitly exposing
   protocol information can be expected to provide more consistent
   decisions by on-path devices than the use of diverse methods to infer
   semantics from other flow properties).  In other cases the
   ossification could frustrate the evolution of the protocol (e.g., a
   mechanism implemented in a network device, such as a firewall, that
   required a header field to have only a specific known set of values
   would prevent the device from forwarding packets using a different
   version of a protocol that introduces a feature that changes the
   value of this field).

   Experience developing Transport Layer Security [RFC8446], required a
   design that recognised that deployed middleboxes relied on the
   exposed information in Transport Layer Security (TLS) 1.2.  Examples
   of the impact of ossification can be found in the development of
   Multipath TCP (MPTCP) and the TCP Fast Open option.  The design of
   MPTCP had to be revised to account for middleboxes, so called "TCP
   Normalizers", that monitor the evolution of the window advertised in
   the TCP headers and that reset connections if the window does not
   grow as expected.  Similarly, TCP Fast Open has had issues with
   middleboxes that remove unknown TCP options, that drop segments with
   unknown TCP options, that drop segments that contain data and have
   the SYN bit set, that drop packets with SYN/ACK that acknowledge
   data, or that disrupt connections that send data before the three-way
   handshake completes.  In both cases, the issue was caused by
   middleboxes that had a hard-coded understanding of transport
   behaviour, and that interacted poorly with transports that tried to
   change that behaviour.  Other examples have included middleboxes that
   rewrite TCP sequence and acknowledgement numbers but are unaware of
   the (newer) SACK option and don't correctly rewrite selective
   acknowledgements to match the changes made to the fixed TCP header.

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2.2.  Encryption of Transport Header Information

   Encryption is expected to form a basis for many future transport
   protocol designs.  There are many motivations for deploying encrypted
   transports [RFC7624] (i.e., transport protocols that use encryption
   to provide confidentiality of some or all of the transport-layer
   header information), and encryption of transport payloads (i.e.
   Confidentiality of the payload data).  Increasing public concerns
   about interference with Internet traffic have led to a rapidly
   expanding deployment of encryption to protect end-user privacy, e.g.,
   QUIC [I-D.ietf-quic-transport].  Using encryption to provide
   confidentiality of the transport layer therefore brings some well-
   known privacy and security benefits.  Although it is important that
   protocols either do not expose information where the usage could
   change in future protocols or that methods that utilise the
   information are robust to potential changes as protocols evolve over
   time.

   Introducing cryptographic integrity checks for header fields can
   prevent undetected manipulation of the field by network devices, or
   undetected addition of information to a packet.  This does not
   prevent inspection of the information by a device on path, and it is
   possible that such devices could develop mechanisms that rely on the
   presence of such a field or a known value in the field.  In this
   context, specification of a non-encrypted transport header field
   explicitly allows protocol designers to make specific header
   information observable in the network.  This supports other uses of
   this information by on-path devices, and at the same time this can be
   expected to lead to ossification of the transport header, because
   network forwarding could evolve to depend on the presence and/or
   value of these fields.  To avoid unwanted inspection, a protocol
   could intentionally vary the format and/or value of exposed header
   fields (sometimes known as Greasing [I-D.thomson-quic-grease]).

   A protocol design that uses header encryption with secure key
   distribution can provide confidentiality of some or all of the
   protocol header information.  This prevents an on-path device from
   observing the header field.  This prevents mechanisms being built
   that directly rely on the information or seek to infer semantics of
   an exposed header field and can therefore help reduce ossification of
   the transport layer.  While encryption can hide transport header
   information, 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, creating 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,

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   presenting it own requirements for collecting and distributing the
   data.

2.3.  Encryption tradeoffs

   The are architectural challenges and considerations in the way
   transport protocols are designed, and the ability to characterise and
   compare different transport solutions [Measure].  The decision about
   which transport headers fields are made observable offers trade-offs
   around authentication and confidentiality versus observability,
   network operations and management, and ossification.  The impact
   differs depending on the activity, for example:

   Network Operations and Research:  Observable transport headers enable
      explicit measure and analysis 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 network segments.

      Concealing transport header information makes performance/
      behaviour unavailable to passive observers along the path,
      Operators will be unable to use this information directly and may
      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 prevent
      operators from attempting to apply practices that were used with
      unencrypted transport headers.

      Confidentiality of the transport payload could be provided while
      leaving some, or all, transport headers unencrypted (or providing
      this information in a network-layer extension), possibly with
      authentication.  This provides many of the privacy and security
      benefits while supporting operations and research, but at the cost
      of ossifying the exposed headers.

   Protection from Denial of Service:  Observable transport headers
      currently provide useful input to classify and detect anomalous
      events (e.g., changes in application behaviour, distributed denial
      of service attacks).  For this application to be effective, this
      needs to be able to uniquely disambiguate unwanted traffic.

      Concealing transport header information would prevent separating
      anomalous traffic based on transport information.  This could
      result in less-efficient identification of unwanted traffic or
      development of different methods (e.g.  rate-limiting of
      uncharacterised traffic).  Additional mechanisms will need to be

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      introduced, such as heuristics to disambiguate any unwanted
      traffic.

   Network Troubleshooting and Diagnostics:   Observable transport
      headers can be used by operators to support network
      troubleshooting and diagnostics.  A flow experiencing packet loss
      or jitter looks like an unaffected flow when only observing
      network layer headers.

      Concealing transport header information eliminates the incentive
      for operators to troubleshoot, since they cannot interpret the
      data.  It can limit understanding of transport dynamics, such as
      the impact of packet loss or latency on the flows, or localizing
      the network segment causing the packet loss or latency.
      Additional mechanisms will be needed to help reconstruct or
      replace transport-level metrics for troubleshooting and
      diagnostics.  These can add complexity and operational costs
      (e.g., in deploying additional functions in equipment or adding
      traffic overhead).

   Network Traffic Analysis:  Observable transport headers can support
      network traffic analysis 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 applications
      end-to-end measurements/traces are sufficient, in other
      applications it is important to relate observations to specific
      equipment/configurations or network segments.

      Concealing transport header information can make analysis harder
      or impossible.  This could impact the ability for an operator to
      anticipate the need for network upgrades and roll-out.  It can
      also impact the 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 directly
      support particular services (e.g., to explore issues relating to
      Quality of Service, QoS; the ability to perform fast re-routing of
      critical traffic, or support to mitigate the characteristics of
      specific radio links).  The more complex the underlying
      infrastructure the more important this impact.

   Open and Verifiable Network Data:   Observable transport headers can
      provide open and verifiable measurement data.  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.

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      Concealing transport header information can reduce the range of
      actors that 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.

   Compliance:  Observable transport headers coupled with published
      transport specifications allow operators and regulators to check
      compliance.  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.

3.  Current uses of Transport Headers within the Network

   Despite transport headers having end-to-end meaning, some of these
   transport headers have come to be used in various ways within the
   Internet.  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 would
   affect how protocol information is used [RFC8404].  To understand
   these implications, it is first necessary to understand how transport
   layer headers are currently observed and/or modified by middleboxes
   within the network.

   Transport protocols can be designed to encrypt or authenticate
   transport header fields.  Authentication at the transport layer can
   be used to detect any changes to an immutable header field that were
   made by a network device along a path.  The intentional modification
   of transport headers by middleboxes (such as Network Address
   Translation, NAT, or Firewalls) is not considered.  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 required to perform the
      observation;

   o  The protocol and version of the header need to be visible, e.g.,
      by defining the wire image [I-D.trammell-wire-image].  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

   Flow identification is a common function.  For example, performed by
   measurement activities, QoS classification, firewalls, Denial of
   Service, DOS, prevention.  This becomes more complex and less easily
   achieved when multiplexing is used at or above the transport layer.

   Observable transport header information (together with information in
   the network header), has been used to identify a flow and the
   connection state of the flow, together with the protocol options
   being used.  In some usages, a low-numbered (well-known) transport
   port number has been used to identify a protocol (although port
   information alone is not sufficient to guarantee identification of a
   protocol, since applications can use arbitrary ports, multiple
   sessions can be multiplexed on a single port, and ports can be re-
   used by subsequent sessions).

   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, with the possibility to negotiate
   additional headers at connection setup, identified by an option
   number in the transport header.  UDP-based protocols can use, but
   sometimes 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].

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   Concealing transport header information can remove information used
   to classify flows by passive observers along the path and 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.  Operators could also turn to more ambitious ways to
   collect, estimate, or infer that data (for example heuristics based
   on the analysis of traffic patterns).  For example, an operator no
   longer has access to Session Description Protocol (SDP) session
   descriptions to classify a flow carry as audio traffic.  Instead, the
   operator might use heuristics to infer that short UDP packets with
   regular spacing carry audio traffic.  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 prevent
   operators from attempting to apply practices that were used with
   unencrypted transport headers.

   Confidentiality of the transport payload could be provided while
   leaving some, or all, transport headers unencrypted (or providing
   this information in a network-layer extension), possibly with
   authentication.  This provides many of the privacy and security
   benefits while supporting operations and research, but at the cost of
   ossifying the exposed headers.

3.1.2.  Metrics derived from Transport Layer Headers

   Observable transport headers enable explicit measure and analysis
   protocol performance, network anomalies, and failure pathologies at
   any point along the Internet path.  Some actors manage their portion
   of the Internet by characterizing the performance of link/network
   segments.  Passive monitoring can observe traffic that does not
   encrypt the transport header information to make inferences from
   transport headers to derive these performance metrics.

   A variety of open source and commercial tools have been deployed that
   utilise this information.  The following metrics can be derived from
   transport header information:

   Traffic Rate and Volume:  Header information (e.g., sequence number
      and packet size) allows derivation of volume measures per-
      application, to characterise the traffic that uses a network
      segment or the pattern of network usage.  This can be measured per
      endpoint or for an aggregate of endpoints (e.g., to assess
      subscriber usage).  It can also be used to trigger measurement-
      based traffic shaping and to implement QoS support within the
      network and lower layers.  Volume measures can be valuable for
      capacity planning and providing detail of trends, rather than the
      volume per subscriber.

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   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., interference on a radio link), buffer overflow
      (e.g., due to congestion), policing (traffic management), buffer
      management (e.g., Active Queue Management, AQM [RFC7567]), and
      inadequate provision of traffic preemption.  Understanding flow
      loss rate requires either maintaining per flow packet counters or
      by observing sequence numbers in transport headers.  Loss can be
      monitored at the interface level by devices in the network.  It is
      often valuable to understand the conditions under which packet
      loss occurs.  This 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 the
      transport did not require 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:  The throughput achieved by a flow can be
      determined even when transport header information is concealed,
      providing the individual flow can be identified.  Goodput
      [RFC7928] is a measure of useful data exchanged (the ratio of
      useful/total volume of traffic sent by a flow).  This requires
      ability to differentiate loss and retransmission of packets (e.g.,
      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 response time and user-perceived response time.  It
      often indirectly impacts throughput and flow completion time.
      Latency 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

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      [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 to measure 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.

      The service offered by network operators can benefit from latency
      information to understand the impact of deployment and 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 [RFC8290] [RFC8289] [RFC8033] often
      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 (e.g., 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
      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

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      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.

      Metrics have been defined that evaluate whether a network has
      maintained packet order on a packet-by-packet basis [RFC4737] and
      [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, e.g. to 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 that use the information 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.  This can provide information

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   to enable a different forwarding treatment by the network, even when
   a transport employs encryption to protect other header information.

   On the one hand, the user of a transport that multiplexes multiple
   sub-flows could wish 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 network needs of individual sub-flows.  There are
   several ways this could be done:

   IP Address:  Applications 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".  To promote privacy, the
      Flow Label assignment needs to avoid introducing linkability that
      a network device may observe.  Once set, a label can provide
      information that can help inform network-layer queuing and
      forwarding [RFC6438](e.g. for Equal Cost Multi-Path, ECMP,
      routing, and Link Aggregation, LAG) [RFC6294].  [RFC6438]
      describes considerations when used with IPsec.

   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.

      Since the DSCP value can impact the quality of experience for a
      flow, observations of service performance need to consider this

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      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 Congestion
      Experienced (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.

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 QUIC, with the obvious exception of
   Virtual Private Networks (VPNs) and IPsec.

   When encryption conceals more layers in each packet, people seeking
   understanding of the network operation rely more on pattern
   inferences and other heuristics reliance on pattern inferences and
   accuracy suffers.  For example, the traffic patterns between server
   and browser are dependent on browser supplier and version, even when
   the sessions use the same server application (e.g., web e-mail
   access).  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]).

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   When measurement datasets are made available by servers or client
   endpoints, additional metadata, such as the state of the network, is
   often required 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.

3.2.2.  Use by Operators to Plan and Provision Networks

   Traffic measurements (e.g., traffic volume, loss, latency) is used by
   operators to help plan deployment of new equipment and configuration
   in their networks.  Data is also valuable to equipment vendors who

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   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 may 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]) may 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).  However, passive measurements can rely on observing
   transport headers which is not possible if those headers are
   encrypted.

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., to
   prevent flows from acquiring excessive network capacity 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
      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].

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      A standards-compliant TCP stack provides congestion control that
      may therefore be 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 transport (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
      safe operation of network infrastructure, and mechanisms 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 are required
      to employ mechanisms to prevent congestion 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
      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.

      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
      the RTP and RTCP header information of real-time flows (see

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      Section 3.1.2, and the Secure RTP extensions [RFC3711] were
      explicitly designed to expose header information to enable such
      observation.

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.  Sections 3.1.2 and 5 of [RFC8404] provide further
   examples.  These tasks seldom involve the need to determine the
   contents of the transport payload, or other application details.

   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
   may 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.

   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

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   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,s 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 bandwidth 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 bandwidth 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 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 it to fall back to compressing only the
   network layer headers, with a significant reduction in efficiency.
   This may have operational impact.

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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.
   Encryption methods can hide information from an eavesdropper in the
   network.  Encryption can also help protect the privacy of a user, by
   hiding data relating to user/device identity or location.  Neither an
   integrity check nor encryption methods 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.  Any header information that has a clear
   definition in the protocol's message format(s), or is implied by that
   definition, and is not cryptographically confidentiality-protected
   can be unambiguously interpreted by on-path observers
   [I-D.trammell-wire-image].

   There are several motivations:

   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 may 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 middlebox use 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), Virtual Private Networks (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 revelation 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, there are now
      activities in the IETF to design new protocols that could include

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      some form of transport header encryption (e.g., QUIC
      [I-D.ietf-quic-transport]).

   Authentication methods (that provide integrity checks of protocols
   fields) have also been specified at the network layer, and this also
   protects transport header fields.  The network layer itself carries
   protocol header fields that are increasingly used to help forwarding
   decisions reflect the need of transport protocols, such as the IPv6
   Flow Label [RFC6437], DSCP, and ECN fields.

   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, and which 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,
      allowing 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

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      the IP pseudo header, TCP header, and TCP data.  TCP-AO protects
      the transport layer, preventing attacks from disabling the TCP
      connection itself and provides replay protection.  TCP-AO may
      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:  Transport layer header information that is observable can
      be observed in the network.  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.

   Encrypting the Transport Payload:  The transport layer payload can be
      encrypted to protect the content of transport segments.  This
      leaves transport protocol header information in the clear.  The
      integrity of immutable transport header fields could be protected
      by combining this with an integrity check.

      Examples of encrypting the payload include Transport Layer
      Security (TLS) over TCP [RFC8446] [RFC7525], Datagram TLS (DTLS)
      over UDP [RFC6347] [RFC7525], Secure RTP [RFC3711], and TCPcrypt
      [I-D.ietf-tcpinc-tcpcrypt] which permits opportunistic encryption
      of the TCP transport payload.

   Encrypting the Transport Headers and Payload:  The network layer
      payload could be encrypted (including the entire transport header
      and the payload).  This method provides confidentiality of the

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      entire transport packet.  It therefore does not expose any
      transport information to devices in the network, which also
      prevents modification along a network path.

      One example of encryption at the network layer is use of IPsec
      Encapsulating Security Payload (ESP) [RFC4303] in tunnel mode.
      This encrypts and authenticates all transport headers, preventing
      visibility of the transport headers by in-network devices.  Some
      Virtual Private Network (VPN) methods also encrypt these headers.

   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
      protocol, where support of optional mechanisms can increase the
      complexity of the protocol and its implementation and in the
      management decisions that are required 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 Protocol Headers

   Some measurements can be made by adding additional protocol headers
   carrying operations, administration and management (OAM) information
   to packets 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])
   and removing the additional header at the egress of the maintenance
   domain.  This approach enables some types of measurements, but does

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   not cover the entire range of measurements described in this
   document.  In some cases, it can be difficult to position measurement
   tools at the required segments/nodes and there can be challenges in
   correlating the downsream/upstream information when in-band OAM data
   is inserted by an on-path device.  This has the advantage that a
   single header can support all transport protocols, but there could
   also be less desirable implications of separating the operation of
   the transport protocol from the measurement framework.

   Another example of a network-layer approach 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 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 measurements suggest it can be undesirable to rely on methods
   requiring the presence of network options or extension headers.  IPv4
   network options are often not supported (or are carried on a slower
   processing path) and some IPv6 networks are also known to drop
   packets that set an IPv6 header extension (e.g., [RFC7872]).  Another
   disadvantage is that protocols that separately expose header
   information do not necessarily have an incentive to expose the
   information that is utilised by the protocol itself, and could
   manipulate the exposed header information to gain an advantage from
   the network.

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.

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6.1.  Independent Measurement

   Independent observation by multiple actors is important for
   scientific analysis.  Encrypting transport header encryption changes
   the ability for other actors 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 - loss
   detection and recovery, congestion detection and congestion control,
   some of these need to work only end-to-end (e.g., parameter
   negotiation, flow-control).

   The majority of present Internet applications use two well-known
   transport protocols, TCP and UDP.  Although TCP represents the
   majority of current traffic, some 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, increasing 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 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 choose what 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 could set the spin bit to reflect to
   explicitly reveal a session's RTT [I-D.ietf-quic-spin-exp]).

   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

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

   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 behavior of a specific mechanism with the
   configuration and traffic using operational equipment (e.g.
   Combining transport and network measurements to explore congestion
   control dynamics or the implications of active queue management).

   For some usage a standardised endpoint-based logging format (e.g.,
   based onQuic-Trace [Quic-Trace]) could offer an alternative to in-
   network measurement.  Such information will have a diversity of uses
   - examples include developers wishing to debug/understand the
   transport/applictaion protocols with which they work, to researchers
   seeking to spot trends, anomalies and to characterise variants of
   protocols.  This use will need to establish the validity and
   provenance of the logging information (e.g., 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.

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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
   may 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 may 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 may 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

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   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 may in turn impact operational mechanisms, and 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 combination 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.

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   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
   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

   Confidentiality and strong integrity checks have properties that are
   being incorporated into new protocols and that 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.

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   The traffic that can be observed by on-path network devices 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 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
   level (ISPs, enterprises, firewall maintainer, etc) to identify
   appropriate operational support functions and procedures.

   Protocols that change their transport header format (wire format) or
   their behaviour (e.g., algorithms that are needed to classify and
   characterise the protocol), will require new tooling to be developed
   to catch-up with the change.  If the currently deployed tools and
   methods are no longer relevant then it may no longer be possible to
   correctly 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 because of being incorrectly classified/
   monitored).

   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 may also be useful to various
   stakeholder (as described in earlier sections), however this document
   does not make recommendations about what information should be
   exposed, to whom it should be observable, or how this will be
   achieved.

   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]).

   There are trade-offs and implications of increased use of encryption.
   Transport protocol designers have often ignored the implications of

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   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 the header information.  It can be expected that a
   lack of visibility of transport header information can 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:

   o  Coexistence of Transport and Network Device Protocols/
      Configuration: Transmission Control Protocol (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 can 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.

   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 understand complex feature
      interactions.  An inability to observe transport protocol
      information can limit the ability 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 need to be
      developed.

   o  Operational Practice: The network operations community relies on
      being able to understand the pattern and requirements of traffic

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      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
      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 this 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 Groups (ICCRG) and research
      publications in reviewing new transport mechanisms and assessing
      the impact of their experimental deployment).

   The choice of whether future transport protocols encrypt their
   protocol headers needs to be taken based not solely on security and
   privacy considerations, but also taking 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 considered [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 in
   the various sections of the document.

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   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.
   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 benefits
   of ossifying common headers, versus the potential demerits 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 can limit the ability to measure and
   characterise traffic.

   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 yo 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 may not finally be delivered by the
   transport layer.  This is sometimes known as a "shadowing attack".

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   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).

   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 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.

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

   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-03 (work in progress), June 2018.

   [I-D.ietf-quic-spin-exp]
              Trammell, B. and M. Kuehlewind, "The QUIC Latency Spin
              Bit", draft-ietf-quic-spin-exp-01 (work in progress),
              October 2018.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-14 (work
              in progress), August 2018.

   [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]
              Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey
              of Transport Security Protocols", draft-ietf-taps-
              transport-security-02 (work in progress), June 2018.

   [I-D.ietf-tcpinc-tcpcrypt]
              Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic protection of TCP Streams
              (tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-12 (work in
              progress), June 2018.

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   [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.thomson-quic-grease]
              Thomson, M., "More Apparent Randomization for QUIC",
              draft-thomson-quic-grease-00 (work in progress), December
              2017.

   [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.

   [I-D.trammell-wire-image]
              Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", draft-trammell-wire-image-04 (work in
              progress), April 2018.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

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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.

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/

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   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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