TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                              C.S. Perkins
Expires: October 10, 2018                          University of Glasgow
                                                          April 10, 2018

The Impact of Transport Header Confidentiality on Network Operation and
                       Evolution of the Internet


   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 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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 10, 2018.

Copyright Notice

   Copyright (c) 2018 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 (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Current uses of Transport Headers within the Network . . . . .  8
     2.1.  Observing Transport Information in the Network . . . . . .  9
       2.1.1.  Flow Identification  . . . . . . . . . . . . . . . . .  9
       2.1.2.  Metrics derived from Transport Layer Headers . . . . . 10
       2.1.3.  Metrics derived from Network Layer Headers . . . . . . 12
     2.2.  Transport Measurement  . . . . . . . . . . . . . . . . . . 14
       2.2.1.  Point of Measurement . . . . . . . . . . . . . . . . . 14
       2.2.2.  Use by Operators to Plan and Provision Networks  . . . 15
       2.2.3.  Service Performance Measurement  . . . . . . . . . . . 15
       2.2.4.  Measuring Transport to Support Network Operations  . . 16
     2.3.  Use for Network Diagnostics and Troubleshooting  . . . . . 17
       2.3.1.  Examples of measurements . . . . . . . . . . . . . . . 18
     2.4.  Observing Headers to Implement Network Policy  . . . . . . 19
   3.  Encryption and Authentication of Transport Headers . . . . . . 19
     3.1.  Authenticating the Transport Protocol Header . . . . . . . 21
     3.2.  Encrypting the Transport Payload . . . . . . . . . . . . . 21
     3.3.  Encrypting the Transport Header  . . . . . . . . . . . . . 21
     3.4.  Authenticating Transport Information and Selectively
           Encrypting the Transport Header  . . . . . . . . . . . . . 22
     3.5.  Optional Encryption of Header Information  . . . . . . . . 22
   4.  Addition of Transport Information to Network-Layer Protocol
       Headers  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.  Implications of Protecting the Transport Headers . . . . . . . 23
     5.1.  Independent Measurement  . . . . . . . . . . . . . . . . . 23
     5.2.  Characterising "Unknown" Network Traffic . . . . . . . . . 24
     5.3.  Accountability and Internet Transport Protocols  . . . . . 24
     5.4.  Impact on Research, Development and Deployment . . . . . . 25
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 29
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 29
   Appendix A. Revision information . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35

1.  Introduction

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   This document describes implications 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 expected
   implications of transport protocol design and network operation.  It
   also considers anticipated implications on transport and application

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

   To achieve stable Internet operations the IETF transport community
   has to date relied heavily on 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]). In turn, the network operations
   community relies on being able to understand the pattern and
   requirements of traffic passing over the Internet, both in aggregate
   and at the flow level.

   There are many motivations for deploying encrypted transports (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). The increasing public concerns about the interference
   with Internet traffic have led to a rapidly expanding deployment of
   encryption to protect end-user privacy, in protocols like QUIC [I-D
   .ietf-quic-transport], but also expected to form a basis of future
   protocol designs.

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   Implementations of network devices are encouraged to avoid side-
   effects when protocols are updated.  Introducing cryptographic
   integrity checks to header fields can also prevent undetected
   manipulation of the field by network devices, or undetected addition
   of information to a packet.  However, 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.  Reliance on the
   presence and semantics of packet headers leads to ossification: An
   endpoint could be required to supply a specific header to receive the
   network service that it desires.  In some cases, this could be benign
   to the protocol (e.g., recognising the start of a connection), but
   not in all cases (e.g., a mechanism implemented in a network device,
   such as a firewall, could require a header field to have only a
   specific known set of values could prevent the device from forwarding
   packets using a different version of a protocol that introduces a new
   feature that changes the value present in this field).

   A protocol design can use header encryption to 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 imply semantics of an exposed header
   field.  Using encryption to provide confidentiality of the transport
   layer brings some well-known privacy and security benefits and can
   therefore help reduce ossification of the transport layer.  In
   particular, it is important that protocols either do not expose
   information where the usage may change in future protocols, or that
   methods that utilise the information are robust to potential changes
   as protocols evolve over time.  To avoid unwanted inspection, a
   protocol could also intentionally vary the format and value of header
   fields (sometimes known as Greasing [I-D.thomson-quic-grease]).

   At the same time, some network operators and access providers, have
   come to rely on the in-network measurement of transport properties
   and the functionality provided by middleboxes to both support network
   operations and enhance performance.  There can therefore be
   implications when working with encrypted transport protocols that

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   hide transport header information from the network.  This present
   architectural challenges and considerations in the way transport
   protocols are designed, and ability to characterise and compare
   different transport solutions [Measure].

   A level of ossification of the header can be advantageous in terms of
   providing a set of specified header fields that become observable by
   in-network devices.  This results in trade-offs around
   authentication, and confidentiality of transport protocol headers and
   the potential support for other uses of this header information.  For
   example, a design that provides confidentiality of protocol header
   information can impact the following activities that rely on
   measurement and analysis of traffic flows:

   Network Operations and Research: Observable transport headers enable
      both operators and the research community to measure and analyse
      protocol performance, network anomalies, and failure pathologies.

      This information can help inform capacity planning, and assist in
      determining the need for equipment and/or configuration changes by
      network operators.

      The data can also inform Internet engineering research, and help
      the development of new protocols, methodologies, and procedures.
      Concealing the transport protocol header information makes the
      stream performance unavailable to passive observers along the
      path, and likely leads to the development of alternative methods
      to collect or infer that data.

      Providing confidentiality of the transport payload, but leaving
      some, or all, of the transport headers unencrypted, possibly with
      authentication, can provide the majority of the privacy and
      security benefits while allowing some measurement.

   Protection from Denial of Service: Observable transport headers
      currently provide useful input to classify traffic and detect
      anomalous events (e.g., changes in application behaviour,
      distributed denial of service attacks). To be effective, this
      protection needs to be able to uniquely disambiguate unwanted
      traffic.  An inability to separate this traffic using packet
      header information may result in less-efficient identification of
      unwanted traffic or development of different methods (e.g.  rate-
      limiting of uncharacterised traffic).

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   Network Troubleshooting and Diagnostics:  Encrypting transport header
      information eliminates the incentive for operators to troubleshoot
      what they cannot interpret.  A flow experiencing packet loss or
      jitter looks like an unaffected flow when only observing network
      layer headers (if transport sequence numbers and flow identifiers
      are obscured). This limits understanding of the impact of packet
      loss or latency on the flows, or even localizing the network
      segment causing the packet loss or latency.  Encrypted traffic may
      imply "don't touch" to some, and could limit a trouble-shooting
      response to "can't help, no trouble found".  The additional
      mechanisms that will need to be introduced to help reconstruct
      transport-level metrics add complexity and operational costs
      (e.g., in deploying additional functions in equipment or adding
      traffic overhead).

   Network Traffic Analysis: Hiding transport protocol header
      information can make it harder to determine which transport
      protocols and features are being used across a network segment and
      to measure trends in the pattern of usage.  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
      the impact may, in many cases, be small there are scenarios where
      operators directly support particular services (e.g., to
      troubleshoot 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

   Open and Verifiable Network Data:  Hiding transport protocol header
      information can reduce the range of actors that can capture useful
      measurement data.  For example, one approach could be to employ an
      existing transport protocol that reveals little information (e.g.,
      UDP), and perform traditional transport functions at higher layers
      protecting the confidentiality of transport information.  Such a
      design, limits the information sources available to the Internet
      community to understand the operation of new transport protocols,
      so preventing access to the information necessary 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 performance cannot be
      independently verified by all parties.  The ability of other

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      stakeholders to review code can help develop deeper insight.  In
      the heterogeneous Internet, this helps extend the range of
      topologies, vendor equipment, and traffic patterns that are

      Independently captured data is important to help ensure the health
      of the research and development communities.  It can provide input
      and test scenarios to support development of new transport
      protocol mechanisms, especially when this analysis can be based on
      the behaviour experienced in a diversity of deployed networks.

      Independently verifiable performance metrics might also be
      important to demonstrate regulatory compliance in some
      jurisdictions, and provides an important basis for informing
      design decisions.

   The last point leads us to consider the impact of hiding transport
   headers in the specification and development of protocols and
   standards.  This has potential impact on:

   o  Understanding Feature Interactions: An appropriate vantage point,
      coupled with timing information about traffic flows, provides a
      valuable tool for benchmarking equipment, functions, and/or
      configurations, and to 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.

   o  Supporting Common Specifications: The 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 clear there are sufficient
      incentives to ensure good practice that benefits the wide
      diversity of requirements for the Internet community as a whole.
      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 specifications, and on the
      ability to verify that others also conform.

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   o  Operational practice: Published transport specifications allow
      operators to check compliance.  This can bring assurance to those
      operating networks, often avoiding the need to deploy complex
      techniques that routinely monitor and manage TCP/IP traffic flows
      (e.g.  Avoiding the capital and operational costs of deploying
      flow rate-limiting and network circuit-breaker methods [RFC8084]).
      When 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.

   o  Restricting research and development: Hiding 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
      ICCRG, and research publications in reviewing new transport
      mechanisms and assessing the impact of their experimental

   In summary, there are tradeoffs.  On the one hand, protocol designers
   have often ignored the implications of whether the information in
   transport header fields can or will be used by in-network devices,
   and the implications this places on protocol evolution.  This
   motivates a design that provides confidentiality of the header
   information.  On the other hand, 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 choice of whether future transport protocols encrypt their
   protocol headers therefore needs to be taken based not solely on
   security and privacy considerations, but also taking into account the
   impact on operations, standards, and research.  Any new Internet
   transport need to provide appropriate transport mechanisms and
   operational support to assure the resulting traffic can not result in
   persistent congestion collapse [RFC2914].  This document suggests
   that the balance between information exposed and concealed should be
   carefully considered when specifying new protocols.

2.  Current uses of Transport Headers within the Network

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   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 [I-D.mm-wg-effect-encrypt].
   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].

2.1.  Observing Transport Information in the Network

   In-network observation of transport protocol headers requires
   knowledge of the format of the transport header:

   o  Flows need to be identified at the level required for monitoring;

   o  The protocol and version of the header need to be observable.  As
      protocols evolve over time and there may be a need to introduce
      new transport headers.  This may require interpretation of
      protocol version information or connection setup information;

   o  Location and syntax of any transport headers needs to be known to
      be observed.  IETF transport protocols specify this information.

   The following subsections describe various ways that observable
   transport information may be utilised.

2.1.1.  Flow Identification

   Transport protocol header information (together with information in
   the network header), can 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 can
   identify a protocol (although port information alone is not
   sufficient to guarantee identification of a protocol). Transport
   protocols, such as TCP and Stream Control Transport Protocol (SCTP)
   specify a standard base header that includes sequence number

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   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 can instead be
   identified by signalling protocols or through the use of magic
   numbers placed in the first byte(s) of the datagram payload.

   Flow identification is more complex and less easily achieved when
   multiplexing is used at or above the transport layer.

2.1.2.  Metrics derived from Transport Layer Headers

   Some actors manage their portion of the Internet by characterizing
   the performance of link/network segments.  Passive monitoring uses
   observed traffic to makes inferences from transport headers to derive
   these measurements.  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,
      length) may allow derivation of volume measures per-application,
      to characterise the traffic that uses a network segment or the
      pattern of network usage.  This may be measured per endpoint or
      for an aggregate of endpoints (e.g., by an operator 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 (providing detail of trends rather than the
      volume per subscriber).

   Loss Rate and Loss Pattern: Flow loss rate may be derived (e.g., from
      sequence number) and is often used as a metric for performance
      assessment and to characterise transport behaviour.  Understanding
      the root cause of loss can help an operator determine whether this
      requires corrective action.  Network operators may also use 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), 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 important 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.

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      Observation of transport feedback information (observing loss
      reports, e.g., RTP Control Protocol (RTCP), TCP SACK) can increase
      understanding of the impact of loss and help identify cases where
      loss may have been wrongly identified, or the transport did not
      require the lost packet.  It is sometimes more important 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 a flow is encrypted, 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 Protocol, RTP, headers

   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.
      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
      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 can be used to locate a source of latency, e.g., by observing
      cases where the ratio of median to minimum RTT is large for a part
      of a path.

      The service offered by 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]
      [I-D.ietf-aqm-fq-codel] and although parameter-less methods are
      desired [RFC7567], current methods [I-D.ietf-aqm-fq-codel] [I-D
      .ietf-aqm-codel] [I-D.ietf-aqm-pie] often cannot scale across all
      possible deployment scenarios.

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

      There have been initiatives in the IETF transport area to reduce
      the impact of reordering within a transport flow, possibly leading
      to a reduction in the requirements for preserving ordering.  These
      have promise 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.

   Operational tools to detect mis-ordered packet flows and quantify the
   degree or reordering.  Key performance indicators are retransmission
   rate, packet drop rate, sector utilisation level, a measure of
   reordering, peak rate, the 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

   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 important to understand the
   context under which the data was collected, including the time,
   observation point, 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.

2.1.3.  Metrics derived from Network Layer Headers

   Some transport information is made visible in the network-layer
   protocol header.  These header fields are not encrypted and can be
   utilised to make flow observations.

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   Use of IPv6 Network-Layer Flow Label: Endpoints are encouraged expose
      flow information in the IPv6 Flow Label field of the network-layer
      header (e.g., [RFC8085]). This can be used to inform network-layer
      queuing, forwarding (e.g., for equal cost multi-path, ECMP,
      routing, and Link Aggregation, LAG). This can provide useful
      information to assign packets to flows in the data collected by
      measurement campaigns.  Although important to characterising a
      path, it does not directly provide performance data.

   Use Network-Layer Differentiated Services Code Point Point: Applicati
      ons can expose their delivery expectations to the network by
      setting the Differentiated Services Code Point (DSCP) field of
      IPv4 and IPv6 packets.  This can be used to inform network-layer
      queuing and forwarding, and can also provide information on the
      relative importance of packet information collected by measurement
      campaigns, but does not directly provide performance data.

      This field provides explicit information that can be used in place
      of inferring traffic requirements (e.g., by inferring QoS
      requirements from port information via a multi-field classifier).
      The DSCP value can therefore impact the quality of experience for
      a flow.  Observations of service performance need to consider this
      field when a network path has support for differentiated service

   Use of Explicit Congestion Marking: ECN [RFC3168] is an optional
      transport mechanism that uses a code point in the network-layer
      header.  Use of ECN can offer gains in terms of increased
      throughput, reduced delay, and other benefits when used over a
      path that includes equipment that supports an AQM method that
      performs Congestion Experienced (CE) marking of IP packets

      ECN exposes the presence of congestion on a network path to the
      transport and network layer.  The reception of CE-marked packets
      can therefore be used to monitor the presence and estimate the
      level of incipient congestion on the upstream portion of the path
      from the point of observation (Section 2.5 of [RFC8087]). Because
      ECN marks are carried in the IP protocol header, it is much easier
      to measure ECN than to measure packet loss.  However, interpreting
      the marking behaviour (i.e., assessing congestion and diagnosing
      faults) requires context from the transport layer (path RTT,
      visibility of loss - that could be due to queue overflow,
      congestion response, etc) [RFC7567].

      Some ECN-capable network devices can provide richer (more frequent
      and fine-grained) indication of their congestion state.  Setting
      congestion marks proportional to the level of congestion (e.g.,
      Data Center TCP, DCTP [RFC8257], and Low Latency Low Loss Scalable

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      throughput, L4S, [I-D.ietf-tsvwg-l4s-arch].

      Use of ECN requires a transport to feed back reception information
      on the path towards the data sender.  Exposure of this Transport
      ECN feedback provides an additional powerful tool to understand
      ECN-enabled AQM-based networks [RFC8087].

      AQM and ECN offer a range of algorithms and configuration options,
      it is therefore important for tools to be available to network
      operators and researchers to understand the implication of
      configuration choices and transport behaviour as use of ECN
      increases and new methods emerge [RFC7567] [RFC8087].  ECN-
      monitoring is expected to become important as AQM is deployed that
      supports ECN [RFC8087].

2.2.  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 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 [I-D.mm-wg-effect-

   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.  Collecting and coordinating
   such metadata is more difficult when the observation point is at a
   different location to the bottleneck/device under evaluation.

   Packet sampling techniques can be 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.

2.2.1.  Point of Measurement

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   Often measurements can only be understood in the context of the other
   flows that share a bottleneck.  A simple example is monitoring of
   AQM. For example, FQ-CODEL [I-D.ietf-aqm-fq-codel], combines sub
   queues (statistically assigned per flow), management of the queue
   length (CODEL), flow-scheduling, and a starvation prevention
   mechanism.  Usually such algorithms are designed to be self-tuning,
   but current methods typically employ heuristics that can result in
   more loss under certain path conditions (e.g., large RTT, effects of
   multiple bottlenecks [RFC7567]).

   In-network measurements can distinguish between upstream and
   downstream metrics with respect to a measurement point.  These are
   particularly useful for locating the source of problems or to assess
   the performance of a network segment or a particular device
   configuration.  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.).

2.2.2.  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 configurations
   in their networks.  Data is also important to equipment vendors who
   need 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 may 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 not be possible 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.

2.2.3.  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

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   users of a network segment, and inform operational practice.

   While active measurements may be used in-network passive measurements
   can have advantages in terms of eliminating unproductive traffic,
   reducing the influence of test traffic on the overall traffic mix,
   and the ability to choose the point of measurement Section 2.2.1.
   However, passive measurements may rely on observing transport

2.2.4.  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 to comply 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].

      A standards-compliant TCP stack provides congestion control 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

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      the other traffic that shares a network.  An observation in the
      network can gain understanding of the dynamics of a flow and its
      congestion control behaviour.  Analysing observed packet sequence
      numbers can be used to help 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,

   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
      Section 2.1.2.

2.3.  Use for Network Diagnostics and Troubleshooting

   Transport header information is useful for a variety of operational
   tasks [I-D.mm-wg-effect-encrypt]: to diagnose network problems,
   assess performance, capacity planning, management of denial of
   service threats, and responding to user performance questions.  These
   tasks seldom involve the need to determine the contents of the
   transport payload, or other application details.

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   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 information may reduce
   the ability for networks to "help" (e.g., in response to tracing
   issues, making appropriate QoS decisions). 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). Although many network operators utilise
   transport information as a part of their operational practice, the
   network will not break because transport headers are encrypted, and
   this may require alternative tools may need to be developed and

2.3.1.  Examples of measurements

   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 debugging and diagnosing the root causes of faults that
   concern a particular user's traffic.  They can also be used to
   support post-mortem inverstigation after an anompoly to determine the
   root cause of a problem.

   In some case, measurements may involve active injection of test
   traffic to complete a measurement.  However, most operators do not
   have access to user equipment, and injection of test traffic may be
   associated with costs in running such tests (e.g., the implications
   of bandwidth tests in a mobile network are obvious). Some active
   measurements (e.g., response under load or particular workloads)
   perturb other traffic, and could require dedicated access to the
   network segment.  An alternative approach is to use in-network
   techniques that observe transport packet headers in operational
   networks to make the measurements.

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   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 to 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.

2.4.  Observing Headers to Implement Network Policy

   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.  Traffic that cannot be classified, will
   typically receive a default treatment.

3.  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.

   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

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      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 ESP. 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 may
      include some form of transport header encryption (e.g., QUIC [I-D

   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], the DSCP and ECN.

   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 of 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 TCP/IP stack.

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   The next subsections briefly review some security design options for
   transport protocols.

3.1.  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 observes these fields.  An
   integrity check can not prevent in-network modification, but can
   avoid a receiving 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 TCP segments,
   including the IP pseudo header, TCP header, and TCP data.  TCP-AO
   protects the transport layer, preventing attacks from disabling the
   TCP connection itself.  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.

3.2.  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 (Section 3.1).

   Examples of encrypting the payload include Transport Layer Security
   (TLS) over TCP [RFC5246] [RFC7525] or Datagram TLS (DTLS) over UDP
   [RFC6347] [RFC7525].

3.3.  Encrypting the Transport Header

   The network layer payload could be encrypted (including the entire
   transport header and payload). This method does not expose any
   transport information to devices in the network, which also prevents
   modification along a network path.

   The IPsec Encapsulating Security Payload (ESP) [RFC4303] is an
   example of encryption at the network layer, it encrypts and
   authenticates all transport headers, preventing visibility of the
   headers by in-network devices.  Some Virtual Private Network (VPN)

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   methods also encrypt these headers.

3.4.  Authenticating Transport Information and Selectively Encrypting
      the Transport Header

   A transport protocol design can encrypt selected header fields, while
   also choosing to authenticate fields in the 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 may 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.

3.5.  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

4.  Addition of Transport Information to Network-Layer Protocol Headers

   Transport protocol information can be made visible in a network-layer
   header.  This has the advantage that this information can then be
   observed by in-network devices.  This has the advantage that a single
   header can support all transport protocols, but there may also be
   less desirable implications of separating the operation of the
   transport protocol from the measurement framework.

   Some measurements may 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
   not cover the entire range of measurements described in this

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   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.

   Another example of a network-layer approach is the IPv6 Performance
   and Diagnostic Metrics (PDM) Destination Option [I-D.ietf-ippm-6man-
   pdm-option].  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.

   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
   advantage to expose the information that is utilised by the protocol
   itself, and could manipulate this header information to gain an
   advantage from the network.

5.  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.  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.

5.1.  Independent Measurement

   Independent observation by multiple actors is important for
   scientific analysis.  Encrypting transport header encryption changes

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   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).

   When encryption conceals information in the transport header, it
   could be possible for an applications to provide summary data on
   performance and usage of the network.  This data could be made
   available to other actors.  However, this data needs to contain
   sufficient detail to understand (and possibly reconstruct the network
   traffic pattern for further testing) and to be correlated with the
   configuration of the network paths being measured.

   Sharing information between actors needs also to consider the privacy
   of the user and the incentives for providing accurate and detailed
   information.  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.
   This becomes important when considering changes to transport
   protocols, changes in network infrastructure, or the emergence of new
   traffic patterns.

5.2.  Characterising "Unknown" Network Traffic

   The patterns and types of traffic that share Internet capacity
   changes with 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.

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5.3.  Accountability and Internet Transport Protocols

   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, and where needed to
   deploy appropriate tools Section 2.2.4.  Obfuscating or hiding this
   information using encryption is expected to lead operators and
   maintainers of middleboxes (firewalls, etc.) to seek other methods to
   classify and mechanisms to condition network traffic.

   A lack of data reduces the level of precision with which mechanisms
   are applied, and this needs to be considered when evaluating the
   impact of designs for transport encryption.  This could lead to
   increased use of rate limiting, circuit breaker techniques [RFC8084],
   or even blocking of uncharacterised traffic.  This would hinder
   deployment of new mechanisms and/or protocols.

5.4.  Impact on Research, Development and Deployment

   The majority of present Internet applications use two well-known
   transport protocols: e.g., TCP and UDP. Although TCP represents the
   majority of current traffic, some important 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.  Over this period, the transport protocol
   headers have mostly changed slowly, and so also the need to develop
   tools track new versions of the protocol.

   Looking ahead, there will be a need to update these protocols and to
   develop and deploy new transport mechanisms and protocols.  There are
   both opportunities and also challenges to the design, evaluation and
   deployment of new transport protocol mechanisms.

   Integrity checks can undetected modification of protocol fields by
   network devices, whereas encryption and obfuscation 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.

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   Hiding headers can limit the ability to measure and characterise
   traffic.  Measurement data is increasingly being used to inform
   design decisions in networking research, during development of new
   mechanisms and protocols and in standardisation.  Measurement has a
   critical role in the design of transport protocol mechanisms and
   their acceptance by the wider community (e.g., as a method to judge
   the safety for Internet deployment). Observation of pathologies are
   also important in 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.

   Evolution and the ability to understand (measure) the impact need to
   proceed hand-in-hand.  Attention needs to be paid to the expected
   scale of deployment of new protocols and protocol mechanisms.
   Whatever the mechanism, experience has shown that it is often
   difficult to correctly implement combination of mechanisms [RFC8085].
   These mechanisms therefore typically evolve as a protocol matures, or
   in response to changes in network conditions, changes in network
   traffic or changes to application usage.

   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.

   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.

6.  Conclusions

   The majority of present Internet applications use two well-known
   transport protocols: e.g., TCP and UDP. Although TCP represents the
   majority of current traffic, some important real-time applications
   have used 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.  Over this period, the
   transport protocol headers have mostly changed slowly, and so also
   the need to develop tools track new versions of the protocol.

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   Confidentiality and strong integrity checks have properties that are
   being incorporated into new protocols and which have important
   benefits.  The pace of development of transports using the WebRTC
   data channel and the rapid deployment of QUIC prototype transports
   can both be attributed to using a combination of UDP transport and
   confidentiality of the UDP payload.

   The traffic that can be observed by devices in a network 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 an operational issue 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 needs to be
   developed to catch-up with the changes.  If the currently deployed
   tools and methods are no longer relevant and performance may not be
   correctly measured.  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 mis-classified and then receiving a default
   treatment by the network.

   A protocol specification 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
   provide tools to manage new variants of protocols.  Several scenarios
   to illustrate different ways this could evolve are provided below:

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   o  One scenario is when transport protocols provide consistent
      information to the network by intentionally exposing a part of the
      transport header.  The design fixes the format of this information
      between versions of the protocol.  This level of ossification
      allows an operator to establish tooling and procedures that allow
      it to provide consistent traffic management as the protocol
      evolves.  In contrast to TCP (where all protocol information is
      exposed), evolution of the transport is facilitated by providing
      cryptographic integrity checks of the transport header fields
      (preventing undetected middlebox changes) and encryption of other
      protocol information (preventing observation within the network).
      The transport information can be used by operators to provide
      troubleshooting, easement and any necessary functions for the
      class of traffic (priority, retransmission, reordering, circuit
      breakers, etc).

   o  An alternative scenario adopts different design goals, with a
      different outcome.  A protocol that encrypts all header
      information forces network operators to act independently from
      apps/transport developments to provide the transport information
      they need.  A range of approaches may proliferate, as in current
      networks, operators can add a shim header to each packet as a flow
      as it crosses the network; other operators/managers could develop
      heuristics and pattern recognition to derive information that
      classifies flows and estimates quality metrics for the service
      being used; some could decide to rate-limit or block traffic until
      new tooling is in place.  In many cases, the derived information
      can be used by operators to provide necessary functions
      appropriate to the class of traffic (priority, retransmission,
      reordering, circuit breakers, etc).  Troubleshooting, and
      measurement becomes more difficult, and more diverse.  This could
      require additional information beyond that visible in the packet
      header and.  In some cases, operators might need access to keying
      information to interpret encrypted data that they observe.  Some
      use cases could demand use of transports that do not use

   The outcome could have significant implications on the way the
   Internet architecture develops.  It exposes a risk that significant
   actors (e.g., developers and transport designers) achieve more
   control of the way in which the Internet architecture develops.In
   particular, there is a possibility that designs could evolve to
   significantly benefit of customers for a specific vendor, and that
   communities with very different network, applications or platforms
   could then suffer at the expense of benefits to their vendors own
   customer base.  In such a scenario, there could be no incentive to
   support other applications/products or to work in other networks
   leading to reduced access for new approaches.

7.  Acknowledgements

   The author would like to thank all who have talked to him face-to-
   face or via email.  ...

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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.

8.  Security Considerations

   This document is about design and deployment considerations for
   transport protocols.  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; no current full-featured standards-track transport protocol
   provides these features on its own.  Therefore, these features are
   not considered in this document, with the exception of native
   authentication capabilities of TCP and SCTP for which the security
   considerations in RFC4895.

   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.  Standard protocols
   and understanding of the interactions between mechanisms and traffic
   patterns can also provide valuable insight into appropriate security
   design.  Like congestion control mechanisms, security mechanisms are
   difficult to design and implement correctly.  It is hence recommended
   that applications employ well-known standard security mechanisms such
   as DTLS, TLS or IPsec, rather than inventing their own.

9.  IANA Considerations


   This memo includes no request to IANA.

10.  References

10.1.  Normative References

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

10.2.  Informative References

              Dolson, D., Snellman, J., Boucadair, M. and C. Jacquenet,
              "Beneficial Functions of Middleboxes", Internet-Draft
              draft-dolson-plus-middlebox-benefits-03, March 2017.


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              Nichols, K., Jacobson, V., McGregor, A. and J. Iyengar,
              "Controlled Delay Active Queue Management", Internet-Draft
              draft-ietf-aqm-codel-10, October 2017.

              Hoeiland-Joergensen, T., McKenney, P.,
              dave.taht@gmail.com, d., Gettys, J. and E. Dumazet, "The
              FlowQueue-CoDel Packet Scheduler and Active Queue
              Management Algorithm", Internet-Draft draft-ietf-aqm-fq-
              codel-06, March 2016.

              Pan, R., Natarajan, P., Baker, F. and G. White, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", Internet-Draft draft-ietf-aqm-pie-10, September

              Elkins, N., Hamilton, R. and m.  mackermann@bcbsm.com,
              "IPv6 Performance and Diagnostic Metrics (PDM) Destination
              Option", Internet-Draft draft-ietf-ippm-6man-pdm-
              option-13, June 2017.

              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", Internet-Draft draft-ietf-
              ippm-ioam-data-02, March 2018.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Internet-Draft draft-ietf-quic-
              transport-03, May 2017.

              Briscoe, B., Kuehlewind, M. and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-
              tcpm-accurate-ecn-06, March 2018.

              Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", Internet-Draft draft-ietf-tsvwg-l4s-
              arch-01, October 2017.

              Moriarty, K. and A. Morton, "Effects of Pervasive
              Encryption on Operators", Internet-Draft draft-mm-wg-
              effect-encrypt-24, March 2018.


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              Thomson, M., "More Apparent Randomization for QUIC",
              Internet-Draft draft-thomson-quic-grease-00, December

              Trammell, B., "Abstract Mechanisms for a Cooperative Path
              Layer under Endpoint Control", Internet-Draft draft-
              trammell-plus-abstract-mech-00, September 2016.

              Kuehlewind, M., Trammell, B. and J. Hildebrand,
              "Transport-Independent Path Layer State Management",
              Internet-Draft draft-trammell-plus-statefulness-02,
              December 2016.

   [Latency]  Briscoe, B., "Reducing Internet Latency: A Survey of
              Techniques and Their Merits", November 2014.

   [Measure]  Fairhurst, G., Kuehlewind, M. and D. Lopez, "Measurement-
              based Protocol Design", June 2017.

   [RFC1273]  Schwartz, M.F., "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, <http://www.rfc-

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, DOI 10.17487/RFC2914, September 2000, <https://www

   [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, <http://www.rfc-editor.org/

   [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, <http://www.rfc-

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,

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   [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-

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G. and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <http://www.rfc-editor.org/info/rfc3449>.

   [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, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, DOI
              10.17487/RFC4302, December 2005, <http://www.rfc-

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, DOI 10.17487/RFC4303, December 2005, <http://www

   [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, <http://www.rfc-editor.org/

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

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008, <http://www.rfc-editor.org/info/

   [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>.

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   [RFC5559]  Eardley, P., Ed., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, DOI 10.17487/RFC5559, June 2009,

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

   [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/

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

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <http://www.rfc-editor.org/info/rfc6679>.

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

   [RFC7567]  Baker, F.Ed.,  and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management", BCP
              197, RFC 7567, DOI 10.17487/RFC7567, July 2015, <http://

   [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, <http://www.rfc-editor.org/

   [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>.

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   [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, <http://www.rfc-editor.org/info/rfc7928>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers", BCP
              208, RFC 8084, DOI 10.17487/RFC8084, March 2017, <http://

   [RFC8085]  Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://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, <http://www.rfc-editor.org/info/rfc8086>.

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

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [Tor]      The Tor Project,  ., "https://www.torproject.org", June

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

   -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.

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

Authors' Addresses

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

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

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

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

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