The Impact of Transport Header Confidentiality on Network Operation and Evolution of the Internet
draft-fairhurst-tsvwg-transport-encrypt-06

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TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                              C.S. Perkins
Expires: August 11, 2018                           University of Glasgow
                                                        February 9, 2018

The Impact of Transport Header Confidentiality on Network Operation and
                       Evolution of the Internet
               draft-fairhurst-tsvwg-transport-encrypt-06

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 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.

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   This Internet-Draft will expire on August 11, 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 . . . . . .  8
       2.1.1.  Flow Identification  . . . . . . . . . . . . . . . . .  9
       2.1.2.  Metrics derived from Transport Layer Headers . . . . .  9
       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 . . . . . . . . . . . . . . . 17
     2.4.  Observing Headers to Implement Network Policy  . . . . . . 18
   3.  Encryption and Authentication of Transport Headers . . . . . . 18
     3.1.  Authenticating the Transport Protocol Header . . . . . . . 20
     3.2.  Encrypting the Transport Payload . . . . . . . . . . . . . 20
     3.3.  Encrypting the Transport Header  . . . . . . . . . . . . . 20
     3.4.  Authenticating Transport Information and Selectively
           Encrypting the Transport Header  . . . . . . . . . . . . . 21
     3.5.  Optional Encryption of Header Information  . . . . . . . . 21
   4.  Addition of Transport Information to Network-Layer Protocol
       Headers  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   5.  Implications of Protecting the Transport Headers . . . . . . . 22
     5.1.  Independent Measurement  . . . . . . . . . . . . . . . . . 22
     5.2.  Characterising "Unknown" Network Traffic . . . . . . . . . 23
     5.3.  Accountability and Internet Transport Protocols  . . . . . 23
     5.4.  Impact on Research, Development and Deployment . . . . . . 24
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 25
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 28
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 28
   Appendix A. Revision information . . . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33

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
   evolution.

   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 - inspecting transport layer headers to help
   understand traffic dynamics.

   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, but
   also expected to forma a basis of future protocol designs.

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   Introducing cryptographic integrity checks to header fields can also
   prevent undetected manipulation of the field by network devices.
   However, it does not prevent inspection of the information by 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.  This leads to ossification of the header: An
   endpoint could be required to supply the 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 in
   other cases, any change to the protocol use of a specific header can
   have undesirable implications (e.g., a mechanism implemented in a
   network device, such as a firewall, that requires a header field to
   have only a specific known set of values 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 that intentionally varies the format and value of
   header fields (sometimes known as Greasing) has been suggested as a
   way to help avoid such ossification of the transport protocol.

   Implementations of network devices are encouraged to avoid side-
   effects when protocols are updated.  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.

   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 (e.g., [I-D.dolson-plus-middlebox-
   benefits]).

   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.  Protocol designers have often ignored these implications and
   this document suggests that exposure of information should be
   carefully considered when specifying new protocols.

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

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   protocol design that encrypts (hides) all the transport information
   can help reduce ossification of the transport layer, it could result
   in ossification of the network service.  There can be advantages in
   providing a level of ossification of the header in terms of providing
   a set of specified header fields that are observable from in-network
   devices.

   There can also be implications when working with encrypted transport
   protocols that 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].  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, it 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.

      This data information can inform Internet engineering research,
      and help the development of new protocols, methodologies, and
      procedures.  Hiding the entire transport protocol, including
      header information, will restrict the availability of data, and
      might lead to the development of alternative, and potentially more
      intrusive, methods to acquire the needed 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 can
      provide useful input to classification of traffic and detection of
      anomalous events, such as 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 is expected to lead to
      less precise pattern matching techniques or resort to
      indiscriminately (rate) limiting 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 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 on the
      flows that share a network segment.  Encrypted traffic therefore
      implies "don't touch", and a likely trouble-shooting response will
      be "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 [I-D.mm-wg-effect-
      encrypt].

   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.
      The trends in 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.  While the impact may, in many cases, be
      small there are scenarios where operators directly support
      particular services (e.g., in radio links, or 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 impact.

   Open and Verifiable Network Data:  The Hiding transport protocol
      header information can reduces the range of actors that can
      capture useful measurement data.  This is, of course, its goal.
      Doing so, however, limits the information sources available to the
      Internet community to understand the operation of transport
      protocols, so preventing access to the information necessary to
      inform design decisions and standards for new protocols and
      related operational practices.

      There are dangers in a model where only endpoints (i.e., at user
      devices and within service platforms) can observe performance, and
      this cannot be independently verified.

      To ensure the health of the standards and research communities, we
      need independently captured data to develop new transport protocol
      mechanisms based on the behaviour experienced in deployed
      networks.

      Independently verifiable performance metrics might also important
      in order to demonstrate regulatory compliance in some
      jurisdictions.

   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:

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   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 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 network.  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 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 network relies on
      accepting common specifications, and on the ability to verify that
      others also conform.

   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

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      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.  Hiding transport header information (e.g., by pervasive
      encryption of transport information) could eliminate the
      independent self-checks 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
      deployment).

   In summary, a lack of visibility of transport header information can
   impact the ways that protocols are designed, standardised, deployed,
   and operated.  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.  A network
   that is secure but unusable due to persistent congestion collapse is
   not an improvement, and while that would be an extreme outcome
   proposals that impose high costs for very limited benefits need to be
   considered carefully, to ensure the benefits outweigh the costs.

2.  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 network protocols are designed and 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 methods 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;

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   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 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 (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 ) 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 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 have a need to characterise 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

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      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 cause of loss, including: corruption on a link
      (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.

      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 - since losses
      can often occur as bursts, rather than randomly-timed events.

   Throughput and Goodput: The throughput observed 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), which 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
      [RFC3550]).

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   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, and determining latency metrics is a key
      driver in the deployment of AQM [RFC7567], DiffServ [RFC2474], and
      Explicit Congestion Notification (ECN) [RFC3168] [RFC8087].

      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 may 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.

      An example usage of this method could identify excessive buffers
      to help deploy or configure AQM [RFC7567] [RFC7928] to effectively
      eliminate unnecessary queuing in routers and other devices.  AQM
      methods need to be deployed at the capacity bottleneck, but are
      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.  The
      service offered by operators can therefore benefit from latency
      information to understand the impact of deployment and tune
      deployed services.

   Jitter: Some network applications are sensitive to changes in packet
      timing.  For such applications, it can be necessary to measure the
      jitter observed along a portion of the path.  The requirements to
      measure jitter 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

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      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.

      As in the drive to reduce network latency, there is a need for
      operational tools to detect mis-ordered packet flows and quantify
      the degree or reordering.  Techniques for measuring reordering
      typically observe packet sequence numbers.  Metrics have been
      defined that evaluate whether a network has maintained packet
      order on a packet-by-packet basis [RFC4737] and [RFC5236].

      There have been initiatives in the IETF transport area to reduce
      the impact of reordering within a transport flow, possibly leading
      to reduce the requirements for 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 level of reordering within
      deployed infrastructure, and inform decisions about how to
      progress such mechanisms.

   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. Key
   performance indicators are retransmission rate, packet drop rate,
   sector utilisation level, a measure of reordering, peak rate, the CE-
   marking rate, etc.  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.

   When information in transport headers is concealed, measurements need
   to rely on pattern inferences and other heuristics grows, and
   accuracy suffers [I-D.mm-wg-effect-encrypt].

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
   used to make flow observations.

   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 any performance data.

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   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 any 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
      treatment.

   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
      [RFC8087].

      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 carried in the IP protocol header, it is much easier to
      measure ECN than metering 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
      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].

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      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
   VPNs and IPsec.  When encryption conceals more layers in a packet,
   people seeking understanding of the network operation need to rely
   more on pattern inferences and other heuristics.  The accuracy of
   measurements therefore suffers, as does the ability to investigate
   and troubleshoot interactions between different anomalies.  For
   example, the traffic patterns between a web server and a browser are
   dependent on browser supplier and version, even use of the
   application (e.g., web e-mail access). Even when measurement datasets
   are made available (e.g., from endpoints) additional metadata, such
   as the state of the network, is often required to interpret the 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

   Often measurements can only be understood in the context of the other

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   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 this 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
   headers.

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.  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 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 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,
      etc.

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   Congestion Control Compliance for UDP traffic UDP provides a minimal
      message-passing 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 UDP 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.

   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 be
   expected to reduce the ability for networks to "help" (e.g., in
   response to tracing issues, making appropriate Quality of Service,
   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 that tools need 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.

2.3.1.  Examples of measurements

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   Future versions of this document may provide more about existing
   tooling at Network Operations Centres that rely upon observing
   transport layer header information.

   Debugging and diagnosing the root causes of faults concern particular
   users traffic is an activity that may depend on connection
   information from the protocol - In some case, this may involve active
   injection of test traffic to complete a measurement.  Most operators
   do not have access to user equipment.  There may also be costs
   associated with 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) may
   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.

   in other cases, measurement involves dissecting 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 - 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.

   XXX Note: The authors are looking for contributions that say more
   about the things people are currently doing with exposed transport
   fields and what problems they are trying to solve, or how they use
   the information they derive.  How problematic is new tools to follow-
   up.  Examples could include: Health monitoring; anomoly/DoS
   detection; Capacity planning, etc XXX

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 QoS management for 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

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   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.

   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 common 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].

   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.  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.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 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.

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   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.

   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

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   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)
   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.

   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.  On the one hand,
   security work typically employs a design technique that seeks to
   expose only what is needed.  On the other hand, there may be
   performance and operational benefits in exposing selected information
   to network tools.

   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
   protection.

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

   The transport 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.

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   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
   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 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

   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
   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).

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   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.

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

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   classify and mechanisms to condition network traffic.

   A lack of data seems likely to reduce the level of precision with
   which these 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

   There are both opportunities and also challenges to the design,
   evaluation and deployment of new transport protocol mechanisms.

   Integrity checks can avoiding network devices undetected modification
   of protocols, whereas encryption and obfuscation can prevent these
   headers being utilised by network devices.  This provides greater
   freedom to update the protocols and can therefore ease
   experimentation with new techniques and their final deployment in
   endpoints.

   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.

   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., 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.

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   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

   XXX Notes for a draft conclusion XXX

   The majority of traffic sent by the present Internet uses 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 data.
   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.

   Encryption (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 encryption 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.

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   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:

   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 for the
      class of traffic (priority, retransmission, reordering, circuit
      breakers, etc). Troubleshooting, and measurement becomes more
      difficult or could require additional information.  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 encryption.

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   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.  ...

   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

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.

10.  References

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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/
              rfc2119>.

10.2.  Informative References

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

   [I-D.ietf-aqm-codel]
              Nichols, K., Jacobson, V., McGregor, A. and J. Jana,
              "Controlled Delay Active Queue Management", Internet-Draft
              draft-ietf-aqm-codel-00, October 2014.

   [I-D.ietf-aqm-fq-codel]
              Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
              J. and E. Dumazet, "FlowQueue-Codel", Internet-Draft
              draft-ietf-aqm-fq-codel-00, January 2015.

   [I-D.ietf-aqm-pie]
              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-00, October
              2014.

   [I-D.ietf-ippm-6man-pdm-option]
              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-10, May 2017.

   [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. and d.  daniel.bernier@bell.ca, "Data Fields
              for In-situ OAM", Internet-Draft draft-ietf-ippm-ioam-
              data-01, October 2017.

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

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M. and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-
              tcpm-accurate-ecn-00, December 2015.

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   [I-D.ietf-tsvwg-l4s-arch]
              Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", Internet-Draft draft-ietf-tsvwg-l4s-
              arch-00, May 2017.

   [I-D.mm-wg-effect-encrypt]
              Moriarty, K. and A. Morton, "Effect of Pervasive
              Encryption on Operators", Internet-Draft draft-mm-wg-
              effect-encrypt-11, April 2017.

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

   [I-D.trammell-plus-statefulness]
              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-
              editor.org/info/rfc2474>.

   [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/
              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, <http://www.rfc-
              editor.org/info/rfc3168>.

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

   [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>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004, <http://www
              .rfc-editor.org/info/rfc3819>.

   [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-
              editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, DOI 10.17487/RFC4303, December 2005, <http://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, <http://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, <http://www.rfc-
              editor.org/info/rfc4737>.

   [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-
              editor.org/info/rfc5236>.

   [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/
              rfc5246>.

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   [RFC5559]  Eardley, P., Ed., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, DOI 10.17487/RFC5559, June 2009,
              <http://www.rfc-editor.org/info/rfc5559>.

   [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/
              info/rfc6269>.

   [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/
              rfc6437>.

   [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-
              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, <http://
              www.rfc-editor.org/info/rfc7567>.

   [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/
              info/rfc7624>.

   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015, <http://www.rfc-
              editor.org/info/rfc7713>.

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   [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, <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://
              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, <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/
              info/rfc8087>.

   [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
              2017.

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.

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   -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.

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   Department of Engineering
   Fraser Noble Building
   Aberdeen, AB24 3UE
   Scotland
   
   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk/

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow, G12 8QQ
   Scotland
   
   Email: csp@csperkins.org
   URI:   https://csperkins.org//

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