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Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9065.
Authors Gorry Fairhurst , Colin Perkins
Last updated 2020-12-04 (Latest revision 2020-11-02)
Replaces draft-fairhurst-tsvwg-transport-encrypt
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd David L. Black
Shepherd write-up Show Last changed 2020-09-15
IESG IESG state Became RFC 9065 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Martin Duke
Send notices to David Black <>
TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                C. Perkins
Expires: May 6, 2021                               University of Glasgow
                                                        November 2, 2020

    Considerations around Transport Header Confidentiality, Network
     Operations, and the Evolution of Internet Transport Protocols


   To protect user data and privacy, Internet transport protocols have
   supported payload encryption and authentication for some time.  Such
   encryption and authentication is now also starting to be applied to
   the transport protocol headers.  This helps avoid transport protocol
   ossification by middleboxes, mitigate attacks against the transport
   protocol, and protect metadata about the communication.  Current
   operational practice in some networks inspect transport header
   information within the network, but this is no longer possible when
   those transport headers are encrypted.

   This document discusses the possible impact when network traffic uses
   a protocol with an encrypted transport header.  It suggests issues to
   consider when designing new transport protocols or features.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2021.

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Current uses of Transport Headers within the Network  . . . .   4
     2.1.  To Identify Transport Protocols and Flows . . . . . . . .   5
     2.2.  To Understand Transport Protocol Performance  . . . . . .   6
     2.3.  To Support Network Operations . . . . . . . . . . . . . .  13
     2.4.  To Support Network Diagnostics and Troubleshooting  . . .  16
     2.5.  To Support Header Compression . . . . . . . . . . . . . .  17
     2.6.  To Verify SLA Compliance  . . . . . . . . . . . . . . . .  18
   3.  Other Uses of Observable Transport Headers  . . . . . . . . .  18
     3.1.  Characterising "Unknown" Network Traffic  . . . . . . . .  19
     3.2.  Accountability and Internet Transport Protocols . . . . .  19
     3.3.  Impact on Tooling and Network Operations  . . . . . . . .  20
     3.4.  Independent Measurement . . . . . . . . . . . . . . . . .  20
     3.5.  Impact on Research, Development and Deployment  . . . . .  22
   4.  Encryption and Authentication of Transport Headers  . . . . .  23
     4.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  24
     4.2.  Approaches to Transport Header Protection . . . . . . . .  26
   5.  Intentionally Exposing Transport Information to the Network .  28
     5.1.  Exposing Transport Information in Extension Headers . . .  28
     5.2.  Common Exposed Transport Information  . . . . . . . . . .  29
     5.3.  Considerations for Exposing Transport Information . . . .  29
   6.  Addition of Transport OAM Information to Network-Layer
       Headers . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     6.1.  Use of OAM within a Maintenance Domain  . . . . . . . . .  30
     6.2.  Use of OAM across Multiple Maintenance Domains  . . . . .  30
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  31
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  36
   11. Informative References  . . . . . . . . . . . . . . . . . . .  36
   Appendix A.  Revision information . . . . . . . . . . . . . . . .  45

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Introduction

   The transport layer supports the end-to-end flow of data across a
   network path, providing features such as connection establishment,
   reliability, framing, ordering, congestion control, flow control,
   etc., as needed to support applications.  One of the core functions
   of an Internet transport: to discover and adapt to the
   characteristics of the network path that is currently being used.

   For some years, it has been common for the transport layer payload to
   be protected by encryption and authentication, but for the transport
   layer headers to be sent unprotected.  Examples of protocols that
   behave in this manner include Transport Layer Security (TLS) over TCP
   [RFC8446], Datagram TLS [RFC6347] [I-D.ietf-tls-dtls13], the Secure
   Real-time Transport Protocol [RFC3711], and tcpcrypt [RFC8548].  The
   use of unencrypted transport headers has led some network operators,
   researchers, and others to develop tools and processes that rely on
   observations of transport headers both in aggregate and at the flow
   level to infer details of the network's behaviour and inform
   operational practice.

   Transport protocols are now being developed that encrypt some or all
   of the transport headers, in addition to the transport payload data.
   The QUIC transport protocol [I-D.ietf-quic-transport] is an example
   of such a protocol.  Such transport header encryption makes it
   difficult to observe transport protocol behaviour within the network.
   This document discusses some implications of transport header
   encryption for network operators, researchers, and others that have
   previously observed transport headers, and highlights some issues to
   consider for transport protocol designers.

   As discussed in [RFC7258], the IETF has concluded that Pervasive
   Monitoring (PM) is a technical attack that needs to be mitigated in
   the design of IETF protocols.  This document supports that
   conclusion.  It also recognises that RFC7258 states "Making networks
   unmanageable to mitigate PM is not an acceptable outcome, but
   ignoring PM would go against the consensus documented here.  An
   appropriate balance will emerge over time as real instances of this
   tension are considered".  This document is written to provide input
   to the discussion around what is an appropriate balance, by
   highlighting some implications of transport header encryption.

   This document explains current uses of transport header information
   in the network, which can be beneficial or malicious.  It is written
   to provide input to the discussion around what is an appropriate

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   balance, by highlighting some implications of transport header

2.  Current uses of Transport Headers within the Network

   In response to pervasive monitoring [RFC7624] revelations and the
   IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
   efforts are underway to increase encryption of Internet traffic.
   Applying confidentiality to transport header fields can improve
   privacy, and can help to mitigate certain attacks, but can also
   affect network operations [RFC8404].

   When considering what parts of the transport headers should be
   encrypted to provide confidentiality, and what parts should be
   visible to the network (including non-encrypted but authenticated
   headers), it is necessary to consider both the impact on network
   operations and management, and the implications for ossification and
   user privacy [Measurement].  Different parties will view the relative
   importance of these concerns differently.  For some, the benefits of
   encrypting all the transport headers outweigh the impact of doing so;
   others might analyse the security, privacy, and ossification impacts
   and arrive at a different trade-off.

   This section reviews examples of the observation of transport layer
   headers within the network.  Unencrypted transport headers provide
   information can support network operations and management, and this
   section notes some ways in which this has been done.  Unencrypted
   transport header information also contributes metadata that can be
   exploited for purposes unrelated to network transport measurement,
   diagnostics or troubleshooting (e.g., to block or to throttle traffic
   from a specific content provider), and this section also notes some
   threats relating to unencrypted transport headers.

   Exposed transport information also provides a source of information
   that contributes to linked data sets, which could be exploited to
   deduce private information, e.g., user patterns, user location,
   tracking behaviour, etc.  This might reveal information the parties
   did not intend to be revealed.  [RFC6973] aims to make designers,
   implementers, and users of Internet protocols aware of privacy-
   related design choices in IETF protocols.

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

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2.1.  To Identify Transport Protocols and Flows

   Information in exposed transport layer headers can be used by the
   network to identify transport protocols and flows [RFC8558].  The
   ability to identify transport protocols, flows, and sessions is a
   common function performed, for example, by measurement activities,
   QoS classifiers, and firewalls.  These functions can be beneficial,
   and performed with the consent of, and in support of, the end user.
   Alternatively, a network operator could use the same mechanisms to
   support practises that are adversarial to the end user, including
   blocking, de-prioritising, and monitoring traffic without consent.

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

   In some uses, an assigned transport port (e.g., 0..49151) can
   identify the upper-layer protocol or service [RFC7605].  However,
   port information alone is not sufficient to guarantee identification.
   Applications can use arbitrary ports and do not need to use assigned
   port numbers.  The use of an assigned port number is also not limited
   to the protocol for which the port is intended.  Multiple sessions
   can also be multiplexed on a single port, and ports can be re-used by
   subsequent sessions.

   Some flows can be identified by observing signalling protocol data
   (e.g., [RFC3261], [I-D.ietf-rtcweb-overview]) or through the use of
   magic numbers placed in the first byte(s) of the datagram payload

   When transport header information cannot be observed, this removes
   information that could have been used to classify flows by passive
   observers along the path.  More ambitious ways could be used to
   collect, estimate, or infer flow information, including heuristics
   based on the analysis of traffic patterns.  For example, an operator
   that cannot access the Session Description Protocol (SDP) session
   descriptions [RFC4566] to classify a flow as audio traffic, might
   instead use (possibly less-reliable) heuristics to infer that short
   UDP packets with regular spacing carry audio traffic.  Operational
   practises aimed at inferring transport parameters are out of scope
   for this document, and are only mentioned here to recognise that
   encryption does not prevent operators from attempting to apply
   practises that were used with unencrypted transport headers.

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   The IAB [RFC8546] have provided a summary of expected implications of
   increased encryption on network functions that use the observable
   headers and describe the expected benefits of designs that explicitly
   declare protocol invariant header information that can be used for
   this purpose.

2.2.  To Understand Transport Protocol Performance

   Information in exposed transport layer headers can be used by the
   network to understand transport protocol performance and behaviour.

2.2.1.  Using Information Derived from Transport Layer Headers

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

   Traffic Rate and Volume:  Volume measures per-application can be used
      to characterise the traffic that uses a network segment or the
      pattern of network usage.  Observing the protocol sequence number
      and packet size offers one way to measure this (e.g., measurements
      observing counters in periodic reports such as RTCP; or
      measurements observing protocol sequence numbers in statistical
      samples of packet flows, or specific control packets, such as
      those observed at the start and end of a flow).

      Measurements can be per endpoint, or for an endpoint aggregate.
      This can be used, for example, to assess subscriber usage or for
      billing purposes.

      Measurements can also be used to trigger traffic shaping, and to
      associate QoS support within the network and lower layers.  This
      can be done with consent and in support of an end user, to improve
      quality of service; or can be used by the network to de-prioritise
      certain flows without user consent.

      Volume measures can also be valuable for capacity planning and
      providing detail of trends in usage.

      The traffic rate and volume can be determined providing that the
      packets belonging to individual flows can be identified, but there
      might be no additional information about a flow when the transport
      headers cannot be observed.

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

      There are various causes of loss, including: corruption of link
      frames (e.g., due to interference on a radio link), buffering loss
      (e.g., overflow due to congestion, Active Queue Management (AQM)
      [RFC7567], or inadequate provision following traffic pre-emption),
      and policing (traffic management) [RFC2475].  Understanding flow
      loss rates requires either observing sequence numbers in network
      or transport headers, or maintaining per-flow packet counters
      (flow identification often requires transport layer information).
      Per-hop loss can also sometimes be monitored at the interface
      level by devices in the network.

      Losses can often occur as bursts, randomly-timed events, etc.  The
      pattern of loss can provide insight into the cause of loss.  It
      can also be valuable to understand the conditions under which loss
      occurs, which usually requires relating loss to the traffic
      flowing at a network node or segment at the time of loss.  This
      can also help identify cases where loss could have been wrongly
      identified, or where the transport did not require transmission of
      a lost packet.

   Throughput and Goodput:  Throughput is the amount of payload data
      sent by a flow per time interval.  Goodput (see Section 2.5 of
      [RFC7928]) is a measure of useful data exchanged (the ratio of
      useful data to total volume of traffic sent by a flow).  The
      throughput of a flow can be determined in the absence of transport
      header information, providing that the individual flow can be
      identified, and the overhead known.  Goodput requires ability to
      differentiate loss and retransmission of packets, for example by
      observing packet sequence numbers in the TCP or RTP headers

   Latency:  Latency is a key performance metric that impacts
      application and user-perceived response times.  It often
      indirectly impacts throughput and flow completion time.  This
      determines the reaction time of the transport protocol itself,
      impacting flow setup, congestion control, loss recovery, and other
      transport mechanisms.  The observed latency can have many
      components [Latency].  Of these, unnecessary/unwanted queueing in

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      network buffers has often been observed as a significant factor
      [bufferbloat].  Once the cause of unwanted latency has been
      identified, this can often be eliminated.

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

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

      Latency and round-trip time information can potentially expose
      some information useful for approximate geolocation, as discussed
      in [PAM-RTT].  Encrypting transport headers can reduce the latency
      information that is available.

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

   Flow Reordering:  Significant packet reordering within a flow can
      impact time-critical applications and can be interpreted as loss
      by reliable transports.  Many transport protocol techniques are
      impacted by reordering (e.g., triggering TCP retransmission or re-
      buffering of real-time applications).  Packet reordering can occur
      for many reasons, from equipment design to misconfiguration of

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      forwarding rules.  Network tools can detect and measure unwanted/
      excessive reordering, and the impact on transport performance.

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

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

      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 assist in
      understanding the context under which the data was collected,
      including the time, observation point [RFC7799], and way in which
      metrics were accumulated.  The RTCP protocol directly reports some
      of this information in a form that can be directly visible in the
      network.  A user of summary measurement data has to trust the
      source of this data and the method used to generate the summary

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

   In some cases, measurements could involve active injection of test
   traffic to perform a measurement (see Section 3.4 of [RFC7799]).
   However, most operators do not have access to user equipment,
   therefore the point of test is normally different from the transport
   endpoint.  Injection of test traffic can incur an additional cost in
   running such tests (e.g., the implications of capacity tests in a
   mobile network are obvious).  Some active measurements [RFC7799]
   (e.g., response under load or particular workloads) perturb other
   traffic, and could require dedicated access to the network segment.

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   Passive measurements (see Section 3.6 of [RFC7799]) can have
   advantages in terms of eliminating unproductive test traffic,
   reducing the influence of test traffic on the overall traffic mix,
   and the ability to choose the point of observation (see
   Section 2.3.1).  Measurements can rely on observing packet headers,
   which is not possible if those headers are encrypted, but could
   utilise information about traffic volumes or patterns of interaction
   to deduce metrics.

   One alternative approach is to use in-network techniques, which does
   not require the cooperation of an endpoint (see Section 6).

2.2.2.  Using Information Derived from Network Layer Header Fields

   Information from the transport header 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, or by firewalls to implement access rules (see
   also Section 2.2.2 of [RFC8404]).  Operators can use such policies to
   support user applications and to protect against unwanted traffic.
   Such policies can also be used without user consent, to de-prioritise
   certain applications or services, for example.

   Network-layer classification methods that rely on a multi-field
   classifier (e.g., inferring QoS from the 5-tuple or choice of
   application protocol) are incompatible with transport protocols that
   encrypt the transport header information.  Traffic that cannot be
   classified typically receives a default treatment.  Some networks
   block or rate traffic that cannot be classified.

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

   The user of a transport that multiplexes multiple sub-flows might
   want to obscure the presence and characteristics of these sub-flows.
   On the other hand, an encrypted transport could set the network-layer
   information to indicate the presence of sub-flows, and to reflect the
   service requirements of individual sub-flows.  There are several ways
   this could be done:

   IP Address:  Applications normally expose the addresses used by
      endpoints, and this is used in the forwarding decisions in network
      devices.  Address and other protocol information can be used by a

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      Multi-Field (MF) classifier to determine how traffic is treated
      [RFC2475], and hence affect the quality of experience for a flow.

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

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

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

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

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

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      field when a network path supports differentiated service

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

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

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

   Network-Layer Options  Network protocols can carry optional headers.
      These can be used to explicitly expose transport header
      information to on-path devices operating at the network layer (as
      discussed further in Section 6).

      IPv4 [RFC0791] has provision for optional header fields identified
      by an option type field.  IP routers can examine these headers and
      are required to ignore IPv4 options that they does not recognise.
      Many current paths include network devices that forward packets
      that carry options on a slower processing path.  Some network
      devices (e.g., firewalls) can be (and are) configured to drop
      these packets [RFC7126].  BCP 186 [RFC7126] provides Best Current
      Practice guidance on how operators should treat IPv4 packets that
      specify options.

      IPv6 can encode optional network-layer information in separate
      headers that may be placed between the IPv6 header and the upper-
      layer header [RFC8200].  The Hop-by-Hop options header, when
      present, immediately follows the IPv6 header.  IPv6 permits this
      header to be examined by any node along the path.  While [RFC7872]
      required all nodes to examine and process the Hop-by-Hop options
      header, it is now expected [RFC8200] that nodes along a path only

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      examine and process the Hop-by-Hop options header if explicitly
      configured to do so.

   Careful use of the network layer features can help provide similar
   information in the case where the network is unable to inspect
   transport protocol headers.  Section 5 describes use of network
   extension headers.

2.3.  To Support Network Operations

   Some network operators make use of on-path observations of transport
   headers to monitor the performance of their networks, and to support
   network operations.  Transport protocols with observable headers
   allow such operators to explicitly measurement and analyse transport
   protocol performance, and in some cases this can help detect and
   locate network problems.  [RFC8517] gives an operator's perspective
   about such use.

   When encryption hides the transport headers, making it difficult to
   directly observe transport behaviour and dynamics, those seeking an
   understanding of network operations might learn to work without that
   information.  Alternatively, they might use more limited measurements
   combined with pattern inference and other heuristics to infer network
   behaviour (see Section 2.1.1 of [RFC8404]).  Operational practises
   aimed at inferring transport parameters are out of scope for this
   document, and are only mentioned here to recognise that encryption
   does not necessarily stop operators from attempting to apply
   practises that have been used with unencrypted transport headers.

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

   Packet sampling techniques are used to scale the processing involved
   in observing packets on high rate links.  This exports only the
   packet header information of (randomly) selected packets.  The
   utility of these measurements depends on the type of bearer and
   number of mechanisms used by network devices.  Simple routers are
   relatively easy to manage, but 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.

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2.3.1.  Problem Location

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

   Sometimes multiple in network observation points have to be used.  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.

2.3.2.  Network Planning and Provisioning

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

   Trends in aggregate traffic can be observed and can be related to the
   endpoint addresses being used, but when transport header information
   is not observable, it might be impossible to correlate patterns in
   measurements with changes in transport protocols.  This increases the
   dependency on other indirect sources of information to inform
   planning and provisioning.

2.3.3.  Service Performance Measurement

   Performance measurements (e.g., throughput, loss, latency) can be
   used by various actors to analyse the service offered to the users of
   a network segment, and to inform operational practice.

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2.3.4.  Compliance with Congestion Control

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

   Operators can manage traffic load (e.g., when the network is severely
   overloaded) by deploying rate-limiters, traffic shaping, or network
   transport circuit breakers [RFC8084].  The information provided by
   observing transport headers is a source of data that can help to
   inform such mechanisms.

   Congestion Control Compliance of Traffic:  Congestion control is a
      key transport function [RFC2914].  Many network operators
      implicitly accept that TCP traffic complies with a behaviour that
      is acceptable for the shared Internet.  TCP algorithms have been
      continuously improved over decades, and have reached a level of
      efficiency and correctness that is difficult to match in custom
      application-layer mechanisms [RFC8085].

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

      However, when anomalies are detected, tools can interpret the
      transport protocol header information to help understand the
      impact of specific transport protocols (or protocol mechanisms) on
      the other traffic that shares a network.  An observation in the
      network can gain an understanding of the dynamics of a flow and
      its congestion control behaviour.  Analysing observed flows can
      help to build confidence that an application flow backs-off its
      share of the network load under 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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      The ability to identify sources and flows that contribute to
      persistent congestion is important to the safe operation of
      network infrastructure, and can inform configuration of network
      devices to complement the endpoint congestion avoidance mechanisms
      [RFC7567] [RFC8084] to avoid a portion of the network being driven
      into congestion collapse [RFC2914].

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

      A network operator can observe the headers of transport protocols
      layered above UDP to understand if the datagram flows comply with
      congestion control expectations.  This can help inform a decision
      on whether it might be appropriate to deploy methods such as rate-
      limiters to enforce acceptable usage.

      UDP flows that expose a well-known header can be observed to gain
      understanding of the dynamics of a flow and its congestion control
      behaviour.  For example, tools exist to monitor various aspects of
      RTP header information and RTCP reports for real-time flows (see
      Section 2.2).  The Secure RTP and RTCP extensions [RFC3711] were
      explicitly designed to expose some header information to enable
      such observation, while protecting the payload data.

2.4.  To Support Network Diagnostics and Troubleshooting

   Transport header information can be utilised for a variety of
   operational tasks [RFC8404]: to diagnose network problems, assess
   network provider performance, evaluate equipment or protocol
   performance, capacity planning, management of security threats
   (including DoS), and responding to user performance questions.
   Section 3.1.2 and Section 5 of [RFC8404] provide further examples.

   Operators can monitor the health of a portion of the Internet, to
   provide early warning and trigger action.  Traffic and performance
   measurements can assist in setting buffer sizes, debugging and
   diagnosing the root causes of faults that concern a particular user's
   traffic.  They can also be used to support post-mortem investigation
   after an anomaly to determine the root cause of a problem.  In other
   cases, measurement involves dissecting network traffic flows.
   Observed transport header information can help identify whether link/

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   network tuning is effective and alert to potential problems that can
   be hard to derive from link or device measurements alone.

   An alternative could rely on access to endpoint diagnostic tools or
   user involvement in diagnosing and troubleshooting unusual use cases
   or to troubleshoot non-trivial problems.

   Another approach is to use traffic pattern analysis.  Such tools can
   provide useful information during network anomalies (e.g., detecting
   significant reordering, high or intermittent loss), however indirect
   measurements would need to be carefully designed to provide
   information for diagnostics and troubleshooting.

   The design trade-offs for radio networks are often very different
   from those of wired networks.  A radio-based network (e.g., cellular
   mobile, enterprise Wireless LAN (WLAN), 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 and differences between traffic
   types, and their correlation with link propagation and interference
   can all have significant impact on the cost and performance of a
   provided service.  For radio links, the use 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.

   Lack of tools and resulting information can reduce the ability of an
   operator to observe transport performance and could limit the ability
   of network operators to trace problems, make appropriate QoS
   decisions, or respond to other queries about the network service.

   A network operator supporting traffic that uses transport header
   encryption is unable to use tools that rely on transport protocol
   information.  However, the use of encryption has the desirable effect
   of preventing unintended observation of the payload data and these
   tools seldom seek to observe the payload, or other application
   details.  A flow that hides its transport header information could
   imply "don't touch" to some operators.  This might limit a trouble-
   shooting response to "can't help, no trouble found".

2.5.  To Support Header Compression

   Header compression saves link capacity by compressing network and
   transport protocol headers on a per-hop basis.  It was widely used
   with low bandwidth dial-up access links, and still finds application
   on wireless links that are subject to capacity constraints.  Examples
   of header compression include use with TCP/IP and RTP/UDP/IP flows

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   [RFC2507], [RFC6846], [RFC2508], [RFC5795].  Successful compression
   depends on observing the transport headers and understanding of the
   way header fields change packet-by-packet, and is hence incompatible
   with header encryption.  Devices that compress transport headers are
   dependent on a stable header format, implying ossification of that

   Introducing a new transport protocol, or changing the format of the
   transport header information, will limit the effectiveness of header
   compression until the network devices are updated.  Encrypting the
   transport protocol headers will tend to cause the header compression
   to a fall back to compressing only the network layer headers, with a
   significant reduction in efficiency.  This can limit connectivity if
   the resulting flow exceeds the link capacity, or if the packets are
   dropped because they exceed the link MTU.

   The Secure RTP (SRTP) extensions [RFC3711] were explicitly designed
   to leave the transport protocol headers unencrypted, but
   authenticated, since support for header compression was considered

2.6.  To Verify SLA Compliance

   Observable transport headers coupled with published transport
   specifications allow operators and regulators to explore and verify
   compliance with Service Level Agreements (SLAs).

   When transport header information cannot be observed, other methods
   have to be found to confirm that the traffic produced conforms to the
   expectations of the operator or developer.

   Independently verifiable performance metrics can be utilised to
   demonstrate regulatory compliance in some jurisdictions, and as a
   basis for informing design decisions.  This can bring assurance to
   those operating networks, often avoiding deployment of complex
   techniques that routinely monitor and manage Internet traffic flows
   (e.g., avoiding the capital and operational costs of deploying flow
   rate-limiting and network circuit-breaker methods [RFC8084]).

3.  Other Uses of Observable Transport Headers

   The choice of which transport header fields to expose and which to
   encrypt is a design decision for the transport protocol.  Selective
   encryption requires trading conflicting goals of observability and
   network support, privacy, and risk of ossification, to decide what
   header fields to protect and which to make visible.

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   Security work typically employs a design technique that seeks to
   expose only what is needed [RFC3552].  This approach provides
   incentives to not reveal any information that is not necessary for
   the end-to-end communication.  The IAB has provided guidelines for
   writing Security Considerations for IETF specifications [RFC3552].

   Endpoint design choices impacting privacy also need to be considered
   as a part of the design process [RFC6973].  The IAB has provided
   guidance for analyzing and documenting privacy considerations within
   IETF specifications [RFC6973].

   There can also be performance and operational trade-offs in exposing
   selected information to network tools.  This section explores key
   implications of working with encrypted transport protocols, but does
   not endorse or condemn these practices.

3.1.  Characterising "Unknown" Network Traffic

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

   If "unknown" or "uncharacterised" traffic patterns form a small part
   of the traffic aggregate passing through a network device or segment
   of the network the path, the dynamics of the uncharacterised traffic
   might not have a significant collateral impact on the performance of
   other traffic that shares this network segment.  Once the proportion
   of this traffic increases, monitoring the traffic can determine if
   appropriate safety measures have 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 has to be considered in determining
   how this activity is performed.  On a shorter timescale, information
   could also have to be collected to manage DoS attacks against the

3.2.  Accountability and Internet Transport Protocols

   Information provided by tools observing transport headers can be used
   to classify traffic, and to limit the network capacity used by
   certain flows, as discussed in Section 2.3.4).  Equally, operators
   could use analysis of transport headers and transport flow state to
   demonstrate that they are not providing differential treatment to
   certain flows.  Obfuscating or hiding this information using
   encryption could lead operators and maintainers of middleboxes

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   (firewalls, etc.) to seek other methods to classify, and potentially
   other mechanisms to condition network traffic.

   A lack of data that reduces the level of precision with which flows
   can be classified also reduces the design space for conditioning
   mechanisms (e.g., rate limiting, circuit breaker techniques
   [RFC8084], or blocking of uncharacterised traffic) [RFC5218].

3.3.  Impact on Tooling and Network Operations

   A variety and open source and proprietary tools have been deployed to
   can make use of the transport header information that's observable in
   widely used protocols such as TCP or RTP/UDP/IP.

   Changes to the transport, whether to protect the transport headers,
   introduce a new transport protocol, protocol feature, or application
   might require changes to such tools, and so could impact operational
   practice and policies.  Such changes have associated costs that are
   incurred by the network operators that need to update their tooling
   or develop alternative practises that work without access to the
   changed/removed information.

   If new protocols, or protocol extensions, are made to closely
   resemble or match existing mechanisms, then these changes and the
   associated costs can be small.  Equally, more extensive changes to
   the transport tend to require more extensive, and more expensive,
   changes to tooling and operational practice.

   Protocol designers can mitigate these costs by explicitly choosing to
   expose selected information as invariants that are guaranteed not to
   change for a particular protocol (e.g., the header invariants and the
   spin-bit in QUIC [I-D.ietf-quic-transport]).  Specification of common
   log formats and development of alternative approaches can also help
   mitigate the costs of transport changes.

3.4.  Independent Measurement

   Independent observation by multiple actors is currently used by the
   transport community to maintain an accurate understanding of the
   network.  Encrypting transport header encryption changes the ability
   to collect and independently analyse data.

   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 assess congestion and
   requests for retransmission) provide an incentive for the sending
   endpoint to provide correct information, since the protocol will not
   work otherwise.  This increases confidence that the observer

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   understands the transport interaction with the network.  For example,
   when TCP is used over an unencrypted network path (i.e., one that
   does not use IPsec or other encryption below the transport), it
   implicitly exposes transport header information that can be used for
   measurement at any point along the path.  This information is
   necessary for the protocol's correct operation, therefore there is no
   incentive for a TCP or RTP implementation to put incorrect
   information in this transport header.  A network device can have
   confidence that the well-known (and ossified) transport header
   information represents the actual state of the endpoints.

   When encryption is used to hide some or all of the transport headers,
   the transport protocol chooses which information to reveal to the
   network about its internal state, what information to leave
   encrypted, and what fields to grease to protect against future
   ossification [RFC8701].  Such a transport could provide summary data
   regarding its performance, congestion control state, etc., or to make
   available explicit measurement information.  For example, a QUIC
   endpoint can optionally set the spin bit to reflect to explicitly
   reveal the RTT of an encrypted transport session to the on-path
   network devices [I-D.ietf-quic-transport]).

   When providing or using such information, it is important to consider
   the privacy of the user and their incentive for providing accurate
   and detailed information.  Protocols that selectively reveal some
   transport state or measurable information are choosing to establish a
   trust relationship with the network operators.  There is no protocol
   mechanism that can guarantee that the information provided represents
   the actual transport state of the endpoints, since those endpoints
   can always send additional information in the encrypted part of the
   header, to update or replace whatever they reveal.  This reduces the
   ability to independently measure and verify that a protocol is
   behaving as expected.  For some operational uses, the information has
   to contain sufficient detail to understand, and possibly reconstruct,
   the network traffic pattern for further testing.  In this case,
   operators have to gain the trust of transport protocol implementers
   if the transport headers are to correctly reveal such information.

   OAM data records [I-D.ietf-ippm-ioam-data] could be embedded into a
   variety of encapsulation methods at different layers to support the
   goals of a specific operational domain.  OAM-related metadata can
   support functions such as performance evaluation, path-tracing, path
   verification information, classification and a diversity of other
   uses.  When encryption is used to hide some or all of the transport
   headers, analysis requires coordination between actors at different
   layers to successfully characterise flows and correlate the
   performance or behaviour of a specific mechanism with the
   configuration and traffic using operational equipment (e.g.,

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   combining transport and network measurements to explore congestion
   control dynamics, the implications of designs for active queue
   management or circuit breakers).

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

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

3.5.  Impact on Research, Development and Deployment

   Transport protocol evolution, and the ability to measure and
   understand the impact of protocol changes, have to proceed hand-in-
   hand.  A transport protocol that provides observable headers can be
   used to provide open and verifiable measurement data.  Observation of
   pathologies has a critical role in the design of transport protocol
   mechanisms and development of new mechanisms and protocols.  This
   helps understanding of the interactions between cooperating protocols
   and network mechanisms, the implications of sharing capacity with
   other traffic and the impact of different patterns of usage.  The
   ability of other stakeholders to review transport header traces helps
   develop insight into performance and traffic contribution of specific
   variants of a protocol.

   Development of new transport protocol mechanisms has to consider the
   scale of deployment and the range of environments in which the
   transport is used.  Experience has shown that it is often difficult
   to correctly implement new mechanisms [RFC8085], and that mechanisms
   often evolve as a protocol matures, or in response to changes in
   network conditions, changes in network traffic, or changes to
   application usage.  Analysis is especially valuable when based on the
   behaviour experienced across a range of topologies, vendor equipment,
   and traffic patterns.

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   New transport protocol formats are expected to facilitate an
   increased pace of transport evolution, and with it the possibility to
   experiment with and deploy a wide range of protocol mechanisms.
   There has been recent interest in a wide range of new transport
   methods, e.g., Larger Initial Window, Proportional Rate Reduction
   (PRR), congestion control methods based on measuring bottleneck
   bandwidth and round-trip propagation time, the introduction of AQM
   techniques and new forms of ECN response (e.g., Data Centre TCP,
   DCTP, and methods proposed for L4S).  The growth and diversity of
   applications and protocols using the Internet also continues to
   expand.  For each new method or application, it is desirable to build
   a body of data reflecting its behaviour under a wide range of
   deployment scenarios, traffic load, and interactions with other
   deployed/candidate methods.

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

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

4.  Encryption and Authentication of Transport Headers

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

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4.1.  Motivation

   There are several motivations for transport header encryption.

   One motive to encrypt transport headers is to prevent network
   ossification from network devices that inspect transport headers.
   Once a network device observes a transport header and becomes reliant
   upon using it, the overall use of that field can become ossified,
   preventing new protocols and mechanisms from being deployed.  One of
   the benefits of encrypting transport headers is that it can help
   improve the pace of transport development by eliminating interference
   by deployed middleboxes.  Examples of this include:

   o  During the development of TLS 1.3 [RFC8446], the design needed to
      be modified to function in the presence of deployed middleboxes
      that relied on the presence of certain header fields exposed in
      TLS 1.2 [RFC5426].

   o  The design of Multipath TCP (MPTCP) [RFC8684] also had to be
      revised to account for middleboxes (known as "TCP Normalizers")
      that monitor the evolution of the window advertised in the TCP
      header and then reset connections when the window did not grow as

   o  TCP Fast Open [RFC7413] can experience problems due to middleboxes
      that modify the transport header of packets by removing "unknown"
      TCP options, segments with unrecognised TCP options can be
      dropped, segments that contain data and set the SYN bit can be
      dropped, or middleboxes that disrupt connections that send data
      before completion of the three-way handshake.

   o  Other examples of ossification have included middleboxes that
      modify transport headers by rewriting TCP sequence and
      acknowledgement numbers, but are unaware of the (newer) TCP
      selective acknowledgement (SACK) option and therefore fail to
      correctly rewrite the SACK information to match the changes that
      were made to the fixed TCP header, preventing SACK from operating

   In all these cases, middleboxes with a hard-coded, but incomplete,
   understanding of transport behaviour, interacted poorly with
   transport protocols after the transport behaviour was changed.  In
   some case, the middleboxes modified or replaced information in the
   transport protocol header.

   Transport header encryption prevents an on-path device from observing
   the transport headers, and therefore stops mechanisms being built
   that directly rely on or infer semantics of the transport header

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   information.  Encryption is normally combined with authentication of
   the protected information.  RFC 8546 summarises this approach,
   stating that it is "The wire image, not the protocol's specification,
   determines how third parties on the network paths among protocol
   participants will interact with that protocol" (Section 1 of
   [RFC8546]), and it can be expected that header information that is
   not encrypted will become ossified.  Encryption can reduce
   ossification of the transport protocol, but does not itself prevent
   ossification of the network service.  People seeking to understand
   network traffic could still come to rely on pattern inferences and
   other heuristics or machine learning to derive measurement data and
   as the basis for network forwarding decisions [RFC8546].  This can
   also create dependencies on the transport protocol, or the patterns
   of traffic it can generate, also in time resulting in ossification of
   the service.

   Another motivation stems from increased concerns about privacy and
   surveillance.  Users value the ability to protect their identity and
   location, and defend against analysis of the traffic.  Revelations
   about the use of pervasive surveillance [RFC7624] have, to some
   extent, eroded trust in the service offered by network operators and
   have led to an increased use of encryption to avoid unwanted
   eavesdropping on communications.  Concerns have also been voiced
   about the addition of information to packets by third parties to
   provide analytics, customisation, advertising, cross-site tracking of
   users, to bill the customer, or to selectively allow or block
   content.  Whatever the reasons, the IETF is designing protocols that
   include transport header encryption (e.g., QUIC
   [I-D.ietf-quic-transport]) to supplement the already widespread
   payload encryption, and to further limit exposure of transport
   metadata to the network.

   The use of transport header authentication and encryption exposes a
   tussle between middlebox vendors, operators, applications developers
   and users:

   o  On the one hand, future Internet protocols that support transport
      header 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,
      and can improve privacy by reducing leakage of transport metadata.

   o  On the other hand, encryption of transport layer 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.

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   A decision to use transport header encryption can improve user
   privacy, and can reduce protocol ossification and help the evolution
   of the transport protocol stack, but is also has implications for
   network operations and management.

4.2.  Approaches to Transport Header Protection

   The designers of a transport protocol have to decide whether to
   encrypt all, or a part of, the transport layer information.
   Section 4 of [RFC8558] states: "Anything exposed to the path should
   be done with the intent that it be used by the network elements on
   the path".

   Protocol designers can decide not to encrypt certain transport header
   fields, making those fields observable in the network, or can define
   new fields designed to explicitly expose observable transport layer
   information to the network.  Where exposed fields are intended to be
   immutable (i.e., can be observed, but not modified by a network
   device), the endpoints are encouraged to use authentication to
   provide a cryptographic integrity check that can detect if these
   immutable fields have been modified by network devices.
   Authentication can also help to prevent attacks that rely on sending
   packets that fake exposed control signals in transport headers (e.g.,
   TCP RST spoofing).  Making a part of a transport header observable or
   exposing new header fields can lead to ossification of that part of a
   header as network devices come to rely on observations of the exposed

   The following briefly reviews some security design options for
   transport protocols.  A Survey of the Interaction between Security
   Protocols and Transport Services [RFC8922] provides more details
   concerning commonly used encryption methods at the transport layer.

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

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

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      authentication might interact with middleboxes, depending on their
      behaviour [RFC3234].

      The IPsec Authentication Header (AH) [RFC4302] was designed to
      work at the network layer and authenticate the IP payload.  This
      approach authenticates all transport headers, and verifies their
      integrity at the receiver, preventing in-network modification.
      The IPsec Encapsulating Security Payload (ESP) [RFC4303] can also
      provide authentication and integrity without confidentiality using
      the NULL encryption algorithm [RFC2410].  SRTP [RFC3711] is
      another example of a transport protocol that allows header

   Selectively Encrypting Transport Headers and Payload:  A transport
      protocol design can encrypt selected header fields, while also
      choosing to authenticate the entire transport header.  This allows
      specific transport header fields to be made observable by network
      devices (explicitly exposed either in a transport header field or
      lower layer protocol header).  A design that only exposes
      immutable fields can also perform end-to-end authentication of
      these fields across the path to prevent undetected modification of
      the immutable transport headers.

      Mutable fields in the transport header provide opportunities where
      network devices can modify the transport behaviour (e.g., the
      extended headers described in [I-D.trammell-plus-abstract-mech]).

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

   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 have to be made to use variable
      format fields.  Instead, fields of a specific type ought to always
      be sent with the same level of confidentiality or integrity

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

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      of the protocol could even become conditional on a specific header
      field value.

      A protocol can intentionally vary the value, format, and/or
      presence of observable transport header fields.  This behaviour,
      known as GREASE (Generate Random Extensions And Sustain
      Extensibility) is designed to avoid a network device ossifying the
      use of a specific observable field.  Greasing seeks to ease
      deployment of new methods.  This seeks to prevent in-network
      devices utilising the information in a transport header, or can
      make an observation robust to a set of changing values, rather
      than a specific set of values.  It is not a security mechanism,
      although use can be combined with an authentication mechanism.

   As seen, different transports use encryption to protect their header
   information to varying degrees.  The trend is towards increased

5.  Intentionally Exposing Transport Information to the Network

   A transport protocol can choose to expose certain transport
   information to on-path devices operating at the network layer by
   sending observable fields.  One approach is to make an explicit
   choice not to encrypt certain transport header fields, making this
   transport information observable by the network.  Another approach is
   to choose to expose transport information through the use of network-
   layer extension headers (see Section 6).  Both are examples of
   explicit information intended to be used by network devices on the
   path [RFC8558].

   Whatever the mechanism used to expose the information, a decision to
   only expose specific transport information, places the transport
   endpoint in control of what to expose or not to expose outside of the
   encrypted transport header.  This decision can then be made
   independently of the transport protocol functionality.  This can be
   done by exposing part of the transport header or as a network layer

5.1.  Exposing Transport Information in Extension Headers

   At the network-layer, packets can carry optional headers (similar to
   Section 6) that may be used to explicitly expose transport header
   information to the on-path devices operating at the network layer
   (Section 2.2.2).  For example, an endpoint that sends an IPv6 Hop-by-
   Hop option [RFC8200] can provide explicit transport layer information
   that can be observed and used by network devices on the path.

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   Network-layer optional headers explicitly indicate the information
   that is exposed, whereas use of exposed transport header information
   first requires an observer to identify the transport protocol and its
   format.  See Section 2.1 for further discussion of transport protocol

   An arbitrary path can include one or more network devices that drop
   packets that include a specific header or option used for this
   purpose (see [RFC7872]).  This could impact the proper functioning of
   the protocols using the path.  Protocol methods can be designed to
   probe to discover whether the specific option(s) can be used along
   the current path, enabling use on arbitrary paths.

5.2.  Common Exposed Transport Information

   There are opportunities for multiple transport protocols to
   consistently supply common observable information [RFC8558].  A
   common approach can result in an open definition of the observable
   fields.  This has the potential that the same information can be
   utilised across a range of operational and analysis tools.

5.3.  Considerations for Exposing Transport Information

   Considerations concerning what information, if any, it is appropriate
   to expose include:

   o  On the one hand, explicitly exposing derived fields containing
      relevant transport information (e.g., metrics for loss, latency,
      etc) can avoid network devices needing to derive this information
      from other header fields.  This could result in development and
      evolution of transport-independent tools around a common
      observable header, and permit transport protocols to also evolve
      independently of this ossified header [RFC8558].

   o  On the other hand, protocols and implementations might be designed
      to avoid consistently exposing external information that reflects
      the actual internal information used by the protocol itself.  An
      endpoint/protocol could choose to expose transport header
      information to optimise the benefit it gets from the network
      [RFC8558].  The value of this information would be enhanced if the
      exposed information could be verified to match the protocol's
      observed behavior.

   The motivation to reflect actual transport header information and the
   implications of network devices using this information has to be
   considered when proposing such a method.  RFC 8558 summarises this as
   "When signals from endpoints to the path are independent from the
   signals used by endpoints to manage the flow's state mechanics, they

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   may be falsified by an endpoint without affecting the peer's
   understanding of the flow's state.  For encrypted flows, this
   divergence is not detectable by on-path devices."  [RFC8558].

6.  Addition of Transport OAM Information to Network-Layer Headers

   If the transport headers are encrypted, on-path devices can make
   measurements by utilising additional protocol headers carrying
   operations, administration and management (OAM) information in an
   additional packet header.  Using network-layer approaches to reveal
   information has the potential that the same method (and hence same
   observation and analysis tools) can be consistently used by multiple
   transport protocols.  This approach also could be applied to methods
   beyond OAM (see Section 5).  There can also be less desirable
   implications from separating the operation of the transport protocol
   from the measurement framework.

6.1.  Use of OAM within a Maintenance Domain

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

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

6.2.  Use of OAM across Multiple Maintenance Domains

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

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

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7.  Conclusions

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

   This document has described some current practises, and the
   implications for some stakeholders, when transport layer header
   encryption is used.  It does not judge whether these practises are
   necessary, or endorse the use of any specific practise.  Rather, the
   intent is to highlight operational tools and practises to consider
   when designing and modifying transport protocols, so protocol
   designers can make informed choice about what transport header fields
   to encrypt, and whether it might be beneficial to make an explicit
   choice to expose certain fields to the network.  In making such a
   decision, it is important to balance:

   o  User Privacy: The less transport header information that is
      exposed to the network, the lower the risk of leaking metadata
      that might have privacy implications for the users.  Transports
      that chose to expose some header fields need to make a privacy
      assessment to understand the privacy cost versus benefit trade-off
      in making that information available.  The process used to define
      and expose the QUIC spin bit to the network is an example of such
      an analysis.

   o  Transport Ossification: Unencrypted transport header fields are
      likely to ossify rapidly, as network devices come to rely on their
      presence, making it difficult to change the transport in future.
      This argues that the choice to expose information to the network
      is made deliberately and with care, since it is essentially
      defining a stable interface between the transport and the network.
      Some protocols will want to make that interface as limited as
      possible; other protocols might find value in exposing certain
      information to signal to the network, or in allowing the network
      to change certain header fields as signals to the transport.  The
      visible wire image of a protocol should be explicitly designed.

   o  Network Ossification: While encryption can reduce ossification of
      the transport protocol, it does not itself prevent ossification of
      the network service.  People seeking to understand network traffic
      could still come to rely on pattern inferences and other
      heuristics or machine learning to derive measurement data and as
      the basis for network forwarding decisions [RFC8546].  This can

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      also create dependencies on the transport protocol, or the
      patterns of traffic it can generate, also in time resulting in
      ossification of the service.

   o  Impact on Operational Practice: The network operations community
      has long relied on being able to understand Internet traffic
      patterns, both in aggregate and at the flow level, to support
      network management, traffic engineering, and troubleshooting.
      Operational practice has developed based on the information
      available from unencrypted transport headers.  The IETF has
      supported this practice by developing operations and management
      specifications, interface specifications, and associated Best
      Current Practises.  Widespread deployment of transport protocols
      that encrypt their information will impact network operations,
      unless operators can develop alternative practises that work
      without access to the transport header.

   o  Pace of Evolution: Removing obstacles to change can enable an
      increased pace of evolution.  If a protocol changes its transport
      header format (wire image) or their transport behaviour, this can
      result in the currently deployed tools and methods becoming no
      longer relevant.  Where this needs to be accompanied by
      development of appropriate operational support functions and
      procedures, it can incur a cost in new tooling to catch-up with
      each change.  Protocols that consistently expose observable data
      do not require such development, but can suffer from ossification
      and need to consider if the exposed protocol metadata has privacy
      implications.  There is no single deployment context, and
      therefore designers need to consider the diversity of operational
      networks (ISPs, enterprises, Distributed DoS (DDoS) mitigation and
      firewall maintainers, etc.).

   o  Supporting Common Specifications: Common, open, specifications can
      stimulate engagement by developers, users, researchers, and the
      broader community.  Increased protocol diversity can be beneficial
      in meeting new requirements, but the ability to innovate without
      public scrutiny risks point solutions that optimise for specific
      cases, but that can accidentally disrupt operations of/in
      different parts of the network.  The social contract that
      maintains the stability of the Internet relies on accepting common
      interworking specifications, and on it being possible to detect
      violations.  It is important to find new ways of maintaining that
      community trust as increased use of transport header encryption
      limits visibility into transport behaviour.

   o  Impact on Benchmarking and Understanding Feature Interactions: An
      appropriate vantage point for observation, coupled with timing
      information about traffic flows, provides a valuable tool for

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      benchmarking network devices, endpoint stacks, functions, and/or
      configurations.  This can also help with understanding complex
      feature interactions.  An inability to observe transport header
      information can make it harder to diagnose and explore
      interactions between features at different protocol layers, a
      side-effect of not allowing a choice of vantage point from which
      this information is observed.  New approaches might have to be

   o  Impact on Research and Development: Hiding transport header
      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 have to be
      evaluated while considering other mechanisms, across a broad range
      of network topologies and with attention to the impact on traffic
      sharing the capacity.  If increased use of transport header
      encryption results in reduced availability of open data, it could
      eliminate the independent checks to the standardisation process
      that have previously been in place from research and academic
      contributors (e.g., the role of the IRTF Internet Congestion
      Control Research Group (ICCRG) and research publications in
      reviewing new transport mechanisms and assessing the impact of
      their deployment).

   Observable transport header information might be useful to various
   stakeholders.  Other sets of stakeholders have incentives to limit
   what can be observed.  This document does not make recommendations
   about what information ought to be exposed, to whom it ought to be
   observable, or how this will be achieved.  There are also design
   choices about where observable fields are placed.  For example, one
   location could be a part of the transport header outside of the
   encryption envelope, another alternative is to carry the information
   in a network-layer option or extension header.  New transport
   protocol designs ought to explicitly identify any fields that are
   intended to be observed, consider if there are alternative ways of
   providing the information, and reflect on the implications of
   observable fields being used by network devices, and how this might
   impact user privacy and protocol evolution when these fields become

   As [RFC7258] notes, "Making networks unmanageable to mitigate
   (pervasive monitoring) is not an acceptable outcome, but ignoring
   (pervasive monitoring) would go against the consensus documented
   here."  Providing explicit information can help avoid traffic being
   inappropriately classified, impacting application performance.  An
   appropriate balance will emerge over time as real instances of this
   tension are analysed [RFC7258].  This balance between information

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   exposed and information hidden ought to be carefully considered when
   specifying new transport protocols.

8.  Security Considerations

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

   Authentication, confidentiality protection, and integrity protection
   are identified as Transport Features by [RFC8095].  As currently
   deployed in the Internet, these features are generally provided by a
   protocol or layer on top of the transport protocol [RFC8922].

   Confidentiality and strong integrity checks have properties that can
   also be incorporated into the design of a transport protocol or to
   modify an existing transport.  Integrity checks can protect an
   endpoint from undetected modification of protocol fields by network
   devices, whereas encryption and obfuscation or greasing can further
   prevent these headers being utilised by network devices [RFC8701].
   Preventing observation of headers provides an opportunity for greater
   freedom to update the protocols and can ease experimentation with new
   techniques and their final deployment in endpoints.  A protocol
   specification needs to weigh the costs of ossifying common headers,
   versus the potential benefits of exposing specific information that
   could be observed along the network path to provide tools to manage
   new variants of protocols.

   Header encryption can provide confidentiality of some or all of the
   transport header information.  This prevents an on-path device from
   knowledge of the header field.  It therefore prevents mechanisms
   being built that directly rely on the information or seeks to infer
   semantics of an exposed header field.  Reduced visibility into
   transport metadata can limit the ability to measure and characterise
   traffic, and conversely can provide privacy benefits.

   Extending the transport payload security context to also include the
   transport protocol header protects both information with the same
   key.  A privacy concern would arise if this key was shared with a
   third party, e.g., providing access to transport header information
   to debug a performance issue, would also result in exposing the
   transport payload data to the same third party.  Such risks would be
   mitigated using a layered security design that provides one domain of
   protection and associated keys for the transport payload and
   encrypted transport headers; and a separate domain of protection and
   associated keys for any observable transport header fields.

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   Exposed transport headers are sometimes utilised as a part of the
   information to detect anomalies in network traffic.  "While PM is an
   attack, other forms of monitoring that might fit the definition of PM
   can be beneficial and not part of any attack, e.g., network
   management functions monitor packets or flows and anti-spam
   mechanisms need to see mail message content."  [RFC7258].  This can
   be used as the first line of defence to identify potential threats
   from DoS or malware and redirect suspect traffic to dedicated nodes
   responsible for DoS analysis, malware detection, or to perform packet
   "scrubbing" (the normalisation of packets so that there are no
   ambiguities in interpretation by the ultimate destination of the
   packet).  These techniques are currently used by some operators to
   also defend from distributed DoS attacks.

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

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

   Open standards motivate a desire for this evaluation to include
   independent observation and evaluation of performance data, which in
   turn suggests control over where and when measurement samples are

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   collected.  This requires consideration of the appropriate balance
   between encrypting all and no transport header information.  Open
   data, and accessibility to tools that can help understand trends in
   application deployment, network traffic and usage patterns can all
   contribute to understanding security challenges.

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

   Addition of observable transport information to the path increases
   the information available to an observer and may, when this
   information can be linked to a node or user, reduce the privacy of
   the user.  See the security considerations of [RFC8558].

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Acknowledgements

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

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

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

11.  Informative References

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

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              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", draft-ietf-ippm-ioam-data-10 (work in
              progress), July 2020.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-29 (work
              in progress), June 2020.

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

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May

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

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

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

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

   [PAM-RTT]  Trammell, B. and M. Kuehlewind, "Revisiting the Privacy
              Implications of Two-Way Internet Latency Data (in Proc.
              PAM 2018)", March 2018.

              "https:QUIC trace utilities //

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   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC2410]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
              Its Use With IPsec", RFC 2410, DOI 10.17487/RFC2410,
              November 1998, <>.

   [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,

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

   [RFC2507]  Degermark, M., Nordgren, B., and S. Pink, "IP Header
              Compression", RFC 2507, DOI 10.17487/RFC2507, February
              1999, <>.

   [RFC2508]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
              Headers for Low-Speed Serial Links", RFC 2508,
              DOI 10.17487/RFC2508, February 1999,

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

   [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,

   [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,

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

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,

   [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, <>.

   [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, <>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,

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

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [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,

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

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

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

   [RFC5426]  Okmianski, A., "Transmission of Syslog Messages over UDP",
              RFC 5426, DOI 10.17487/RFC5426, March 2009,

   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
              March 2009, <>.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,

   [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,

   [RFC6294]  Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
              the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
              2011, <>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

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   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,

   [RFC6846]  Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West,
              "RObust Header Compression (ROHC): A Profile for TCP/IP
              (ROHC-TCP)", RFC 6846, DOI 10.17487/RFC6846, January 2013,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7126]  Gont, F., Atkinson, R., and C. Pignataro, "Recommendations
              on Filtering of IPv4 Packets Containing IPv4 Options",
              BCP 186, RFC 7126, DOI 10.17487/RFC7126, February 2014,

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

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

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

   [RFC7594]  Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
              Aitken, P., and A. Akhter, "A Framework for Large-Scale
              Measurement of Broadband Performance (LMAP)", RFC 7594,
              DOI 10.17487/RFC7594, September 2015,

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <>.

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   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <>.

   [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,

   [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, <>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,

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

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <>.

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   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095,
              DOI 10.17487/RFC8095, March 2017,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC8250]  Elkins, N., Hamilton, R., and M. Ackermann, "IPv6
              Performance and Diagnostic Metrics (PDM) Destination
              Option", RFC 8250, DOI 10.17487/RFC8250, September 2017,

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,

   [RFC8290]  Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
              J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
              and Active Queue Management Algorithm", RFC 8290,
              DOI 10.17487/RFC8290, January 2018,

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8462]  Rooney, N. and S. Dawkins, Ed., "Report from the IAB
              Workshop on Managing Radio Networks in an Encrypted World
              (MaRNEW)", RFC 8462, DOI 10.17487/RFC8462, October 2018,

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   [RFC8517]  Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
              Jacquenet, "An Inventory of Transport-Centric Functions
              Provided by Middleboxes: An Operator Perspective",
              RFC 8517, DOI 10.17487/RFC8517, February 2019,

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <>.

   [RFC8548]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic Protection of TCP Streams
              (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <>.

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,

   [RFC8922]  Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
              Wood, "A Survey of the Interaction between Security
              Protocols and Transport Services", RFC 8922,
              DOI 10.17487/RFC8922, October 2020,

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Appendix A.  Revision information

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

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

   -02 This draft fixes textual errors.

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

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

   Comments from the community are welcome on the text and

   -05 Corrections received and helpful inputs from Mohamed Boucadair.

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

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

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

   -09 Updated security considerations.

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

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

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

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

   -04 Revised following SecDir Review

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

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

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

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

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

   o  Updated section 6 and added more explanation of impact on

   o  Other comments addressed.

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

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

   -07 Addressed feedback from Ruediger and Thomas.

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

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

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

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

   -10 Updated following additional feedback from 1st WGLC.  Comments
   from David Black; Tommy Pauly; Ian Swett; Mirja Kuehlewind; Peter
   Gutmann; Ekr; and many others via the TSVWG list.  Some people

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   thought that "needed" and "need" could represent requirements in the
   document, etc. this has been clarified.

   -11 Updated following additional feedback from Martin Thomson, and
   corrections from other reviewers.

   -12 Updated following additional feedback from reviewers.

   -13 Updated following 2nd WGLC with comments from D.L.Black; T.
   Herbert; Ekr; and other reviewers.

   -14 Update to resolve feedback to rev -13.  This moves the general
   discussion of adding fields to transport packets to section 6, and
   discusses with reference to material in RFC8558.

   -15 Feedback from D.L.  Black, T.  Herbert, J.  Touch, S.  Dawkins
   and M.  Duke.  Update to add reference to RFC7605.  Clarify a focus
   on immutable transport fields, rather than modifying middleboxes with
   Tom H.  Clarified Header Compression discussion only provides a list
   of examples of HC methods for transport.  Clarified port usage with
   Tom H/Joe T.  Removed some duplicated sentences, and minor edits.
   Added NULL-ESP.  Improved after initial feedback from Martin Duke.

   -16 Editorial comments from Mohamed Boucadair.  Added DTLS 1.3.

   -17 Revised to satisfy ID-NITs and updates REFs to latest rev,
   updated HC REFs; cited IAB guidance on security and privacy within
   IETF specs.

   -18 Revised based on AD review.

Authors' Addresses

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


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


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