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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 2020-03-21 (Latest revision 2020-02-27)
Replaces draft-fairhurst-tsvwg-transport-encrypt
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Waiting for WG Chair Go-Ahead
Revised I-D Needed - Issue raised by WGLC
Document shepherd David L. Black
IESG IESG state Became RFC 9065 (Informational)
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Send notices to David Black <>
TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                                C. Perkins
Expires: September 21, 2020                        University of Glasgow
                                                          March 20, 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, while also protecting metadata about the
   communication.  Current operational practice in some networks inspect
   transport header information within the network, but this is no
   longer possible when those transport headers are encrypted.  This
   document discusses the possible impact when network traffic uses a
   protocol with an encrypted transport header.  It suggests issues to
   consider when designing new transport protocols or features.  These
   considerations arise from concerns such as network operations,
   prevention of network ossification, enabling transport protocol
   evolution and respect for user privacy.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   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 September 21, 2020.

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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
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   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.  Context and Rationale . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Use of Transport Header Information in the Network  . . .   6
     2.2.  Authentication of Transport Header Information  . . . . .   7
     2.3.  Perspectives on Observable Transport Header Fields  . . .   8
   3.  Current uses of Transport Headers within the Network  . . . .  11
     3.1.  Observing Transport Information in the Network  . . . . .  12
     3.2.  Transport Measurement . . . . . . . . . . . . . . . . . .  20
     3.3.  Use for Network Diagnostics and Troubleshooting . . . . .  23
     3.4.  Header Compression  . . . . . . . . . . . . . . . . . . .  24
   4.  Encryption and Authentication of Transport Headers  . . . . .  25
     4.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  25
     4.2.  Approaches to Transport Header Protection . . . . . . . .  26
   5.  Addition of Transport Information to Network-Layer Headers  .  28
     5.1.  Use of OAM within a Maintenance Domain  . . . . . . . . .  28
     5.2.  Use of OAM across Multiple Maintenance Domains  . . . . .  28
     5.3.  Exposing Transport Information at the Network Layer . . .  29
   6.  Implications of Protecting the Transport Headers  . . . . . .  30
     6.1.  Independent Measurement . . . . . . . . . . . . . . . . .  30
     6.2.  Characterising "Unknown" Network Traffic  . . . . . . . .  32
     6.3.  Accountability and Internet Transport Protocols . . . . .  32
     6.4.  Impact on Network Operations  . . . . . . . . . . . . . .  33
     6.5.  Impact on Research, Development and Deployment  . . . . .  34
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  35
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  38
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  40
   11. Informative References  . . . . . . . . . . . . . . . . . . .  40
   Appendix A.  Revision information . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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1.  Introduction

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

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

   A further level of protection can be achieved by encrypting the
   entire network layer payload, including both the transport headers
   and the transport payload data.  This does not expose any transport
   header information to devices in the network, and therefore also
   prevents modification along a network path.  An example of encryption
   at the network layer is the IPsec Encapsulating Security Payload
   (ESP) [RFC4303] in tunnel mode.  Virtual Private Networks (VPNs)
   typically also operate in this way.  This form of encryption is not
   further discussed in this document.

   There is also a middle ground, comprising transport protocols that
   encrypt some, or all, of the transport layer header information, in
   addition to encrypting the transport payload data.  An example of
   such a protocol, that is now seeing widespread interest and
   deployment, is the QUIC transport protocol [I-D.ietf-quic-transport].
   The encryption and authentication of transport header information can
   prevent unwanted modification of transport header information by
   network devices, reducing the risk of protocol ossification.  It also
   reduces the amount of metadata about the progress of the transport
   connection that is visible to the network [RFC8558].  In this
   document, the term "transport header information" is used to describe
   transport layer information concerning the operation of the transport
   protocol (i.e., information used by the transport protocol that might
   be carried in a protocol header).  This does not refer to transport
   payload data (i.e., information transferred by the transport
   service), which itself could be encrypted.

   There is also, however, some impact, in that the widespread use of
   transport header encryption requires changes to network operations
   and other practises.  Operators could choose to do nothing special
   with encrypted traffic.  In some cases, encryption could drive

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   changes to the design of network measurement for research,
   operational, and standardisation purposes.

   The direction in which the use of transport header encryption evolves
   could have significant implications on the way the Internet
   architecture develops, and therefore needs to be considered as a part
   of protocol design and evolution.  This include considering whether
   the endpoints permit (or are able to permit) network devices to
   observe a specific information by explicitly exposing a transport
   header field (or a field derived from transport header information)
   to the network; whether it is intended that a network device can
   modify the field, whether the devices are able to modify that field;
   and whether any modification along the network path can be detected
   by the receiving endpoint.

   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, but RFC7528 also notes that "Making
   networks unmanageable to mitigate PM is not an acceptable outcome,
   but ignoring PM would go against the consensus documented here.  An
   appropriate balance will emerge over time as real instances of this
   tension are considered".  In support of achieving that balance, this
   document discusses design and deployment considerations for use of
   transport header encryption to protect against pervasive monitoring.

   The transport protocols developed for the Internet are used across a
   wide range of paths across network segments with many different
   regulatory, commercial, and engineering considerations.  This
   document considers some of the costs and changes to network
   management and research that are implied by widespread use of
   transport protocols that encrypt their transport header information.
   It reviews the implications of developing transport protocols that
   use end-to-end encryption to provide confidentiality of their
   transport layer headers, and considers the effect of such changes on
   transport protocol design, transport protocol evolution, and network
   operations.  It also considers some anticipated implications on
   application evolution.  This provides considerations relating to the
   design of transport protocols and features where the transport
   protocol encrypts some or all of their header information.

2.  Context and Rationale

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

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   This simple architectural view does not present 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.
   The design of Internet transport protocols is as much about trying to
   avoid the unwanted side effects of congestion on a flow and other
   capacity-sharing flows, avoiding congestion collapse, adapting to
   changes in the path characteristics, etc., as it is about end-to-end
   feature negotiation, flow control, and optimising for performance of
   a specific application.

   Transport headers have end-to-end meaning, but have often been
   observed by equipment within the network.  Transport protocol
   specifications have not tended to consider this, and have often
   failed to indicate what parts of the transport header are intended to
   be invariant across protocol versions and visible to the network; to
   specify what parts of the transport header can be modified by the
   network to signal to the transport, and in what way; and to define
   which parts of the header are private and/or expected to change in
   future and which need to be protected for privacy or to prevent
   protocol ossification.  This motivates a need to change the way
   transport protocols are designed, modified, and specified.

   Increasing concern about pervasive network monitoring
   [RFC7258][RFC7624], and growing awareness of the problem of protocol
   ossification caused by middlebox interference with Internet traffic,
   has motivated a shift in transport protocol design.  Recent transport
   protocols, such as QUIC [I-D.ietf-quic-transport], encrypt the
   majority of their transport headers to prevent observation and
   protect against modification by the network, and to make explicit
   their invariants and what is intended to be visible to the network.

   Transport header encryption is expected to form a core part of future
   transport protocol designs.  It can help to protect against pervasive
   monitoring, improve privacy, and reduce protocol ossification.
   Transport protocols that use header encryption with secure key
   distribution can provide confidentiality and protection for some, or
   all, of the transport header, controlling what is visible to, and can
   be modified by, the network.

   The increased use of transport header encryption has benefits, but
   also has implications for the broader ecosystem.  The transport
   community has, to date, relied heavily on measurements and insights
   from the network operations community to understand protocol
   behaviour, and to inform the selection of appropriate mechanisms to
   ensure a safe, reliable, and robust Internet.  In turn, network
   operators and access providers have relied upon being able to observe
   traffic patterns and requirements, both in aggregate and at the flow
   level, to help understand and optimise the behaviour of their

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   networks.  Transport header encryption can be used to intentionally
   limit the information available to network observers.  The widespread
   use of transport header encryption would therefore limit such
   observations, unless transport protocols are modified to selectively
   expose transport header information outside of the encrypted
   transport header.  It is important to understand how transport header
   information is used by networks, to allow future protocol designs to
   make an informed choice on what, if any, transport layer information
   to expose to the network.

2.1.  Use of Transport Header Information in the Network

   In-network measurement of transport flow characteristics can be used
   to enhance performance, control cost and improve service reliability.
   To support network operations and enhance performance, some operators
   have deployed functionality that utilises on-path observations of the
   transport headers of packets passing through their network.

   When network devices rely on the presence of a header field or the
   semantics of specific header information, this can lead to
   ossification where an endpoint has to supply a specific header to
   receive the network service that it desires.

   In some cases, network-layer use of transport layer information can
   be benign or advantageous to the protocol (e.g., recognising the
   start of a TCP connection, providing header compression for a Secure
   RTP flow, or explicitly using exposed protocol information to provide
   consistent decisions by on-path devices).  Header compression (e.g.,
   [RFC5795]) depends understanding of transport header and the way
   fields change packet-by-packet; as also do techniques to improve TCP
   performance by transparent modification of acknowledgement traffic
   [RFC3449].  Introducing a new transport protocol or changes to
   existing transport header information prevent these methods being
   used or require the network devices to be updated.

   However, in other cases, ossification can have unwanted outcomes.
   Ossification can frustrate the evolution of a transport protocol.  A
   mechanism implemented in a network device, such as a firewall, that
   requires a header field to have only a specific known set of values
   can prevent the device from forwarding packets using a different
   version of the protocol that introduces a feature that changes to a
   new value for the observed field.

   An example of this type ossification was observed in the development
   of Transport Layer Security (TLS) 1.3 [RFC8446], where the design
   needed to function in the presence of deployed middleboxes that
   relied on the presence of certain header fields exposed in TLS 1.2.
   The design of MPTCP also had to be revised to account for middleboxes

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   (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 expected.  Similarly, issues have been
   reported using TCP.  For example, TCP Fast Open can experience
   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.

   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 selective
   acknowledgement header 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 contrast, 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 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"
   [RFC8546], and it can be expected that header information that is not
   encrypted will become ossified.

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

2.2.  Authentication of Transport Header Information

   The designers of a transport protocol 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" [RFC8558].

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   Protocol designs can decide not to encrypt certain transport header
   fields, making those fields observable in the network, or can choose
   to expose 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 includes these
   immutable fields to detect any manipulation by network devices.

   Making 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 fields.  A
   protocol design that provides an observable field might want to
   restrict the choice of usable values in a field by intentionally
   varying the format and/or value of the field to reduce the chance of
   ossification (see Section 4).

2.3.  Perspectives on Observable Transport Header Fields

   Transport headers fields have been observed within the network for a
   variety of purposes.  Some of these are related to network management
   and operations.  The lists below, and in the following section, seek
   to identify some of these uses and the implications of the increased
   use of transport header encryption.  This analysis does not judge
   whether specific practises are necessary, or endorse the use of any
   specific approach.

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

                       When transport header information is not
                       observable, it cannot be used by network
                       operators.  Some operators might work without
                       that information, or some might turn to more
                       ambitious ways to collect, estimate, or infer
                       this data.  (Operational practises aimed at
                       guessing transport parameters are out of scope
                       for this document, and are only mentioned here to
                       recognise that encryption does not stop operators
                       from attempting to apply practises that have been
                       used with unencrypted transport headers.)

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                       See also Section 3, Section 5, Section 6.4 and s
                       (Section 6.5).

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

                       This information can help anticipate the demand
                       for network upgrades and roll-out, or affect on-
                       going traffic engineering activities performed by
                       operators such as determining which parts of the
                       path contribute delay, jitter, or loss.

                       Tools that rely upon observing specific headers,
                       could fail to produce useful data when those
                       headers are encrypted.  While this impact could,
                       in many cases, be small, there are scenarios
                       where operators have actively monitored and
                       supported particular services, e.g., to explore
                       issues relating to Quality of Service (QoS), to
                       perform fast re-routing of critical traffic, to
                       mitigate the characteristics of specific radio
                       links, and so on.

                       See also Section 3.1 to Section 3.2 and
                       Section 5.

   Troubleshooting:    Observable transport headers have been utilised
                       by operators as a part of network troubleshooting
                       and diagnostics.  Metrics derived from this
                       observed header information can help localise the
                       network segment introducing the loss or latency.
                       Effective troubleshooting often requires
                       understanding of transport behaviour.  Flows
                       experiencing packet loss or jitter are hard to
                       distinguish from unaffected flows when only
                       observing network layer headers.

                       Observable transport feedback information (e.g.,
                       RTP Control Protocol (RTCP) reception reports

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                       [RFC3550]) can explicitly make loss metrics
                       visible to operators.  Loss metrics can also be
                       deduced with more complexity from other header
                       information (e.g., by observing TCP SACK blocks).
                       When the transport header information is
                       encrypted, explicit observable fields could also
                       be made available at the network or transport
                       layers to provide these functions.  [RFC8558]
                       motivates the design of signals to focus on their
                       usage, decoupled from the internal design of the
                       protocol state machine.  This could avoid
                       ossifying the protocol around the design of a
                       specific protocol mechanism.

                       See also Section 3.3 and Section 5.

   Network Protection: Observable transport headers currently provide
                       information that is useful input to classify and
                       detect anomalous events, such as changes in
                       application behaviour or distributed denial of
                       service attacks.  Operators often seek to
                       uniquely disambiguate unwanted traffic.

                       Where flows cannot be disambiguated based on
                       transport header information, this could result
                       in less-efficient identification of unwanted
                       traffic, the introduction of rate limits for
                       uncharacterised traffic, or the use of heuristics
                       to identify anomalous flows.

                       See also Section 6.2 and Section 6.3.

   Verifiable Data:    Observable transport headers can be used to
                       provide open and verifiable measurements to
                       support operations, research, and protocol
                       development.  The ability of multiple stake
                       holders to review transport header traces helps
                       develop insight into performance and traffic
                       contribution of specific variants of a protocol.
                       Independently observed data is important to help
                       ensure the health of the research and development

                       When transport header information can not be
                       observed, this can reduce the range of actors
                       that can observe data.  This limits the
                       information sources available to the Internet
                       community to understand the operation of

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                       transport protocols, reducing information to
                       inform design decisions and standardisation of
                       the new protocols/features and related
                       operational practises

                       See also Section 6.

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

                       When transport header information can not 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]).

                       See also Section 5 and Section 6.1 to
                       Section 6.4.

   Note, again, that this is a list of example uses that have been made
   of transport header information.  It is not an endorsement of any
   particular practice.

3.  Current uses of Transport Headers within the Network

   In response to pervasive monitoring [RFC7624] revelations and the
   IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
   efforts are underway to increase encryption of Internet traffic.
   Applying confidentiality to transport header fields affects how
   protocol information is used [RFC8404], requiring consideration of
   the trade-offs discussed in Section 2.3.

   There are architectural challenges and considerations in the way
   transport protocols are designed, and the ability to characterise and
   compare different transport solutions [Measurement].  The decision
   about which transport headers fields are made observable offers

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   trade-offs around header confidentiality versus header observability
   (including non-encrypted but authenticated header fields) for network
   operations and management, and the implications for ossification and
   user privacy.  Different parties will view the relative importance of
   these differently.  For some, the benefits of encrypting all
   transport headers outweigh the impact of doing so; others might
   analyse the security, privacy and ossification impacts, and arrive at
   a different trade-off.

   To understand the implications, it is necessary to understand how
   transport layer headers are currently observed and/or modified by
   middleboxes within the network.  This section therefore reviews
   examples of current usage.  It does not consider the intentional
   modification of transport headers by middleboxes (such as in Network
   Address Translation, NAT, or Firewalls).  Common issues concerning IP
   address sharing are described in [RFC6269].

3.1.  Observing Transport Information in the Network

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

   o  Flows have to be identified at the level where observation is
      performed.  This implies visibility of the protocol and version of
      the header, e.g., by defining the wire image [RFC8546].  As
      protocols evolve over time, new transport headers could be
      introduced.  Detecting this could require interpretation of
      protocol version information or connection setup information;

   o  Observing transport header information depends on the observer
      knowing the location and the syntax of the observable transport
      headers.  IETF transport protocols can specify this information.

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

3.1.1.  Flow Identification Using Transport Layer Headers

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

   Observable transport header information, together with information in
   the network header, has been used to identify flows and their
   connection state, together with the 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

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   the possibility to negotiate additional headers at connection setup,
   identified by an option number in the transport header.

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

   When transport header information can not 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 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.  The IAB have provided
   a summary of expected implications of increased encryption on network
   functions that use the observable headers [RFC8546] and describe the
   expected benefits of designs that explicitly declare protocol
   invariant header information that can be used for this purpose.

3.1.2.  Metrics derived from Transport Layer Headers

   Observable transport headers enable explicit measurement and analysis
   of protocol performance, network anomalies, and failure pathologies
   at any point along the Internet path.  Some operators use passive
   monitoring to manage their portion of the Internet by 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

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      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 (e.g., to assess
      subscriber usage).  Measurements can also be used to trigger
      traffic shaping, and to associate QoS support within the network
      and lower layers.  Volume measures can also be valuable for
      capacity planning and providing detail of trends in usage.  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.

   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).  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 on the network node/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 [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

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      overhead known.  Goodput requires ability to differentiate loss
      and retransmission of packets, for example by observing packet
      sequence numbers in the TCP or the Real-time Transport Protocol
      (RTP) headers [RFC3550].

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

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

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

   Variation in delay:  Some network applications are sensitive to
      (small) changes in packet timing (jitter).  Short and long-term
      delay variation can impact on the latency of a flow and hence the
      perceived quality of applications using the network.  For example,
      jitter metrics are often cited when characterising paths
      supporting real-time traffic.  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.

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

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   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.

   Passive measurements (see Section 3.6 of [RFC7799]) can have
   advantages in terms of eliminating unproductive test traffic,
   reducing the influence of test traffic on the overall traffic mix,
   and the ability to choose the point of observation (see
   Section 3.2.1).  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.

   An alternative approach is to use in-network techniques to add and
   observe packet headers to facilitate measurements while traffic
   traverses an operational network.  This approach does not require the
   cooperation of an endpoint.

3.1.3.  Transport use of Network Layer Header Fields

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

   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:

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

   Using the IPv6 Network-Layer Flow Label:  A number of Standards Track
      and Best Current Practice RFCs (e.g., [RFC8085], ,
      [RFC6437][RFC6438]) encourage endpoints to set the IPv6 Flow label
      field of the network-layer header.  IPv6 "source nodes SHOULD
      assign each unrelated transport connection and application data
      stream to a new flow" [RFC6437].  A multiplexing transport could
      choose to use multiple Flow labels to allow the network to
      independently forward 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).

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      Since the DSCP value can impact the quality of experience for a
      flow, observations of service performance has to consider this
      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 inSection 5).

      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].  RFC 7126 provides Best Current Practice
      guidance on how operators should treat IPv4 packets that specify

      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

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      header, it is now expected [RFC8200] that nodes along a path only
      examine and process the Hop-by-Hop Options header if explicitly
      configured to do so.

   When transport headers cannot be observed, operators are unable to
   use this information directly.  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.

3.2.  Transport Measurement

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

   When encryption hides more layers in each packet, people seeking
   understanding of the network operation rely more on pattern inference
   and other heuristics.  It remains to be seen whether more complex
   inferences can be mastered to produce the same monitoring accuracy
   (see Section 2.1.1 of [RFC8404]).

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

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

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

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3.2.1.  Point of Observation

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

   Sometimes multiple on-path observation points 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.

3.2.2.  Use by Operators to Plan and Provision Networks

   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.

3.2.3.  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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3.2.4.  Measuring Transport to Support Network Operations

   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 less.  The
   desire to understand the traffic and protocol interactions typically
   grows as the proportion of traffic increases in volume.  The
   challenges increase when multiple instances of an evolving protocol
   contribute to the traffic that share network capacity.

   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 3.1.2).  The Secure RTP and RTCP extensions [RFC3711] were
      explicitly designed to expose some header information to enable
      such observation, while protecting the payload data.

3.3.  Use for 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 denial of service), and responding to user performance
   questions.  Section 3.1.2 and Section 5 of [RFC8404] provide further

   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.

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   In other cases, measurement involves dissecting network traffic
   flows.  Observed transport header information can help identify
   whether link/network tuning is effective and alert to potential
   problems that can be hard to derive from link or device measurements

   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 reliable
   signals 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 WiFi, satellite access/back-haul, point-to-point
   radio) has the complexity of a subsystem that performs radio resource
   management, with direct impact on the available capacity, and
   potentially loss/reordering of packets.  The impact of the pattern of
   loss and congestion, differs for different traffic types, correlation
   with propagation and interference can all have significant impact on
   the cost and performance of a provided service.  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".

3.4.  Header Compression

   Header compression saves link capacity by compressing network and
   transport protocol headers on a per-hop basis.  It was widely used
   with low bandwidth dial-up access links, and still finds application

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   on wireless links that are subject to capacity constraints.  Header
   compression has been specified for use with TCP/IP and RTP/UDP/IP
   flows [RFC2507], [RFC2508], [RFC4995].

   While it is possible to compress only the network layer headers,
   significant savings can be made if both the network and transport
   layer headers are compressed together as a single unit.  The Secure
   RTP extensions [RFC3711] were explicitly designed to leave the
   transport protocol headers unencrypted, but authenticated, since
   support for header compression was considered important.  Encrypting
   the transport protocol headers does not break such header
   compression, but does cause a fall back to compressing only the
   network layer headers, with a significant reduction in efficiency.

4.  Encryption and Authentication of Transport Headers

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

4.1.  Motivation

   There are several motivations for encryption:

   o  One motive to encrypt transport headers is in response to a
      growing awareness of the implications of network ossification from
      network devices that inspect transport headers.  Once a network
      devices 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.

   o  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

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      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.

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

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

      An example transport authentication mechanism is TCP-
      Authentication (TCP-AO) [RFC5925].  This TCP option authenticates
      the IP pseudo header, TCP header, and TCP data.  TCP-AO protects
      the transport layer, preventing attacks from disabling the TCP
      connection itself and provides replay protection.  TCP-AO might
      interact with middleboxes, depending on their behaviour [RFC3234].

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

   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
      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

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      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.  Addition of Transport Information to Network-Layer Headers

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

5.1.  Use of OAM within a Maintenance Domain

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

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

5.2.  Use of OAM across Multiple Maintenance Domains

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

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

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

5.3.  Exposing Transport Information at the Network Layer

   Network packets can carry optional headers may be used to explicitly
   expose transport header information to the on-path devices operating
   at the network layer Section 3.1.3.  For example, an endpoint that
   provides 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.

   When the path includes a network device that drops packets that
   include a specific option used for this purpose (e.g., see[RFC7872]),
   this impacts 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.

   The transport header information exposed by a transport endpoint can
   be different to the (hidden) transport header fields used by the
   protocol state machine and could instead be specifically designed to
   meet the needs of the network layer [RFC8558].  There are
   opportunities for the format and usage of this transport information
   to be standardised, providing an opportunity for common
   specifications and tools to be used with more than one transport

   o  On the one hand, protocols do not necessarily have an incentive to
      expose the actual information that is used by the protocol itself
      and could therefore manipulate the exposed transport header
      information to gain an advantage from the network.  The incentive
      to reflect actual transport header information has to be
      considered when proposing a method.

   o  On the other hand, explicit approaches can simplify tools by
      exposing the relevant metrics (loss, latency, etc), rather having
      to derive this from other fields.  This could stimulate the
      development of proocol-independent tools and also permits
      protocols to evolve independently of the ossified observable
      header [RFC8558].

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6.  Implications of Protecting the 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.

   Security work typically employs a design technique that seeks to
   expose only what is needed.  This approach provides incentives to not
   reveal any information that is not necessary for the end-to-end
   communication.  However, there can be performance and operational
   benefits in exposing selected information to network tools.

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

6.1.  Independent Measurement

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

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

   Protocols that expose the state information used by the transport
   protocol in their header information (e.g., timestamps used to
   calculate the RTT, packet numbers used to asses congestion and
   requests for retransmission) provide an incentive for the sending
   endpoint to provide correct information, since the protocol will not
   work otherwise.  This increases confidence that the observer
   understands the transport interaction with the network.  For example,
   when TCP is used over an unencrypted network path (i.e., one that
   does not use IPsec or other encryption below the transport), it
   implicitly exposes 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

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   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.  Such a transport could be designed (or an existing
   transport modified), for example, to provide summary data regarding
   its performance, congestion control state, etc., or to make an
   explicit measurement signal available.  For example, a QUIC endpoint
   can optionally set the spin bit to reflect to explicitly reveal the
   RTT of an encrypted transport session to the on-path network devices

   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 measurement signals are choosing to establish a
   trust relationship with the network operators.  There is no protocol
   mechanism that can guarantee that the information provided represents
   the actual transport state of the endpoints, since those endpoints
   can always send additional information in the encrypted part of the
   header, to update or replace whatever they reveal.  This reduces the
   ability to independently measure and verify that a protocol is
   behaving as expected.  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.

   Operations, Administration, and Maintenance (OAM) data records
   [I-D.ietf-ippm-ioam-data] could be embedded into a variety of
   encapsulation methods at different layers to support the goals of a
   specific operational domain.  OAM-related metadata can support
   functions such as performance evaluation, path-tracing, path
   verification information, classification and a diversity of other
   uses.  When encryption is used to 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.,
   combining transport and network measurements to explore congestion
   control dynamics, the implications of designs for active queue
   management or circuit breakers).

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   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.

6.2.  Characterising "Unknown" Network Traffic

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

   If "unknown" or "uncharacterised" traffic patterns form a small part
   of the traffic aggregate passing through a network device or segment
   of the network the path, the dynamics of the uncharacterised traffic
   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 denial of service attacks
   against the infrastructure.

6.3.  Accountability and Internet Transport Protocols

   Information provided by tools observing transport headers can be used
   to classify traffic, and to limit the network capacity used by
   certain flows, as discussed in Section 3.2.4).  Equally, operators
   could use analysis of transport headers and transport flow state to

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   demonstrate that they are not providing differential treatment to
   certain flows.  Obfuscating or hiding this information using
   encryption could lead operators and maintainers of middleboxes
   (firewalls, etc.) to seek other methods to classify, and potentially
   other mechanisms to condition, network traffic.

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

6.4.  Impact on Network Operations

   Some network operators currently use observed transport header
   information as a part of their operational practice, and have
   developed tools and techniques that use information observed in
   currently deployed transports and their applications.  A variety of
   open source and proprietary tools have been deployed that use this
   information for a variety of short and long term measurements.
   Encryption of the transport header information prevents tooling from
   directly observing the transport header information, limiting its

   Alternative diagnostic and troubleshooting tools would have to be
   developed and deployed is transport header encryption is widely
   deployed.  Introducing a new protocol or application might then
   require these tool chains and practises to be updated, and could in
   turn impact operational mechanisms, and policies.  Each change can
   introduce associated costs, including the cost of collecting data,
   and the tooling to handle multiple formats (possibly as these co-
   exist in the network, when measurements span time periods during
   which changes are deployed, or to compare with historical data).
   These costs are incurred by an operator to manage the service and
   debug network issues.

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

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6.5.  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 the interactions between cooperating protocols
   and network mechanism, the implications of sharing capacity with
   other traffic and the impact of different patterns of usage.  The
   ability of other stake holders to review transport header traces
   helps develop insight into performance and traffic contribution of
   specific variants of a protocol.

   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.

   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

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   devices and within service platforms) can observe performance, and
   when performance cannot be independently verified by all parties.

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

7.  Conclusions

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

   This document has described some current practises, and the
   implications for some stake holders, 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  Protocol 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

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      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  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 might 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, DDoS mitigation and firewall maintainers,

   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.

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   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
      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 self-checks to the standardisation
      process that have previously been in place from research and
      academic contributors (e.g., the role of the IRTF Internet
      Congestion Control Research Group (ICCRG) and research
      publications in reviewing new transport mechanisms and assessing
      the impact of their deployment).

   Observable transport header information might be useful to various
   stake holders.  Other stake holders 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 ossified.

   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

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   appropriate balance will emerge over time as real instances of this
   tension are analysed [RFC7258].  This balance between information
   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

   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.  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.  Reduces visibility into
   transport metadata can limit the ability to measure and characterise
   traffic.  It can also provide privacy benefits in some cases.

   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

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   encrypted transport headers; and a separate domain of protection and
   associated keys for any observable transport header fields.

   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 can not be predicted (see Section 5.1
   of [RFC8085]).  A receiver could also require additional information
   to be used as a part of a validation check before accepting packets
   at the transport layer (e.g., utilising a part of the sequence number
   space that is encrypted; or by verifying an encrypted token not
   visible to an attacker).  This would also mitigate against on-path
   attacks.  An additional processing cost can be incurred when
   decryption has to be attempted before a receiver is able to discard
   injected packets.

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   Open standards motivate a desire for this evaluation to include
   independent observation and evaluation of performance data, which in
   turn suggests control over where and when measurement samples are
   collected.  This requires consideration of the appropriate balance
   between encrypting all and no transport 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.

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, Martin Thomson, David Black, and other members
   of the TSVWG for their comments and feedback.

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

   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., Pignataro, C., Gredler, H.,
              Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
              P., Chang, R.,, d., and J. Lemon,
              "Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
              data-06 (work in progress), July 2019.

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

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

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

              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.

              "https:QUIC trace utilities //

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

   [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,

   [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,

   [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,

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   [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, <>.

   [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,

   [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,

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

   [RFC4995]  Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
              Header Compression (ROHC) Framework", RFC 4995,
              DOI 10.17487/RFC4995, July 2007,

   [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,

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   [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, <>.

   [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,

   [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,

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

   [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,

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   [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, <>.

   [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,

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

   [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,

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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.

Authors' Addresses

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


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


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