The Impact of Transport Header Encryption on Operation and Evolution of the Internet
draft-fairhurst-tsvwg-transport-encrypt-02

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TSVWG                                                       G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                             June 07, 2017
Expires: December 07, 2017

The Impact of Transport Header Encryption on Operation and Evolution of
                              the Internet
               draft-fairhurst-tsvwg-transport-encrypt-02

Abstract

   This document describes the implications of applying end-to-end
   encryption at the transport layer.  It identifies some in-network
   uses of transport layer header information that can be used with a
   transport header integrity check.  It reviews the implication of
   developing encrypted end-to-end transport protocols and examines the
   implication of developing and deploying encrypted end-to-end
   transport protocols.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 07, 2017.

Copyright Notice

   Copyright (c) 2017 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 (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Internet Transports and Pervasive Encryption . . . . . . . . .  4
     2.1.  Authenticating the Transport Protocol Header . . . . . . .  5
     2.2.  Encrypting the Transport Payload . . . . . . . . . . . . .  6
     2.3.  Encrypting the Transport Header  . . . . . . . . . . . . .  6
     2.4.  Authenticating Transport Information and Selectively
           Encrypting the Transport Header  . . . . . . . . . . . . .  6
     2.5.  Adding Transport Information to Network-Layer Protocol
           Headers  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Use of Transport Headers in the Network  . . . . . . . . . . .  8
     3.1.  Use to Identify Flows and Packet Formats . . . . . . . . .  9
     3.2.  Measurements derived from Transport Header Information . .  9
       3.2.1.  Use to Characterise Traffic Rate and Volume  . . . . .  9
       3.2.2.  Measuring Loss Rate and Loss Pattern . . . . . . . . . 10
       3.2.3.  Measuring Throughput and Goodput . . . . . . . . . . . 10
       3.2.4.  Measuring Latency (Network Transit Delay and Jitter) . 10
       3.2.5.  Measuring Flow Reordering  . . . . . . . . . . . . . . 11
     3.3.  Measurements derived from Network-Transport Information  . 11
       3.3.1.  Use of IPv6 Network-Layer Flow Label . . . . . . . . . 12
       3.3.2.  Use Network-Layer Differentiated Services Code Point
               Point  . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.3.3.  Use of Explicit Congestion Marking . . . . . . . . . . 12
   4.  Transport Measurement  . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Point of Measurement . . . . . . . . . . . . . . . . . . . 13
     4.2.  Use by Operators to Plan and Provision Networks  . . . . . 14
     4.3.  Service Performance Measurement  . . . . . . . . . . . . . 14
     4.4.  Use for Network Diagnostics and Troubleshooting  . . . . . 14
     4.5.  Acceptable Response to Congestion  . . . . . . . . . . . . 15
       4.5.1.  Measuring Compliance of UDP Traffic  . . . . . . . . . 15
       4.5.2.  Measuring Transport to Support Network Operations  . . 16
   5.  Observing Transport Flows with Encrypted Transport Header Fiel 16
     5.1.  Transport Information at the Network Layer . . . . . . . . 16
     5.2.  An Observable Transport Flow Identifier  . . . . . . . . . 17
       5.2.1.  A Method to Determine Header Format  . . . . . . . . . 17
       5.2.2.  Use of a Transport as a Substrate  . . . . . . . . . . 17
       5.2.3.  Support for Mobility and Flow Migration  . . . . . . . 18
       5.2.4.  Flow Start and Stop  . . . . . . . . . . . . . . . . . 18
     5.3.  Observable Transport Sequence Number . . . . . . . . . . . 19
     5.4.  Observable Transport Reception . . . . . . . . . . . . . . 19
     5.5.  Observable Transport Timestamps  . . . . . . . . . . . . . 20
     5.6.  Observable ECN Transport Feedback Information  . . . . . . 20
     5.7.  Other Observable Transport Fields  . . . . . . . . . . . . 20
     5.8.  Interpretation of Transport Header Fields  . . . . . . . . 21
     5.9.  Requirements for Transport Measurement . . . . . . . . . . 21
   6.  The Effect of Encrypting Transport Header Fields . . . . . . . 22
     6.1.  Independent Measurement  . . . . . . . . . . . . . . . . . 22
     6.2.  Characterising "Unknown" Network Traffic . . . . . . . . . 23

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     6.3.  Accountability and Internet Transport Portocols  . . . . . 23
   7.  Implications on Evolution of the Internet Transport  . . . . . 24
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 28
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 28
   Appendix A. Revision information . . . . . . . . . . . . . . . . . 32
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32

1.  Introduction

   This document discusses the implications of end-to-end encryption
   applied at the transport layer, and examines the impact on transport
   protocol design, transport use, and network operations and
   management.  It also considers some anticipated implications on
   transport and application evolution.

   The transport layer is the first end-to-end layer in the network
   stack.  Despite headers having end-to-end meaning, some transport
   headers have come to be used in various ways within the Internet.  In
   response to pervasive monitoring [RFC7624] revelations and the IETF
   consensus that "Pervasive Monitoring is an Attack" [RFC7258], efforts
   are underway to increase encryption of Internet traffic, which would
   prevent visibility of transport headers.  This has implications on
   how network protocols are designed and used [I-D.mm-wg-effect-
   encrypt].

   Transport information that is sent without end-to-end integrity check
   could be modified by "middleboxes" - defined as any intermediary box
   performing functions apart from normal, standard functions of an IP
   router on the data path between a source host and destination host
   [RFC3234].  When transport headers are modified by network devices on
   the path, this can change the end-to-end protocol transport protocol
   behaviour in a way that may have benefits (e.g., to user performance/
   cost) or may hinder (e.g., disrupting application experience).
   Whatever the outcome, modification of packets by a middlebox was not
   usually intended when the protocol was specified and is usually not
   known by the sending or receiving endpoints.

   Middleboxes have been deployed for a variety of reasons [RFC3234],
   including protocol enhancement, proxies such as Protocol Enhancing
   Proxies (PEPs) [RFC3135], TCP acknowledgement (ACK) enhancement
   [RFC3449], use by application protocol caches [I-D.mm-wg-effect-
   encrypt], application layer gateways [I-D.mm-wg-effect-encrypt], etc.
   [I-D.dolson-plus-middlebox-benefits] summarizes some of the functions
   provided by such middleboxes, and benefits that may arise when used
   in specific deployment scenarios.  Such methods, which involve in-
   network modification of transport headers, are not further discussed.

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   Transport protocols can be designed to encrypt or authenticate
   transport header fields.  Authentication methods at the transport
   layer can detect any changes to an immutable header field that were
   made by a network device along a path.  These methods do not require
   encryption of the header fields, and hence authenticated fields may
   remain visible to network devices.  A receiving transport endpoint
   can use an integrity check to avoid accepting modified protocol
   headers.  This document therefore considers the case where transport
   header fields are not altered as a packet traverses the network path.

   Authentication methods (that provide integrity checks of protocols
   fields) have also been specified at the network layer, and this also
   protects transport header fields.  The network layer itself carries
   protocol header fields that are increasingly used to help forwarding
   decisions reflect the need of transport protocols, such the IPv6 Flow
   Label [RFC6437], the Differentiated Services Code Point (DSCP)
   [RFC2474] and Explicit Congestion Notification (ECN) [RFC3168].

   Encryption methods can hide information from an eavesdropper in the
   network.  Encryption can also help protect the privacy of a user, by
   hiding data relating to user/device identity or location.  Neither an
   integrity check nor encryption methods prevent traffic analysis, and
   usage needs to reflect that profiling of users and fingerprinting of
   behaviour can take place even on encrypted traffic flows.

   This document seeks to identify the implications of various
   approaches to transport protocol authentication and encryption.

2.  Internet Transports and Pervasive Encryption

   End-to-end encryption can be applied at various protocol layers.  It
   can be applied above the transport to encrypt the transport payload.
   One motive to use encryption is a response to perceptions that the
   network has become ossified by over-reliance on middleboxes that
   prevent new protocols and mechanisms from being deployed.  This has
   lead to a common perception that there is too much "manipulation" of
   protocol headers within the network, and that designing to deploy in
   such networks is preventing transport evolution.  In the light of
   this, a method that authenticates transport headers may help improve
   the pace of transport development, by eliminating the need to always
   consider deployed middleboxes [I-D.trammell-plus-abstract-mech], or
   potentially to only explicitly enable middlebox use for particular
   paths with particular middleboxes [RFC3135].

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   Another perspective stems from increased concerns about privacy and
   surveillance.  Some Internet users have valued the ability to protect
   identity and defend against traffic analysis, and have used methods
   such as IPsec ESP and Tor [Tor].  Revelations about the use of
   pervasive surveillance [RFC7624] have, to some extent, eroded trust
   in the service offered by network operators, and following the
   Snowden revelation in the USA in 2013 has led to an increased desire
   for people to employ encryption to avoid unwanted "eavesdropping" on
   their communications.  Whatever the reasons, there are now activities
   in the IETF to design new protocols that may include some form of
   transport header encryption (e.g., QUIC [I-D.ietf-quic-transport]).

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

   o  On the one hand, future Internet protocols that enable large-scale
      encryption assist in the restoration of the end-to-end nature of
      the Internet by returning complex processing to the endpoints,
      since middleboxes cannot modify what they cannot see.
   o  On the other hand, encryption of transport layer header
      information has implications to people responsible for operating
      networks and researchers and analysts seeking to understand the
      dynamics of protocols and traffic patterns.

   Whatever the motives, a decision to use pervasive of transport header
   encryption will have implications on the way in which design and
   evaluation is performed, and which can in turn impact the direction
   of evolution of the TCP/IP stack.

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

2.1.  Authenticating the Transport Protocol Header

   Transport layer header information can be authenticated.  An
   integrity check that protects the immutable transport header fields,
   but can still expose the transport protocol header information in the
   clear, allowing in-network devices to observes these fields.  An
   integrity check can not prevent in-network modification, but can
   avoid accepting changes and avoid impact on the transport protocol
   operation.

   An example transport authentication mechanism is TCP-Authentication
   (TCP-AO) [RFC5925].  This TCP option authenticates TCP segments,
   including the IP pseudo header, TCP header, and TCP data.  TCP-AO
   protects the transport layer, preventing attacks from disabling the
   TCP connection itself.  TCP-AO may interact with middleboxes,

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   depending on their behavior [RFC3234].

   The IPSec Authentication Header (AH) [RFC4302] works at the network
   layer and authenticates the IP payload.  This therefore also
   authenticates all transport headers, and verifies their integrity at
   the receiver, preventing in-network modification.

2.2.  Encrypting the Transport Payload

   The transport layer payload can be encrypted to protect the content
   of transport segments.  This leaves transport protocol header
   information in the clear.  The integrity of immutable transport
   header fields could be protected by combining this with an integrity
   check (Section 2.1).

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

2.3.  Encrypting the Transport Header

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

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

2.4.  Authenticating Transport Information and Selectively Encrypting
      the Transport Header

   A transport protocol design can encrypt selected header fields, while
   also choosing to authenticate fields in the transport header.  This
   allows specific transport header fields to be made observable by
   network devices.  End-to end integrity checks can prevent an endpoint
   from undetected modification of the immutable transport headers.

   The choice of which fields to expose and which to encrypt is a design
   choice for the transport protocol.  Any selective encryption method
   requires trading two conflicting goals for a transport protocol
   designer to decide which header fields to encrypt.  On the one hand,
   security work typically employs a design technique that seeks to
   expose only what is needed.  On the other hand, there may be
   performance and operational benefits in exposing selected information
   to network tools.

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   Mutable fields in the transport header provide opportunities for
   middleboxes to modify the transport behaviour (e.g., the extended
   headers described in [I-D.trammell-plus-abstract-mech]). This
   considers only immutable fields in the transport headers, that is,
   fields that may be authenticated end-to-end across a path.

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

2.5.  Adding Transport Information to Network-Layer Protocol Headers

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

   Some measurements may be made by adding additional protocol headers
   carrying operations, administration and management (OAM) information
   to packets at the ingress to a maintenance domain (e.g., an Ethernet
   protocol header with timestamps and sequence number information using
   a method such as 802.11ag) and removing the additional header at the
   egress of the maintenance domain.  This approach enables some types
   of measurements, but does not cover the entire range of measurements
   described in this document.

   Another example of a network-layer approach is the IPv6 Performance
   and Diagnostic Metrics (PDM) Destination Option [I-D.ietf-ippm-6man-
   pdm-option].  This allows a sender to optionally include a
   destination option that caries header fields that can be used to
   observe timestamps and packet sequence numbers.  This information
   could be authenticated by receiving transport endpoints when the
   information is added at the sender and visible at the receiving
   endpoint, although methods to do this have not currently been
   proposed.  This method needs to be explicitly enabled at the sender.

   A drawback of using extension headers is that IPv4 network options
   are often not supported (or are carried on a slower processing path)
   and some IPv6 networks are also known to drop packets that set an
   IPv6 header extension.  Another disadvantage is that protocols that
   separately expose header information do not necessarily have an
   advantage to expose the information that is utilised by the protocol
   itself, and could manipulate this header information to gain an

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   advantage from the network.

3.  Use of Transport Headers in the Network

   This section identifies ways that actors can benefit by observing
   (non-encrypted) transport header fields at devices in the network.
   The list of actors who perform measurements include:

   o  Protocol developers and implementors of TCP/IP stacks;
   o  Researchers working on new mechanisms;
   o  Use of new applications using existing applications;
   o  Analysis researching the impact of mechanisms on network equipment
      or specific network topologies;
   o  Staff supporting operation of a network.

   One approach is to use active measurement using dedicated tools to
   generate and measure test traffic.  To test a transport path, such
   active tools need to be run from an endpoint, and most operators do
   not have access to user equipment.  There may also be costs
   associated with running such tests (e.g., the implications of
   bandwidth tests in a mobile network are obvious). Some active
   measurements (e.g., response under load or particular workloads) may
   perturb other traffic, and could require dedicated access to the
   network segment.  An alternative approach is to use in-network
   techniques that observe transport packet headers in operational
   networks to make the measurements.

   Transport layer information can help identify whether the link/
   network tuning is effective and alert to potential problems that can
   be hard to derive from link or device measurements alone.  The design
   trade offs for radio networks are often very different to those of
   wired networks.  A radio-based network (e.g., cellular mobile,
   enterprise WiFi, satellite access/backhaul, point-to-point radio) has
   the complexity of a subsystem that performs radio resource management
   - with direct impact on the available capacity, and potentially loss/
   reordering of packets.  The impact of the pattern of loss and
   congestion, differs for different traffic types, correlation with
   propagation and interference can all have significant impact on the
   cost and performance of a provided service.  The need for this type
   of information is expected to increase as operators bring together
   heterogeneous types of network equipment and seek to deploy
   opportunistic methods to access radio spectrum.

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

   o  Flows, need to be identified at the level required for monitoring;
   o  The protocol and version of the header that is being used.  As
      protocols evolve over time and there may be a need to introduce
      new transport headers.  This may require interpretation of
      protocol version information or connection setup information;
   o  The position and syntax of any transport headers that need to be
      observed.  IETF transport protocols specify this information.

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

3.1.  Use to Identify Flows and Packet Formats

   Transport protocol header information can identify a flow and the
   connection state of the flow, together with the protocol options
   being used.  In some usages, a low-numbered (well-known ) port that
   can identify a protocol (although port information alone is not
   sufficient to guarantee identification of a protocol). Transport
   protocols, such as TCP and SCTP specify a standard base header that
   includes sequence number information and other data, with the
   possibility to negotiate additional headers at connection setup and
   identified by an option number in the transport header.  UDP-based
   protocols sometimes do not use well-known ports but also can instead
   be identified by signalling protocols or through the use of magic
   numbers placed in the first byte(s) of the datagram payload.

3.2.  Measurements derived from Transport Header Information

   Some actors have a need to characterise the performance of link/
   network segments.  Passive monitoring uses observed traffic to makes
   inferences from transport headers to derive measurements.  A variety
   of open source and commercial tools can utilise this information.
   Transport fields in the Real Time Protocol (RTP) header[RFC3550]
   [RFC4585] can be observed to derive traffic volume measurements and
   provide information on the progress and quality of a session using
   RTP. Key performance indicators are retransmission rate, packet drop
   rate, sector utilization level, a measure of reordering, peak rate,
   the CE-marking rate, etc.  Metadata is often important to understand
   the context under which the data was collected, including the time,
   observation point, and way in which metrics were accumulated.

   Some Internet transports report summary performance data that is
   observable in the network (e.g., RTCP feedback[RFC3550]). A user of
   summary measurement data needs to trust the source of this data and
   the method used to generate the summary information.

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

3.2.1.  Use to Characterise Traffic Rate and Volume

   Transport headers may be observed to derive volume measures per-
   application, to characterise the traffic using a network segment and
   pattern of network usage.  This may be measured per endpoint or

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   aggregate of endpoint (e.g., by an operator to assess subscriber
   usage). This can also be used to trigger measurement-based traffic
   shaping and to implement QoS support within the network and lower
   layers.  Volume measures can be valuable for capacity planning
   (providing detail of trends rather than the volume per subscriber).

3.2.2.  Measuring Loss Rate and Loss Pattern

   Flow loss rate is often used as a metric for performance assessment
   and to characterise the transport behaviour.  Understanding the root
   cause of loss can help an operator determine whether this requires
   corrective action.

   There are various cause of loss, including: corruption on a link
   (e.g., interference on a radio link), buffer overflow (e.g., due to
   congestion), policing (traffic management), buffer management (e.g.,
   Active Queue Management (AQM). Loss can be monitored at the interface
   level by devices in the network.  It is often important to understand
   the conditions under which packet loss occurs, which usually means
   relating loss to the traffic flowing on the network segment at the
   time of loss.  Understanding flow loss rate requires either
   maintaining per flow packet counters or by observing sequence numbers
   in transport headers.

   Observation of transport feedback information (observing loss
   reports, e.g., RTCP, TCP SACK) can increase understanding of the
   impact of loss and help identify cases where loss may have been
   wrongly identified, or the transport did not require the lost packet.
   It is sometimes more important to understand the pattern of loss,
   than the loss rate - since losses can often occur as bursts, rather
   than randomly timed events.

3.2.3.  Measuring Throughput and Goodput

   The throughput observed by a flow can be determined even when a flow
   is encrypted, providing the individual flow can be identified.
   Goodput [RFC7928] is a measure of useful data exchanged (the ratio of
   useful/total volume of traffic sent by a flow), which requires
   ability to differentiate loss and retransmission of packets (e.g., by
   observing packet sequence numbers in TCP or RTP).

3.2.4.  Measuring Latency (Network Transit Delay and Jitter)

   Latency is a key performance metric that impacts application response
   time and user-perceived response time.  It also often indirectly
   impacts throughput and flow completion time.  Latency determines the
   reaction time of the transport protocol itself, impacting flow setup,
   congestion control, loss recovery, and other transport mechanisms.
   The observed latency can have many components [Latency].  Of these,
   unnecessary/unwanted queuing in network buffers has often been
   observed as a significant factor.  Once the cause of unwanted latency
   has been identified, this can often be eliminated, and determining
   latency metrics is a key driver in the deployment of AQM [RFC7567],
   DiffServ [RFC2474], and ECN [RFC3168] [RFC8087].

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

   An example usage of this method could be to identify excessive
   buffers and to deploy or configure Active Queue Management (AQM)
   [RFC7567] [RFC7928].  Operators deploying such tools can effectively
   eliminate unnecessary queuing in routers and other devices.  AQM
   methods need to be deployed at the capacity bottleneck, but are often
   deployed in combination with other techniques, such as scheduling
   [RFC7567] [I-D.ietf-aqm-fq-codel] and although parameter-less methods
   are desired [RFC7567], current methods [I-D.ietf-aqm-fq-codel] [I-D
   .ietf-aqm-codel] [I-D.ietf-aqm-pie] often cannot scale across all
   possible deployment scenarios.  The service offered by operators can
   therefore benefit from latency information to understand the impact
   of deployment and tune deployed services.

   Some network applications are sensitive to packet jitter, and it can
   be necessary to measure the jitter observed along a portion of the
   path.  The requirements to measure jitter resemble those for the
   measurement of latency.

3.2.5.  Measuring Flow Reordering

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

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

   There has been initiatives in the IETF transport area to reduce the
   impact of reordering withing a transport flow, possibly leading to
   reduced the requirements for ordering.  These have promise to
   simplify network equipment design as well as the potential to improve
   robustness of the transport service.  Measurements of reordering can
   help understand the level of reordering within deployed
   infrastructure, and inform decisions about how to progress such
   mechanisms.

3.3.  Measurements derived from Network-Transport Information

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   This section describes transport information that is already
   observable in network-layer header fields.

3.3.1.  Use of IPv6 Network-Layer Flow Label

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

3.3.2.  Use Network-Layer Differentiated Services Code Point Point

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

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

3.3.3.  Use of Explicit Congestion Marking

   Explicit Congestion Notification (ECN)[RFC3168] uses a codepoint in
   the network-layer header.  Use of ECN can offer gains in terms of
   increased throughput, reduced delay, and other benefits when used
   over a path that includes equipment that supports an AQM method that
   performs Congestion Experienced (CE) marking of IP packets [RFC8087].

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

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   Some ECN-capable network devices can provide richer (more frequent
   and fine-grained) indication of their congestion state.  Setting
   congestion marks proportional to the level of congestion (e.g., DCTP
   [I-D.ietf-tcpm-dctcp], and L4S [I-D.ietf-tsvwg-l4s-arch]).

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

   Section 5.6 describes the transport layer feedback information that
   accompanies the use of ECN.

4.  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 analyze.  For most packets, this has been transport
   layer, until the emergence of QUIC, with the obvious exception of
   VPNs and IPsec.  When encryption conceals more layers in a packet,
   people seeking understanding of the network operation need to rely
   more on pattern inferences and other heuristics.  The accuracy of
   measurements therefore suffers, as does the ability to investigate
   and troubleshoot interactions between different anomalies.  For
   example, the traffic patterns between server and browser are
   dependent on browser supplier and version, even when the sessions use
   the same server application (e.g., web e-mail access). Even when
   measurement datasets are made available (e.g., from endpoints)
   additional metadata, such as the state of the network, is often
   required to interpret the data.  Collecting and coordinating such
   metadata is more difficult when the observation point is at a
   different location to the bottleneck/device under evaluation.

   Packet sampling techniques can be used to scale processing involved
   in observing packets on high rate links.  This only exports 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 transport measurement.

4.1.  Point of Measurement

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

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

4.2.  Use by Operators to Plan and Provision Networks

   Traffic measurements (e.g.  Traffic volume, loss, latency) is used by
   operators to help plan deployment of new equipment and configurations
   in their networks.  Data is also important to equipment vendors who
   need to understand traffic trends traffic and patterns of usage as
   inputs to decisions about planning products and provisioning for new
   deployments.  This measurement information can also be correlated
   with billing information when this is also collected by an operator.

   A network operator supporting traffic that uses transport header
   encryption may not have access to per-flow measurement data.  Trends
   in aggregate traffic can be observed and can be related this to the
   endpoint addresses being used, but it may not be possible to
   correlate patterns in measurements with changes in transport
   protocols (e.g., the impact of changes in introducing a new transport
   protocol mechanism). This increases the dependency on other indirect
   sources of information to inform planning and provisioning.

4.3.  Service Performance Measurement

   Traffic measurements (e.g., traffic volume, loss, latency) can be
   used by various actors to help understand the performance available
   to users of a network segment.  While active measurements may be used
   in-network passive measurements can have advantages in terms of
   eliminating unproductive traffic, reducing the influence of test
   traffic on the overall traffic mix, and the ability to choose the
   point of measurement Section 4.1.

4.4.  Use for Network Diagnostics and Troubleshooting

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

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   A network operator supporting traffic that uses transport header
   encryption can see only encrypted transport headers.  This prevents
   deployment of performance measurement tools that rely on transport
   protocol information.  Choosing to encrypt all information may be
   expected to reduce the ability for networks to "help" (e.g., in
   response to tracing issues, making appropriate Quality of Service,
   QoS, decisions). For some this will be blessing, for others it may be
   a curse.  For example, operational performance data about encrypted
   flows needs to be determined by traffic pattern analysis, rather than
   relying on traditional tools.  This can impact the ability of the
   operator to respond to faults, it could require reliance on endpoint
   diagnostic tools or user involvement in diagnosing and
   troubleshooting unusual use cases or non-trivial problems.  Although
   many network operators utilise transport information as a part of
   their operational practice, the network will not break because
   transport headers are encrypted.

4.5.  Acceptable Response to Congestion

   Congestion control is a key transport function.  Many network
   operators implicitly accept that TCP traffic to comply with a
   behaviour that is acceptable for use in the shared Internet.  TCP
   algorithms have been continuously improved over decades, and they
   have reached a level of efficiency and correctness that custom
   application-layer mechanisms will struggle to easily duplicate
   [RFC8085].  A standards-compliant TCP stack provides congestion
   control that is therefore judged safe for use across the Internet.
   Applications developed on top of well-designed transports can be
   expected to appropriately control their network usage, reacting when
   the network experiences congestion, by back-off and reduce the load
   placed on the network.  This is the normal expected behaviour for TCP
   and other IETF-defined transports.

   Tools exist that can interpret the transport protocol header
   information to help understand the impact of specific transport
   protocols (or protocol mechanisms) on other traffic that shares their
   network.  An observation in the network can gain understanding of the
   dynamics of a flow and its congestion control behaviour.  Analysing
   observed packet sequence numbers can be used to help build confidence
   that an application flow backs-off its share of the network load in
   the face of persistent congestion, and hence to understand whether
   the behaviour is appropriate for sharing limited network capacity.
   For example, it is common to visualise plots of TCP sequence numbers
   versus time for a flow to understand how a flow shares available
   capacity, deduce its dynamics in response to congestion, etc.

4.5.1.  Measuring Compliance of UDP Traffic

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

   A network operator has no way of knowing the specific methods used by
   a UDP application, unless the header format can be determined.  Tools
   are needed to understand if UDP flows comply with congestion control
   expectations and therefore whether there is a need to deploy methods
   such as rate-limiters, transport circuit breakers or other methods to
   enforce acceptable usage.  UDP flows that expose a well-known header
   by specifying the format of header fields can allow information to be
   observed that gains understanding of the dynamics of a flow and its
   congestion control behaviour.  For example, tools exist to monitor
   various aspects of the RTP and RTCP header information of real-time
   flows (see Section 3.2).

4.5.2.  Measuring Transport to Support Network Operations

   By correlating observations 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).

   Information provided by tools can help determine whether mechanisms
   are needed in the network to prevent flows from acquiring excessive
   network capacity.  Operators can manage traffic flows (e.g., to
   prevent flows from acquiring excessive network capacity under severe
   congestion) by deploying rate-limiters, traffic shaping or network
   transport circuit breakers [RFC8084].

5.  Observing Transport Flows with Encrypted Transport Header Fields

   This section examines implications of encrypting specific transport
   header information.

5.1.  Transport Information at the Network Layer

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   Some transport information is made visible in the network-layer
   protocol header.  These header fields are not encrypted and can be
   used to make flow observations.  Endpoints should expose flow
   information in the IPv6 Flow Label Section 3.3.1 in the network-layer
   header.  This can be used to inform network-layer queuing, forwarding
   (e.g., for equal cost multi-path (ECMP) routing, and Link Aggregation
   (LAG)). For transport measurement, this can provide useful
   information to assign packets to flows in the data collected by
   measurement campaigns, but does not directly provide any performance
   data.  Similarly the Differentiated Services CodePoint (DCSP)
   indicates expected forwarding treatment Section 3.3.2. The ECN field
   provides observable congestion data and can help inform measurement
   of flow congestion Section 3.3.3.

5.2.  An Observable Transport Flow Identifier

   To measure and analyse a transport protocol, a measurement tool needs
   to be able to identify traffic flows.  Aggregation of sessions, and
   persistent use of established transport flows by multiple sessions
   means that a flow at the transport layer is not necessarily the same
   as a flow seen at the application layer.  This is usually not a
   consequence.  Data is measured for the aggregate transport flow.

   Some measurement methods sample traffic, rather than collecting all
   packets passing through a measurement point.  These methods still
   require a way to determine the presence, size and position of any
   observable header fields - but may need to do this without observing
   a protocol exchange for a connection setup.

5.2.1.  A Method to Determine Header Format

   If flow information is observed from transport headers, then there
   needs to be a way to identify the format of the header Section 3.1.
   Some IETF transport protocols are identified by an IP protocol number
   (e.g.  ,TCP, SCTP, UDP). All IETF-defined transport protocols include
   a transport port field in their transport header.  Higher layer
   protocols (e.g., HTTP) can be sometimes be observed by a well-known
   port value, which can be indicative of the protocol being
   encapsulated, but there is no way to enforce this usage.  This can be
   used to configure decapsulation, alternatives include a "magic"
   number placed at the start of each UDP datagram.

   Once the protocol has been determined, the transport header can be
   determined from a published specification.  If multiple formats are
   permitted, this may also require observing the protocol version being
   used and possibly parameter negotiation at connection setup.

5.2.2.  Use of a Transport as a Substrate

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   When a transport is used as a substrate, the transport provides an
   encapsulation that allows another transport flow to be within the
   payload of a transport flow.  The transported protocol header may
   provide additional information for multiplexing multiple flows over
   the same 5-tuple.  The UDP Guidelines [RFC8085] provides some
   guidance on using UDP as a substrate protocol.  If there is no
   additional information about the protocol transported by the
   substrate, this may be viewed as an opaque traffic aggregate, and
   prevents transport measurement in the network.  Examples include GRE-
   in-UDP [RFC8086], SCTP-in-UDP. The GRE-in-UDP encapsulation may
   encrypt the payload, but does not encrypt the GRE protocol header.

5.2.3.  Support for Mobility and Flow Migration

   With the proliferation of mobile connected devices, there is a stated
   need for connection-oriented protocols to maintain connections after
   a network migration by an endpoint.  The ability and desirability of
   in-network devices to track such migration depends on the context.
   On the one hand, a load-balancer device in front of server may find
   it useful to map a migrated connection to the same server endpoint.
   On the other hand, a user performing migration to avoid detection may
   prefer the network not to be able to correlate the different parts of
   a migrating session.  Care must then be exercised to make sure that
   the information encoded by the endpoints is not sufficient to
   identify unique flows and facilitate a persistent surveillance attack
   vector [I-D.mm-wg-effect-encrypt].

   The impact of flow migration on measurement activities depends on the
   data being measured, rate of migration and level of encryption that
   is employed.  Requirements for load balancing and mobility can lead
   to complex protocol interactions.

5.2.4.  Flow Start and Stop

   Transports can expose that start and end of flows in a transport
   header field (e.g., TCP SYN, FIN, RST). This can also help
   measurement devices identify the start of flows, or to remove stale
   flow information.  This information is supplemental - flows can start
   and end at any time, the Internet network layer provides only a best
   effort service that allows alternate routing, reordering, loss, etc,
   so a network measurement tool can not rely upon observing these
   indicators.  The time to complete a protocol connection and/or
   session setup can be reported.

   Flow information can provide in-network devices to manage their
   forwarding state [I-D.trammell-plus-statefulness].  It can assist a
   firewall in deciding which flows are permitted through a security
   gateway, or to help maintain the network address translation (NAT)
   bindings in a NAPT or application layer gateway.  This information
   may also find use in load balancers, where visibility of the 5-tuple
   could assist in selecting a server [I-D.mm-wg-effect-encrypt].

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   Access to flow information and an observable start/stop indication
   [I-D.trammell-plus-statefulness] can avoid stateful middleboxes
   relying on timeouts to remove old state.  Without this, middleboxes
   are unaware when a particular flow ceases to be used by an
   application[RFC8085].  This can lead to the state table entries
   keeping state for less time for flows that are not identifiable.

5.3.  Observable Transport Sequence Number

   The TCP or RTP sequence number can be observed in one direction (the
   direction that carries data segments). An authenticated header
   prevents this field being modified or terminated/split [RFC3135] by a
   network device, but allows this still to be used to observe progress
   of the network flow.

   An incrementing sequence number enables detection of loss (either by
   correlating ingress and egress value, or when assuming that all
   packets follow a single path), duplication and reordering (with
   understanding that not necessarily all packets of a flow follow the
   same path, and reordering can complicate processing of observations).
   Tools are widely available to interpret RTP and TCP sequence numbers,
   ranging from open source tools to dedicated commercial packages.  As
   for TCP, use by in-network measurement devices needs to account for
   the impact of load-balancing of flows, changes in forwarding
   behaviour, measurement loss (rather than observed packet loss), etc.

5.4.  Observable Transport Reception

   Acknowledgement (ACK) data provides information about the path from
   the network device to the remote endpoint.  The information can help
   identify packet loss (or the point of loss), RTT, and other network-
   related performance parameters (e.g., throughput, jitter,
   reordering). Unless this information is correlated with other data
   there is no way to disambiguate the cause of impairments (congestion
   loss, link transmission loss, equipment failure).

   An in-network device must not modify the flow of end-to-end ACK data
   when using an authenticated protocol.  That is, must not use the in-
   network methods described in [RFC3449].  This can impact the
   performance and/or efficiency (e.g., cost) of using paths where the

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   return capacity is limited or has implications on the overall design
   (e.g., using TCP with cellular mobile uplinks, DOCSIS uplinks).

   The TCP stream can be observed by correlating the stream of TCP ACKs
   that flow from a receiver in the return direction.  Although these
   ACKs are cumulative, and are not necessarily sent on the same path as
   the forward data, when visible, their sequence can confirm successful
   transmission and the path RTT. In the case of TCP they may also
   indicate packet loss (duplicate ACKs).

   An RTP session can provide reception information [RFC3550] [RFC4585]
   feedback using the RTCP framework.  This reception information and
   can be observed by in-network measurement devices and can be
   interpreted to provide a variety of quality of experience information
   for the related RTP flow, as well as basic network performance data
   (RTT, loss, jitter, etc).

5.5.  Observable Transport Timestamps

   The use of timestamps for latency and jitter measurements Section
   3.2.4 is discussed in other sections of the current version of the
   document.

5.6.  Observable ECN Transport Feedback Information

   Transport protocols that use ECN Section 3.3.3 need to provide ECN
   feedback information in the transport header to inform the sender
   whether packets have been received with an ECN CE-mark [RFC3819].
   This information can be in the form of feedback once each RTT
   [RFC3819] or more frequent.  The latter may involve sending a
   detailed list of all ECN-marked packets (e.g., [I-D.ietf-tcpm-
   accurate-ecn] and [RFC6679]). The detailed information can provide
   detail about the pattern and rate of marking.  The information
   provided in these protocol headers can help a network operator to
   understand the congestion status of the forward path and the impact
   of marking algorithms on the traffic that is carried [RFC8087].

   IETF specifications for Congestion Exposure (CONVEX) [RFC7713] is an
   example of a framework that monitors reception reports for CE-marked
   packets to support network operations.

5.7.  Other Observable Transport Fields

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   This section is not complete - later revision may determine other
   fields or remove this section.

5.8.  Interpretation of Transport Header Fields

   Understanding and analysing transport protocol behaviour typically
   demands tracking changes to the protocol state at the transport
   endpoints.  Although protocols communicate state information in their
   protocol headers, a protocol implementation typically also contains
   internal state that is not directly visible from observing transport
   protocol headers.  Effective measurement tools need to consider that
   not all packets may be observed (due to drops at the capture tap or
   because packets take an alternate route that does not pass the tap).
   Some flows of packets may also be encapsulated within a maintenance
   domain in other protocols, which further complicates analysis.

   Some examples of using network measurements of transport headers to
   infer internal TCP transport state information include:

   o  The TCP congestion window (cwnd) and slow start threshold
      (ssthresh). Tools for analysing in-network performance of TCP may
      observe sequence number to infer the current congestion controller
      state.
   o  The TCP RTT estimator and TCP Retransmission Time Out (RTO) value.
      This can be estimated by correlating sequence and acknowledgement
      numbers, or possibly by observing TCP timestamp options.
   o  Use of pacing (and pacing rate) and use of methods such as
      Proportional Rate Reduction (PRR) and Congestion Window validation
      (CWV). This may be estimated from observing timing of segments
      with TCP sequence numbers.  This is important to some congestion
      control mechanisms and can be important for applications that are
      rate limited or send traffic bursts.
   o  Receiver window and flow control state.  This may be inferred from
      information in TCP ACK segments.  It is important to applications
      where the remote endpoint is resource constrained, or the path
      exhibits a large RTT.
   o  Retransmission state and receiver buffer.  This may be inferred
      from information in TCP ACK segments (especially when SACK blocks
      are provided), this can be important to the performance of
      applications that send traffic bursts.
   o  Use of ACK delay and Nagle algorithm.  This may be estimated from
      observing timing of segments with TCP sequence numbers, and is
      important to the performance of thin application flows.

5.9.  Requirements for Transport Measurement

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   Transport measurement introduces the following requirements to
   identify the observable information:

   o  Observable protocol type and version information is needed to
      identify the protocol being used when characterising the traffic,
      and to enable further observation of the flow.
   o  Observable format information is needed to allow an observer to
      determine the presence of any observable header fields.
   o  A published specification is needed to allow an observer to
      determine the size and position of any observable header fields so
      that these fields may be decoded by a measurement tool.
   o  Observable flow start/stop information can assist some forms of
      measurement and has utility for middleboxes that track state.

   The need for in-network transport measurement introduces the
   following requirements for observable information in transport header
   fields:

   o  Observable transport information to determine the progress of
      flows for each direction of communication.  This requires
      observable packet numbers.
   o  Observable transport information to determine loss, and understand
      the response to congestion for a network segment.  This requires
      observable reception information (e.g., packet acknowledgment
      information).
   o  Observable transport information is needed for more advanced
      measurement of latency, jitter, etc.  This requires an observable
      field and a method to correlating return information with the
      observed field.  This could utilise a packet number and/or
      transmission timestamp information.  This information needs to be
      available in both directions of transmission.
   o  Exposure of Transport ECN feedback provides a powerful tool to
      understand ECN-enabled AQM-based networks.  (Forward ECN
      information is already observable in the network header).

6.  The Effect of Encrypting Transport Header Fields

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

6.1.  Independent Measurement

   Independent observation by multiple actors is important for
   scientific analysis.  Encrypting transport header encryption changes
   the ability for other actors to collect and independently analyse
   data.  Internet transport protocols employ a set of mechanisms.  Some
   of these need to work in cooperation with the network layer - loss
   detection and recovery, congestion detection and congestion control,
   some of these need to work only end-to-end (e.g., parameter

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   negotiation, flow-control).

   When encryption conceals information in the transport header, it
   could be possible for an applications to provide summary data on
   performance and usage of the network.  This data could be made
   available to other actors.  However, this data needs to contain
   sufficient detail to understand (and possibly reconstruct the network
   traffic pattern for further testing) and to be correlated with the
   configuration of the network paths being measured.  Sharing
   information between actors needs also to consider the privacy of the
   user and the incentives for providing accurate and detailed
   information.  Protocols that expose the state information used by the
   transport protocol in their header information (e.g., timestamps used
   to calculate RTT, packet numbers used to asses congestion and
   requests for retransmission) provide an incentive for the sending
   endpoint to provide correct information, increasing confidence that
   the observer understands the transport interaction with the network.
   This becomes important when considering changes to transport
   protocols, changes in network infrastructure, or the emergence of new
   traffic patterns.

6.2.  Characterising "Unknown" Network Traffic

   If "unknown" or "uncharacterised" traffic patterns form a small part
   of the traffic aggregate passing through a network device or segment
   of the network the path, the dynamics of the uncharacterised traffic
   may not have a significant collateral impact on the performance of
   other traffic that shares this network segment.  Once the proportion
   of this traffic increases, the need to monitor the traffic and
   determine if appropriate safety measures need to be put in place.

   Tracking the impact of new mechanisms and protocols requires traffic
   volume to be measured and new transport behaviours to be identified.
   This is especially true of protocols operating over a UDP substrate.
   The level and style of encryption needs to be considered in
   determining how this activity is performed.  On a shorter timescale,
   information may also need to be collected to manage denial of service
   attacks against the infrastructure.

6.3.  Accountability and Internet Transport Portocols

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

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   The growth and diversity of applications and protocols using the
   Internet continues to expand - and 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 [I-D.ietf-tcpm-dctcp], and methods
   proposed for Low Latency Low Loss Scalable throughput, L4S).  For
   each new method 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.

   Measurement therefore has a critical role in the design of transport
   protocol mechanisms and their acceptance by the wider community
   (e.g., as a method to judge the safety for Internet deployment.  Open
   standards suggest that such evaluation needs to include independent
   observation and evaluation of performance data.

7.  Implications on Evolution of the Internet Transport

   The transport layer provides the first end-to-end interactions across
   the Internet.  Transport protocols are layered directly over the
   network service and are sent in the payload of network-layer packets.
   However, this simple architectural view hides one of the core
   functions of the transport - to discover and adapt to the properties
   of the Internet path that is currently being used.  The design of
   Internet transport protocols is as much about trying to avoid the
   unwanted side effects of congestion on a flow and other capacity-
   sharing flows, avoiding congestion collapse, adapting to changes in
   the path characteristics, etc., as it is about end-to-end feature
   negotiation, flow control and optimising for performance of a
   specific application.

   To achieve stable Internet operations the IETF transport community,
   has to date, relied heavily on measurement and insight provided from
   the wider community to understand the trade-offs and to inform
   selection of select appropriate mechanisms to ensure a safe, reliable
   and robust Internet since the 1990's.

   There are many motivations for deploying encrypted transports, and
   encryption of transport payloads.  The increasing public concerns
   about the interference with Internet traffic have led to a rapidly
   expanding deployment of encryption to protect end-user privacy, in
   protocols like QUIC. At the same time, network operators and access
   providers, especially in mobile networks, have come to rely on the
   in-network functionality provided by middleboxes both to enhance
   performance and support network operations.

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   This document has expanded upon the expected implications on
   operational practices when working with encrypted transport
   protocols, and offers insight into the potential benefit of
   authentication, encryption and techniques that require in-network
   devices to interpret specific protocol header fields.  It presents a
   need for architectural changes and consideration of approaches to the
   way network transport protocols are designed when using
   encryption[Measure].

   The use of encryption at the transport layer comes with implications
   that need to be considered:

   Troubleshooting and diagnostics: Encrypting all transport information
      eliminates the incentive for operators to troubleshoot what they
      cannot interpret: one flow experiencing packet loss looks like any
      other.  When transport header encryption prevents decoding the
      transport header (if sequence numbers and flow ID are obscured),
      and hence understanding the impact on a particular flow or flows
      that share a common network segment.  Encrypted traffic therefore
      implies "don't touch", and a likely first response will be "can't
      help, no trouble found", or the need to add complexity that comes
      with an additional operational cost [I-D.mm-wg-effect-encrypt].
      

   Open verifyable data: The use of transport header encryption may
      reduce the range of actors who can capture useful measurement
      data.  This may in future restrict the information sources
      available to the Internet community to understand the operation of
      the network and transport protocols, necessary to inform
      standardisation and design decisions for new protocols, equipment
      and operational practices.  There are dangers in a model where
      transport information is only observable at endpoints: i.e., at
      user devices and within service platforms and a need for
      independently captured data to develop open standards and
      stimulate research into new methods.
      

   Operational practice: Published transport specifications allow
      operators to check compliance.  This can bring assurance to those
      operating networks, often avoiding the need to deploy complex
      techniques that routinely monitor and manage TCP/IP traffic flows
      (e.g.  Avoiding the capital and operational costs of deploying
      flow rate-limiting and network circuit-breaker methods).  This
      should continue when encrypted transport headers are used, but
      methods need to confirm that the traffic produced conforms to the
      expectations of the operator or developer.
      

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   Traffic analysis: The use of encryption could make it harder to
      determine which transport methods are being used across a network
      segment and the trends in usage.  This could impact the ability
      for an operator to anticipate the need for network upgrades and
      roll-out.  It can also impact on-going traffic engineering
      activities.  Although the impact in many case may be small, there
      are cases where operators directly support services (e.g., in
      radio links, or to troubleshoot QoS-related issues). The more
      complex the underlying infrastructure the more important this
      impact.
      

   Interactions between mechanisms: An appropriate vantage point,
      coupled with timing information for the flow (fine-grained
      timestamps) is a valuable tool in benchmarking equipment/
      configurations and understanding non-trivial interactions.
      Encryption restricts the ability to explore interactions between
      functions at different protocol layers.  This is a side-effect of
      not allowing a choice of the vantage point from which this
      information is observed.  This can be important (e.g., in
      examining collateral impact of flows sharing a bottleneck, or
      where the intention is to understand the interaction between a
      layer 2 function (e.g., radio resource management policy, a
      channel impairment, an AQM configuration, a Per Hop Behaviour
      (PHB) or scheduling method, and a transport protocol).
      

   Common specifications: Since the introduction of congestion control,
      TCP has continued to be the predominate transport, with a
      consistent approach to avoiding congestion collapse.  There is a
      risk that the diversity of transport mechanisms could also
      increase, with incentives to use a wide range of methods, this is
      not in itself a problem, nor is this a direct result of
      encryption.  Encryption of all headers places the onus on
      validation in the hands of developers.  While there is little to
      doubt that developers will seek to produce high quality code for
      their target use, it is not clear whether there is sufficient
      incentive to ensure good practice that benefits the wide diversity
      of requirements from the Internet community as a whole.  The use
      of encryption needs to be weighed against the reduced visibility
      of the interactions between traffic, the network and the
      mechanisms.  Especially, if a development cycle could focus on
      specific protocols/applications and then offer incentives for
      optimisations that could prove suboptimal for users or operators
      that utilise a network segments with different characteristics
      than targeted by the developer.
      

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   Restricting research and development: The use of encryption may
      impede independent research into new mechanisms, measurement of
      behaviour, and development initiatives.  Experience shows that
      transport protocols are complicated to design and complex to
      deploy, and that individual mechanisms need to be evaluated while
      considering other mechanism, across a broad range of network
      topologies and with attention to the impact on traffic sharing the
      capacity.  Adopting pervasive encryption of transport information
      could eliminate the independent self-checks that have previously
      been in place from research and academic contributors (e.g., the
      role of the IRTF ICCRG, and research publications in reviewing new
      transport mechanisms and assessing the impact of their
      experimental deployment).

   Pervasive use of transport header encryption can impact the ways that
   future protocols are designed and deployed.  The choice of whether
   candidate transport designs should encrypt their protocol headers
   therefore needs to be taken based not just on security
   considerations, but also on the impact on operating networks and the
   constrictions this may place on evolution of Internet protocols.
   While encryption of all transport information can help reduce
   ossification of the transport layer, it could result in ossification
   of the network service.  There can be advantages in providing a level
   of ossification of the header in terms of providing a set of open
   specified header fields that are observable from in-network devices.

8.  Acknowledgements

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

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

9.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.

10.  Security Considerations

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   This document is about design and deployment considerations for
   transport protocols.  Authentication, confidentiality protection, and
   integrity protection are identified as Transport Features by
   RFC8095".  As currently deployed in the Internet, these features are
   generally provided by a protocol or layer on top of the transport
   protocol; no current full-featured standards-track transport protocol
   provides these features on its own.  Therefore, these features are
   not considered in this document, with the exception of native
   authentication capabilities of TCP and SCTP for which the security
   considerations in RFC4895.

   Like congestion control mechanisms, security mechanisms are difficult
   to design and implement correctly.  It is hence recommended that
   applications employ well-known standard security mechanisms such as
   DTLS, TLS or IPsec, rather than inventing their own.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997, <http://www.rfc-editor.org/info/
              rfc2119>.

11.2.  Informative References

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

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

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

   [I-D.ietf-aqm-pie]
              Pan, R., Natarajan, P., Baker, F. and G. White, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", Internet-Draft draft-ietf-aqm-pie-00, October
              2014.

   [I-D.ietf-ippm-6man-pdm-option]
              Elkins, N., Hamilton, R. and m.  mackermann@bcbsm.com,
              "IPv6 Performance and Diagnostic Metrics (PDM) Destination
              Option", Internet-Draft draft-ietf-ippm-6man-pdm-
              option-10, May 2017.

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

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

   [I-D.ietf-tcpm-dctcp]
              Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", Internet-Draft draft-ietf-tcpm-
              dctcp-06, May 2017.

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

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

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

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

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

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

   [RFC2474]  Nichols, K., Blake, S., Baker, F. and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI
              10.17487/RFC2474, December 1998, <http://www.rfc-
              editor.org/info/rfc2474>.

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   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135, DOI
              10.17487/RFC3135, June 2001, <http://www.rfc-editor.org/
              info/rfc3135>.

   [RFC3168]  Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
              Explicit Congestion Notification (ECN) to IP", RFC 3168,
              DOI 10.17487/RFC3168, September 2001, <http://www.rfc-
              editor.org/info/rfc3168>.

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

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R. and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

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

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

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, DOI
              10.17487/RFC4302, December 2005, <http://www.rfc-
              editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, DOI 10.17487/RFC4303, December 2005, <http://www
              .rfc-editor.org/info/rfc4303>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C. and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI
              10.17487/RFC4585, July 2006, <http://www.rfc-editor.org/
              info/rfc4585>.

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, S.
              and J. Perser, "Packet Reordering Metrics", RFC 4737, DOI
              10.17487/RFC4737, November 2006, <http://www.rfc-
              editor.org/info/rfc4737>.

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   [RFC5236]  Jayasumana, A., Piratla, N., Banka, T., Bare, A. and R.
              Whitner, "Improved Packet Reordering Metrics", RFC 5236,
              DOI 10.17487/RFC5236, June 2008, <http://www.rfc-
              editor.org/info/rfc5236>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008, <http://www.rfc-editor.org/info/
              rfc5246>.

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

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

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/
              RFC6437, November 2011, <http://www.rfc-editor.org/info/
              rfc6437>.

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

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7525]  Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations
              for Secure Use of Transport Layer Security (TLS) and
              Datagram Transport Layer Security (DTLS)", BCP 195, RFC
              7525, DOI 10.17487/RFC7525, May 2015, <http://www.rfc-
              editor.org/info/rfc7525>.

   [RFC7567]  Baker, F.Ed.,  and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management", BCP
              197, RFC 7567, DOI 10.17487/RFC7567, July 2015, <http://
              www.rfc-editor.org/info/rfc7567>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C. and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624, DOI
              10.17487/RFC7624, August 2015, <http://www.rfc-editor.org/
              info/rfc7624>.

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

   [RFC7928]  Kuhn, N., Ed., Natarajan, P., Ed., Khademi, N.Ed.,  and D.
              Ros, "Characterization Guidelines for Active Queue
              Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July
              2016, <http://www.rfc-editor.org/info/rfc7928>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers", BCP
              208, RFC 8084, DOI 10.17487/RFC8084, March 2017, <http://
              www.rfc-editor.org/info/rfc8084>.

   [RFC8085]  Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://www.rfc-editor.org/info/rfc8085>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X. and T. Herbert, "GRE-in-
              UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, March
              2017, <http://www.rfc-editor.org/info/rfc8086>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087, DOI
              10.17487/RFC8087, March 2017, <http://www.rfc-editor.org/
              info/rfc8087>.

   [Tor]      The Tor Project,  ., "https://www.torproject.org", June
              2017.

Appendix A.  Revision information

   -00 This is an individual draft for the IETF community

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

   -01 This draft fixes textua errors.

   Comments from the community are welcome on the text and
   recommendations.

Author's Address

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

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