< draft-ietf-tsvwg-transport-encrypt-04.txt   draft-ietf-tsvwg-transport-encrypt-05.txt >
TSVWG G. Fairhurst TSVWG G. Fairhurst
Internet-Draft University of Aberdeen Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins Intended status: Informational C. Perkins
Expires: August 22, 2019 University of Glasgow Expires: September 10, 2019 University of Glasgow
February 18, 2019 March 9, 2019
The Impact of Transport Header Confidentiality on Network Operation and The Impact of Transport Header Confidentiality on Network Operation and
Evolution of the Internet Evolution of the Internet
draft-ietf-tsvwg-transport-encrypt-04 draft-ietf-tsvwg-transport-encrypt-05
Abstract Abstract
This document describes implications of applying end-to-end This document describes implications of applying end-to-end
encryption at the transport layer. It identifies in-network uses of encryption at the transport layer. It identifies in-network uses of
transport layer header information. It then reviews the implications transport layer header information. It then reviews the implications
of developing end-to-end transport protocols that use authentication of developing end-to-end transport protocols that use authentication
to protect the integrity of transport information or encryption to to protect the integrity of transport information or encryption to
provide confidentiality of the transport protocol header and expected provide confidentiality of the transport protocol header and expected
implications of transport protocol design and network operation. implications of transport protocol design and network operation.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 22, 2019. This Internet-Draft will expire on September 10, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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3.1. Observing Transport Information in the Network . . . . . 10 3.1. Observing Transport Information in the Network . . . . . 10
3.2. Transport Measurement . . . . . . . . . . . . . . . . . . 16 3.2. Transport Measurement . . . . . . . . . . . . . . . . . . 16
3.3. Use for Network Diagnostics and Troubleshooting . . . . . 20 3.3. Use for Network Diagnostics and Troubleshooting . . . . . 20
3.4. Header Compression . . . . . . . . . . . . . . . . . . . 21 3.4. Header Compression . . . . . . . . . . . . . . . . . . . 21
4. Encryption and Authentication of Transport Headers . . . . . 21 4. Encryption and Authentication of Transport Headers . . . . . 21
5. Addition of Transport Information to Network-Layer Protocol 5. Addition of Transport Information to Network-Layer Protocol
Headers . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Headers . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6. Implications of Protecting the Transport Headers . . . . . . 26 6. Implications of Protecting the Transport Headers . . . . . . 26
6.1. Independent Measurement . . . . . . . . . . . . . . . . . 26 6.1. Independent Measurement . . . . . . . . . . . . . . . . . 26
6.2. Characterising "Unknown" Network Traffic . . . . . . . . 28 6.2. Characterising "Unknown" Network Traffic . . . . . . . . 28
6.3. Accountability and Internet Transport Protocols . . . . . 28 6.3. Accountability and Internet Transport Protocols . . . . . 29
6.4. Impact on Operational Cost . . . . . . . . . . . . . . . 29 6.4. Impact on Operational Cost . . . . . . . . . . . . . . . 29
6.5. Impact on Research, Development and Deployment . . . . . 30 6.5. Impact on Research, Development and Deployment . . . . . 30
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33 8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
11. Informative References . . . . . . . . . . . . . . . . . . . 35 11. Informative References . . . . . . . . . . . . . . . . . . . 35
Appendix A. Revision information . . . . . . . . . . . . . . . . 42 Appendix A. Revision information . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
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This document discusses some consequences of applying end-to-end This document discusses some consequences of applying end-to-end
encryption at the transport layer. It reviews the implications of encryption at the transport layer. It reviews the implications of
developing end-to-end transport protocols that use encryption to developing end-to-end transport protocols that use encryption to
provide confidentiality of the transport protocol header, and provide confidentiality of the transport protocol header, and
considers the effect of such changes on transport protocol design and considers the effect of such changes on transport protocol design and
network operations. It also considers anticipated implications on network operations. It also considers anticipated implications on
transport and application evolution. transport and application evolution.
Transports are increasingly encrypting and authenticating the payload Transports are increasingly encrypting and authenticating the payload
(i.e., the application data carried within the transport connection) (i.e., the application data carried within the transport connection)
end-to-end. Such protection is encouraged, and the implications are end-to-end. Such protection is encouraged, and the implications of
not further discussed in this memo. protecting the payload are not further discussed in this memo.
2. Context and Rationale 2. Context and Rationale
The transport layer provides end-to-end interactions between The transport layer provides end-to-end interactions between
endpoints (processes) using an Internet path. Transport protocols endpoints (processes) using an Internet path. Transport protocols
layer directly over the network-layer service and are sent in the layer directly over the network-layer service and are sent in the
payload of network-layer packets. They support end-to-end payload of network-layer packets. They support end-to-end
communication between applications, supported by higher-layer communication between applications, supported by higher-layer
protocols, running on the end systems (or transport endpoints). This protocols, running on the end systems (or transport endpoints). This
simple architectural view hides one of the core functions of the simple architectural view hides one of the core functions of the
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and at the flow level. and at the flow level.
There are many motivations for deploying encrypted transports There are many motivations for deploying encrypted transports
[RFC7624] (i.e., transport protocols that use encryption to provide [RFC7624] (i.e., transport protocols that use encryption to provide
confidentiality of some or all of the transport-layer header confidentiality of some or all of the transport-layer header
information), and encryption of transport payloads (i.e. information), and encryption of transport payloads (i.e.
Confidentiality of the payload data). The increasing public concerns Confidentiality of the payload data). The increasing public concerns
about interference with Internet traffic have led to a rapidly about interference with Internet traffic have led to a rapidly
expanding deployment of encryption to protect end-user privacy, e.g., expanding deployment of encryption to protect end-user privacy, e.g.,
QUIC [I-D.ietf-quic-transport]. Encryption is also expected to form QUIC [I-D.ietf-quic-transport]. Encryption is also expected to form
a basis of future transport protocol designs. a basis for future transport protocol designs.
Some network operators and access providers have come to rely on the Some network operators and access providers have come to rely on the
in-network measurement of transport properties and the functionality in-network measurement of transport properties and the functionality
provided by middleboxes to both support network operations and provided by middleboxes to both support network operations and
enhance performance. There can therefore be implications when enhance performance. There can therefore be implications when
working with encrypted transport protocols that hide transport header working with encrypted transport protocols that hide transport header
information from the network. These present architectural challenges information from the network. These present architectural challenges
and considerations in the way transport protocols are designed, and and considerations in the way transport protocols are designed, and
ability to characterise and compare different transport solutions ability to characterise and compare different transport solutions
[Measure]. Implementations of network devices are encouraged to [Measure]. Implementations of network devices are encouraged to
avoid side-effects when protocols are updated. Introducing avoid side-effects when protocols are updated. Introducing
cryptographic integrity checks to header fields can also prevent cryptographic integrity checks to header fields can also prevent
undetected manipulation of the field by network devices, or undetected manipulation of the field by network devices, or
undetected addition of information to a packet. However, this does undetected addition of information to a packet. However, this does
not prevent inspection of the information by a device on path, and it not prevent inspection of the information by a device on path, and it
is possible that such devices could develop mechanisms that rely on is possible that such devices could develop mechanisms that rely on
the presence of such a field, or a known value in the field. the presence of such a field or a known value in the field.
Reliance on the presence and semantics of specific header information Reliance on the presence and semantics of specific header information
leads to ossification. An endpoint could be required to supply a leads to ossification. An endpoint could be required to supply a
specific header to receive the network service that it desires. In specific header to receive the network service that it desires. In
some cases, this could be benign or advantageous to the protocol some cases, this could be benign or advantageous to the protocol
(e.g., recognising the start of a connection, or explicitly exposing (e.g., recognising the start of a connection, or explicitly exposing
protocol information can be expected to provide more consistent protocol information can be expected to provide more consistent
decisions by on-path devices than the use of diverse methods to infer decisions by on-path devices than the use of diverse methods to infer
semantics from other flow properties); in other cases this is not semantics from other flow properties); in other cases this is not
beneficial (e.g., a mechanism implemented in a network device, such beneficial (e.g., a mechanism implemented in a network device, such
as a firewall, that required a header field to have only a specific as a firewall, that required a header field to have only a specific
known set of values could prevent the device from forwarding packets known set of values could prevent the device from forwarding packets
using a different version of a protocol that introduces a new feature using a different version of a protocol that introduces a new feature
that changes the value present in this field, preventing evolution of that changes the value present in this field, preventing the
the protocol). Experience developing Transport Layer Security evolution of the protocol). Experience developing Transport Layer
[RFC8446], required a design that recognised that deployed Security [RFC8446], required a design that recognised that deployed
middleboxes relied on the exposed information in TLS 1.2 middleboxes relied on the exposed information in TLS 1.2
Examples of the impact of ossification on transport protocol design Examples of the impact of ossification on transport protocol design
and ease of deployment can be seen in the case of Multipath TCP and ease of deployment can be seen in the case of Multipath TCP
(MPTCP) and the TCP Fast Open option. The design of MPTCP had to be (MPTCP) and the TCP Fast Open option. The design of MPTCP had to be
revised to account for middleboxes, so called "TCP Normalizers", that revised to account for middleboxes, so called "TCP Normalizers", that
monitor the evolution of the window advertised in the TCP headers and monitor the evolution of the window advertised in the TCP headers and
that reset connections if the window does not grow as expected. that reset connections if the window does not grow as expected.
Similarly, TCP Fast Open has had issues with middleboxes that remove Similarly, TCP Fast Open has had issues with middleboxes that remove
unknown TCP options, that drop segments with unknown TCP options, unknown TCP options, that drop segments with unknown TCP options,
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coded understanding of transport behaviour, and that interacted coded understanding of transport behaviour, and that interacted
poorly with transports that tried to change that behaviour. Other poorly with transports that tried to change that behaviour. Other
examples have included middleboxes that rewrite TCP sequence and examples have included middleboxes that rewrite TCP sequence and
acknowledgement numbers but are unaware of the (newer) SACK option acknowledgement numbers but are unaware of the (newer) SACK option
and don't correctly rewrite selective acknowledgements to match the and don't correctly rewrite selective acknowledgements to match the
changes made to the fixed TCP header. changes made to the fixed TCP header.
A protocol design that uses header encryption can provide A protocol design that uses header encryption can provide
confidentiality of some or all of the protocol header information. confidentiality of some or all of the protocol header information.
Encryption with secure key distribution prevents an on-path device Encryption with secure key distribution prevents an on-path device
from observing the header field. It therefore prevents mechanisms from observing the header field. It, therefore, prevents mechanisms
being built that directly rely on the information or seek to infer being built that directly rely on the information or seek to infer
semantics of an exposed header field. Using encryption to provide semantics of an exposed header field. Using encryption to provide
confidentiality of the transport layer brings some well-known privacy confidentiality of the transport layer brings some well-known privacy
and security benefits and can therefore help reduce ossification of and security benefits and can therefore help reduce ossification of
the transport layer. In particular, it is important that protocols the transport layer. In particular, it is important that protocols
either do not expose information where the usage could change in either do not expose information where the usage could change in
future protocols, or that methods that utilise the information are future protocols or that methods that utilise the information are
robust to potential changes as protocols evolve over time. To avoid robust to potential changes as protocols evolve over time. To avoid
unwanted inspection, a protocol could also intentionally vary the unwanted inspection, a protocol could also intentionally vary the
format and/or value of header fields (sometimes known as Greasing format and/or value of header fields (sometimes known as Greasing
[I-D.thomson-quic-grease]). However, while encryption hides the [I-D.thomson-quic-grease]). However, while encryption hides the
protocol header information, it does not prevent ossification of the protocol header information, it does not prevent ossification of the
network service. People seeking understanding of network traffic network service. People seeking to understand network traffic could
could come to rely on pattern inferences and other heuristics as the come to rely on pattern inferences and other heuristics as the basis
basis for network decision and to derive measurement data, creating for network decision and to derive measurement data, creating new
new dependencies on the transport protocol. dependencies on the transport protocol.
Specification of non-encrypted transport header fields explicitly Specification of non-encrypted transport header fields explicitly
allows protocol designers to make specific header information allows protocol designers to make specific header information
observable in the network. This supports other uses of this observable in the network. This supports other uses of this
information by on-path devices, and at the same time this can be information by on-path devices, and at the same time this can be
expected to lead to ossification of the transport header, because expected to lead to ossification of the transport header, because
network forwarding could evolve to depend on the presence and/or network forwarding could evolve to depend on the presence and/or
value of these fields. The decision about which transport headers value of these fields. The decision about which transport headers
fields are made observable offers trade-offs around authentication fields are made observable offers trade-offs around authentication
and confidentiality versus observability, network operations and and confidentiality versus observability, network operations and
management, and ossification. For example, a design that provides management, and ossification. For example, a design that provides
confidentiality of protocol header information can impact the confidentiality of protocol header information can impact the
following activities that rely on measurement and analysis of traffic following activities that rely on measurement and analysis of traffic
flows: flows:
Network Operations and Research: Observable transport headers enable Network Operations and Research: Observable transport headers enable
both operators and the research community to explicitly measure both operators and the research community to explicitly measure
and analyse protocol performance, network anomalies, and failure and analyse protocol performance, network anomalies, and failure
pathologies. pathologies.
This information can help inform capacity planning, and assist in This information can help inform capacity planning and assist in
determining the need for equipment and/or configuration changes by determining the need for equipment and/or configuration changes by
network operators. network operators.
The data can also inform Internet engineering research, and help The data can also inform Internet engineering research, and help
in the development of new protocols, methodologies, and in the development of new protocols, methodologies, and
procedures. Concealing the transport protocol header information procedures. Concealing the transport protocol header information
makes the stream performance unavailable to passive observers makes the stream performance unavailable to passive observers
along the path, and likely leads to the development of alternative along the path, and likely leads to the development of alternative
methods to collect or infer that data (for example heuristics methods to collect or infer that data (for example heuristics
based on analysis of traffic patterns). based on the analysis of traffic patterns).
Providing confidentiality of the transport payload, but leaving Providing confidentiality of the transport payload, but leaving
some, or all, of the transport headers unencrypted, possibly with some, or all, of the transport headers unencrypted, possibly with
authentication, can provide many of the privacy and security authentication, can provide many of the privacy and security
benefits while supporting operations and research, but at the cost benefits while supporting operations and research, but at the cost
of ossifying the transport headers. of ossifying the transport headers.
Protection from Denial of Service: Observable transport headers Protection from Denial of Service: Observable transport headers
currently provide useful input to classify traffic and detect currently provide useful input to classify traffic and detect
anomalous events (e.g., changes in application behaviour, anomalous events (e.g., changes in application behaviour,
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functions in equipment or adding traffic overhead). functions in equipment or adding traffic overhead).
Network Traffic Analysis: Hiding transport protocol header Network Traffic Analysis: Hiding transport protocol header
information can make it harder to determine which transport information can make it harder to determine which transport
protocols and features are being used across a network segment and protocols and features are being used across a network segment and
to measure trends in the pattern of usage. This could impact the to measure trends in the pattern of usage. This could impact the
ability for an operator to anticipate the need for network ability for an operator to anticipate the need for network
upgrades and roll-out. It can also impact the on-going traffic upgrades and roll-out. It can also impact the on-going traffic
engineering activities performed by operators (such as determining engineering activities performed by operators (such as determining
which parts of the path contribute delay, jitter or loss). While which parts of the path contribute delay, jitter or loss). While
the impact could, in many cases, be small there are scenarios this impact could, in many cases, be small, there are scenarios
where operators directly support particular services (e.g., to where operators directly support particular services (e.g., to
troubleshoot issues relating to Quality of Service, QoS; the troubleshoot issues relating to Quality of Service, QoS; the
ability to perform fast re-routing of critical traffic, or support ability to perform fast re-routing of critical traffic, or support
to mitigate the characteristics of specific radio links). The to mitigate the characteristics of specific radio links). The
more complex the underlying infrastructure the more important this more complex the underlying infrastructure the more important this
impact. impact.
Open and Verifiable Network Data: Hiding transport protocol header Open and Verifiable Network Data: Hiding transport protocol header
information can reduce the range of actors that can capture useful information can reduce the range of actors that can capture useful
measurement data. This limits the information sources available measurement data. This limits the information sources available
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transport protocols, so preventing access to the information transport protocols, so preventing access to the information
necessary to inform design decisions and standardisation of the necessary to inform design decisions and standardisation of the
new protocols and related operational practices. new protocols and related operational practices.
The cooperating dependence of network, application, and host to The cooperating dependence of network, application, and host to
provide communication performance on the Internet is uncertain provide communication performance on the Internet is uncertain
when only endpoints (i.e., at user devices and within service when only endpoints (i.e., at user devices and within service
platforms) can observe performance, and when performance cannot be platforms) can observe performance, and when performance cannot be
independently verified by all parties. The ability of other independently verified by all parties. The ability of other
stakeholders to review transport header traces can help develop stakeholders to review transport header traces can help develop
deeper insight into performance. In the heterogeneous Internet, deeper insight into performance and traffic contribution of
specific varients of a protocol. In the heterogeneous Internet,
this helps extend the range of topologies, vendor equipment, and this helps extend the range of topologies, vendor equipment, and
traffic patterns that are evaluated. traffic patterns that are evaluated.
Independently captured data is important to help ensure the health Independently captured data is important to help ensure the health
of the research and development communities. It can provide input of the research and development communities. It can provide input
and test scenarios to support development of new transport and test scenarios to support the development of new transport
protocol mechanisms, especially when this analysis can be based on protocol mechanisms, especially when this analysis can be based on
the behaviour experienced in a diversity of deployed networks. the behaviour experienced in a diversity of deployed networks.
Independently verifiable performance metrics might also be Independently verifiable performance metrics might also be
utilised to demonstrate regulatory compliance in some utilised to demonstrate regulatory compliance in some
jurisdictions, and to provide a basis for informing design jurisdictions, and to provide a basis for informing design
decisions. decisions.
The last point leads us to consider the impact of hiding transport The last point leads us to consider the impact of hiding transport
headers in the specification and development of protocols and headers in the specification and development of protocols and
standards. This has potential impact on: standards. This has a potential impact on:
o Understanding Feature Interactions: An appropriate vantage point, o Understanding Feature Interactions: An appropriate vantage point,
coupled with timing information about traffic flows, provides a coupled with timing information about traffic flows, provides a
valuable tool for benchmarking equipment, functions, and/or valuable tool for benchmarking equipment, functions, and/or
configurations, and to understand complex feature interactions. configurations, and to understand complex feature interactions.
An inability to observe transport protocol information can limit An inability to observe transport protocol information can limit
the ability to diagnose and explore interactions between features the ability to diagnose and explore interactions between features
at different protocol layers, a side-effect of not allowing a at different protocol layers, a side-effect of not allowing a
choice of vantage point from which this information is observed. choice of vantage point from which this information is observed.
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operator might use heuristics to guess that short UDP packets with operator might use heuristics to guess that short UDP packets with
regular spacing are carrying audio traffic. Operational practices regular spacing are carrying audio traffic. Operational practices
aimed at guessing transport parameters are out of scope for this aimed at guessing transport parameters are out of scope for this
document, and are only mentioned here to recognize that encryption document, and are only mentioned here to recognize that encryption
may not prevent operators from attempting to apply the same may not prevent operators from attempting to apply the same
practices they used with unencrypted transport headers. practices they used with unencrypted transport headers.
o Compliance: Published transport specifications allow operators and o Compliance: Published transport specifications allow operators and
regulators to check compliance. This can bring assurance to those regulators to check compliance. This can bring assurance to those
operating networks, often avoiding the need to deploy complex operating networks, often avoiding the need to deploy complex
techniques that routinely monitor and manage TCP/IP traffic flows techniques that routinely monitor and manage Internet traffic
(e.g., avoiding the capital and operational costs of deploying flows (e.g., avoiding the capital and operational costs of
flow rate-limiting and network circuit-breaker methods [RFC8084]). deploying flow rate-limiting and network circuit-breaker methods
When it is not possible to observe transport header information, [RFC8084]). When it is not possible to observe transport header
methods are still needed to confirm that the traffic produced information, methods are still needed to confirm that the traffic
conforms to the expectations of the operator or developer. produced conforms to the expectations of the operator or
developer.
o Restricting research and development: Hiding transport information o Restricting research and development: Hiding transport information
can impede independent research into new mechanisms, measurement can impede independent research into new mechanisms, measurement
of behaviour, and development initiatives. Experience shows that of behaviour, and development initiatives. Experience shows that
transport protocols are complicated to design and complex to transport protocols are complicated to design and complex to
deploy, and that individual mechanisms need to be evaluated while deploy, and that individual mechanisms need to be evaluated while
considering other mechanisms, across a broad range of network considering other mechanisms, across a broad range of network
topologies and with attention to the impact on traffic sharing the topologies and with attention to the impact on traffic sharing the
capacity. If this results in reduced availability of open data, capacity. If this results in reduced availability of open data,
it could eliminate the independent self-checks to the it could eliminate the independent self-checks to the
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information concealed ought to be carefully considered when information concealed ought to be carefully considered when
specifying new transport protocols. specifying new transport protocols.
3. Current uses of Transport Headers within the Network 3. Current uses of Transport Headers within the Network
Despite transport headers having end-to-end meaning, some of these Despite transport headers having end-to-end meaning, some of these
transport headers have come to be used in various ways within the transport headers have come to be used in various ways within the
Internet. In response to pervasive monitoring [RFC7624] revelations Internet. In response to pervasive monitoring [RFC7624] revelations
and the IETF consensus that "Pervasive Monitoring is an Attack" and the IETF consensus that "Pervasive Monitoring is an Attack"
[RFC7258], efforts are underway to increase encryption of Internet [RFC7258], efforts are underway to increase encryption of Internet
traffic,. Applying confidentiality to transport header fields would traffic. Applying confidentiality to transport header fields would
affect how protocol information is used [RFC8404]. To understand affect how protocol information is used [RFC8404]. To understand
these implications, it is first necessary to understand how transport these implications, it is first necessary to understand how transport
layer headers are currently observed and/or modified by middleboxes layer headers are currently observed and/or modified by middleboxes
within the network. within the network.
Transport protocols can be designed to encrypt or authenticate Transport protocols can be designed to encrypt or authenticate
transport header fields. Authentication at the transport layer can transport header fields. Authentication at the transport layer can
be used to detect any changes to an immutable header field that were be used to detect any changes to an immutable header field that were
made by a network device along a path. The intentional modification made by a network device along a path. The intentional modification
of transport headers by middleboxes (such as Network Address of transport headers by middleboxes (such as Network Address
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o Flows need to be identified at the level required to perform the o Flows need to be identified at the level required to perform the
observation; observation;
o The protocol and version of the header need to be visible, e.g., o The protocol and version of the header need to be visible, e.g.,
by defining the wire image [I-D.trammell-wire-image]. As by defining the wire image [I-D.trammell-wire-image]. As
protocols evolve over time and there could be a need to introduce protocols evolve over time and there could be a need to introduce
new transport headers. This could require interpretation of new transport headers. This could require interpretation of
protocol version information or connection setup information; protocol version information or connection setup information;
o The location and syntax of any observed transport headers needs to o The location and syntax of any observed transport headers need to
be known. IETF transport protocols can specify this information. be known. IETF transport protocols can specify this information.
The following subsections describe various ways that observable The following subsections describe various ways that observable
transport information has been utilised. transport information has been utilised.
3.1.1. Flow Identification 3.1.1. Flow Identification
Transport protocol header information (together with information in Transport protocol header information (together with information in
the network header), has been used to identify a flow and the the network header), has been used to identify a flow and the
connection state of the flow, together with the protocol options connection state of the flow, together with the protocol options
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Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements
could identify excessively large buffers, indicating where to could identify excessively large buffers, indicating where to
deploy or configure AQM. An AQM method is often deployed in deploy or configure AQM. An AQM method is often deployed in
combination with other techniques, such as scheduling [RFC7567] combination with other techniques, such as scheduling [RFC7567]
[RFC8290] and although parameter-less methods are desired [RFC8290] and although parameter-less methods are desired
[RFC7567], current methods [RFC8290] [RFC8289] [RFC8033] often [RFC7567], current methods [RFC8290] [RFC8289] [RFC8033] often
cannot scale across all possible deployment scenarios. cannot scale across all possible deployment scenarios.
Variation in delay: Some network applications are sensitive to small Variation in delay: Some network applications are sensitive to small
changes in packet timing (jitter). Short and long-term delay changes in packet timing (jitter). Short and long-term delay
variation can impact on the latency of a flow and the hence the variation can impact on the latency of a flow and hence the
perceived quality of applications using the network (e.g., jitter perceived quality of applications using the network (e.g., jitter
metrics are often cited when characterising paths supporting real- metrics are often cited when characterising paths supporting real-
time traffic). To assess the performance of such applications, it time traffic). To assess the performance of such applications, it
can be necessary to measure the variation in delay observed along can be necessary to measure the variation in delay observed along
a portion of the path [RFC3393] [RFC5481]. The requirements a portion of the path [RFC3393] [RFC5481]. The requirements
resemble those for the measurement of latency. resemble those for the measurement of latency.
Flow Reordering: Significant packet reordering within a flow can Flow Reordering: Significant packet reordering within a flow can
impact time-critical applications and can be interpreted as loss impact time-critical applications and can be interpreted as loss
by reliable transports. Many transport protocol techniques are by reliable transports. Many transport protocol techniques are
impacted by reordering (e.g., triggering TCP retransmission, or impacted by reordering (e.g., triggering TCP retransmission or re-
re-buffering of real-time applications). Packet reordering can buffering of real-time applications). Packet reordering can occur
occur for many reasons, from equipment design to misconfiguration for many reasons, from equipment design to misconfiguration of
of forwarding rules. Since this impacts transport performance, forwarding rules. Since this impacts transport performance,
network tools are needed to detect and measure unwanted/excessive network tools are needed to detect and measure unwanted/excessive
reordering. reordering.
There have been initiatives in the IETF transport area to reduce There have been initiatives in the IETF transport area to reduce
the impact of reordering within a transport flow, possibly leading the impact of reordering within a transport flow, possibly leading
to a reduction in the requirements for preserving ordering. These to a reduction in the requirements for preserving ordering. These
have promise to simplify network equipment design as well as the have potential to simplify network equipment design as well as the
potential to improve robustness of the transport service. potential to improve robustness of the transport service.
Measurements of reordering can help understand the present level Measurements of reordering can help understand the present level
of reordering within deployed infrastructure, and inform decisions of reordering within deployed infrastructure, and inform decisions
about how to progress such mechanisms. Key performance indicators about how to progress such mechanisms. Key performance indicators
are retransmission rate, packet drop rate, sector utilisation are retransmission rate, packet drop rate, sector utilisation
level, a measure of reordering, peak rate, the ECN congestion level, a measure of reordering, peak rate, the ECN congestion
experienced (CE) marking rate, etc. experienced (CE) marking rate, etc.
Metrics have been defined that evaluate whether a network has Metrics have been defined that evaluate whether a network has
maintained packet order on a packet-by-packet basis [RFC4737] and maintained packet order on a packet-by-packet basis [RFC4737] and
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multiple Flow labels to allow the network to independently forward multiple Flow labels to allow the network to independently forward
subflows. RFC6437 provides further guidance on choosing a flow subflows. RFC6437 provides further guidance on choosing a flow
label value, stating these "should be chosen such that their bits label value, stating these "should be chosen such that their bits
exhibit a high degree of variability", and chosen so that "third exhibit a high degree of variability", and chosen so that "third
parties should be unlikely to be able to guess the next value that parties should be unlikely to be able to guess the next value that
a source of flow labels will choose". To promote privacy, the a source of flow labels will choose". To promote privacy, the
Flow Label assignment needs to avoid introducing linkability that Flow Label assignment needs to avoid introducing linkability that
a network device may observe. Once set, a label can provide a network device may observe. Once set, a label can provide
information that can help inform network-layer queuing and information that can help inform network-layer queuing and
forwarding [RFC6438](e.g. for Equal Cost Multi-Path, ECMP, forwarding [RFC6438](e.g. for Equal Cost Multi-Path, ECMP,
routing, and Link Aggregation, LAG) [RFC6294]. [RFC6438] includes routing, and Link Aggregation, LAG) [RFC6294]. [RFC6438]
describes considerations when used with IPsec. describes considerations when used with IPsec.
Using the Network-Layer Differentiated Services Code Point: Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to the network Applications can expose their delivery expectations to the network
by setting the Differentiated Services Code Point (DSCP) field of by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications
identify different forwarding treatments for individual sub-flows identify different forwarding treatments for individual sub-flows
(audio vs. video) based on the value of the DSCP field (audio vs. video) based on the value of the DSCP field
[I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information [I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information
to inform network-layer queuing and forwarding, rather than an to inform network-layer queuing and forwarding, rather than an
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application headers via a multi-field classifier. application headers via a multi-field classifier.
Since the DSCP value can impact the quality of experience for a Since the DSCP value can impact the quality of experience for a
flow, observations of service performance need to consider this flow, observations of service performance need to consider this
field when a network path has support for differentiated service field when a network path has support for differentiated service
treatment. treatment.
Using Explicit Congestion Marking: ECN [RFC3168] is a transport Using Explicit Congestion Marking: ECN [RFC3168] is a transport
mechanism that utilises the ECN field in the network-layer header. mechanism that utilises the ECN field in the network-layer header.
Use of ECN explicitly informs the network-layer that a transport Use of ECN explicitly informs the network-layer that a transport
is ECN-capable, and requests ECN treatment of the flows packets. is ECN-capable, and requests ECN treatment of the flow. An ECN-
An ECN-capable transport can offer benefits when used over a path capable transport can offer benefits when used over a path with
with equipment that implements an AQM method with Congestion equipment that implements an AQM method with Congestion
Experienced (CE) marking of IP packets [RFC8087], since it can Experienced (CE) marking of IP packets [RFC8087], since it can
react to congestion without also having to recover from lost react to congestion without also having to recover from lost
packets. packets.
ECN exposes the presence of congestion. The reception of CE- ECN exposes the presence of congestion. The reception of CE-
marked packets can be used to estimate the level of incipient marked packets can be used to estimate the level of incipient
congestion on the upstream portion of the path from the point of congestion on the upstream portion of the path from the point of
observation (Section 2.5 of [RFC8087]). Interpreting the marking observation (Section 2.5 of [RFC8087]). Interpreting the marking
behaviour (i.e., assessing congestion and diagnosing faults) behaviour (i.e., assessing congestion and diagnosing faults)
requires context from the transport layer (such as path RTT). requires context from the transport layer (such as path RTT).
AQM and ECN offer a range of algorithms and configuration options. AQM and ECN offer a range of algorithms and configuration options.
Tools therefore need to be available to network operators and Tools therefore need to be available to network operators and
researchers to understand the implication of configuration choices researchers to understand the implication of configuration choices
and transport behaviour as use of ECN increases and new methods and transport behaviour as the use of ECN increases and new
emerge [RFC7567]. methods emerge [RFC7567].
Careful use of the network layer features can therefore help address Careful use of the network layer features can therefore help address
some of the reasons why the network inspects transport protocol some of the reasons why the network inspects transport protocol
headers. headers.
3.2. Transport Measurement 3.2. Transport Measurement
The common language between network operators and application/content The common language between network operators and application/content
providers/users is packet transfer performance at a layer that all providers/users is packet transfer performance at a layer that all
can view and analyse. For most packets, this has been the transport can view and analyse. For most packets, this has been the transport
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inferences and other heuristics reliance on pattern inferences and inferences and other heuristics reliance on pattern inferences and
accuracy suffers. For example, the traffic patterns between server accuracy suffers. For example, the traffic patterns between server
and browser are dependent on browser supplier and version, even when and browser are dependent on browser supplier and version, even when
the sessions use the same server application (e.g., web e-mail the sessions use the same server application (e.g., web e-mail
access). It remains to be seen whether more complex inferences can access). It remains to be seen whether more complex inferences can
be mastered to produce the same monitoring accuracy (see section be mastered to produce the same monitoring accuracy (see section
2.1.1 of [RFC8404]). 2.1.1 of [RFC8404]).
When measurement datasets are made available by servers or client When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network, is endpoints, additional metadata, such as the state of the network, is
often required to interpret this data. Collecting and coordinating often required to interpret this data to answer questions about
such metadata is more difficult when the observation point is at a network performance or understand a pathology. Collecting and
different location to the bottleneck/device under evaluation. coordinating such metadata is more difficult when the observation
point is at a different location to the bottleneck/device under
evaluation.
Packet sampling techniques can be used to scale the processing Packet sampling techniques are used to scale the processing involved
involved in observing packets on high rate links. This exports only in observing packets on high rate links. This exports only the
the packet header information of (randomly) selected packets. The packet header information of (randomly) selected packets. The
utility of these measurements depends on the type of bearer and utility of these measurements depends on the type of bearer and
number of mechanisms used by network devices. Simple routers are number of mechanisms used by network devices. Simple routers are
relatively easy to manage, a device with more complexity demands relatively easy to manage, a device with more complexity demands
understanding of the choice of many system parameters. This level of understanding of the choice of many system parameters. This level of
complexity exists when several network methods are combined. complexity exists when several network methods are combined.
This section discusses topics concerning observation of transport This section discusses topics concerning observation of transport
flows, with a focus on transport measurement. flows, with a focus on transport measurement.
3.2.1. Point of Observation 3.2.1. Point of Observation
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Sometimes multiple on-path observation points are needed. By Sometimes multiple on-path observation points are needed. By
correlating observations of headers at multiple points along the path correlating observations of headers at multiple points along the path
(e.g., at the ingress and egress of a network segment), an observer (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 can determine the contribution of a portion of the path to an
observed metric, to locate a source of delay, jitter, loss, observed metric, to locate a source of delay, jitter, loss,
reordering, congestion marking, etc. reordering, congestion marking, etc.
3.2.2. Use by Operators to Plan and Provision Networks 3.2.2. Use by Operators to Plan and Provision Networks
Traffic measurements (e.g., traffic volume, loss, latency) is used by Traffic measurements (e.g., traffic volume, loss, latency) is used by
operators to help plan deployment of new equipment and configurations operators to help plan deployment of new equipment and configuration
in their networks. Data is also valuable to equipment vendors who in their networks. Data is also valuable to equipment vendors who
want to understand traffic trends and patterns of usage as inputs to want to understand traffic trends and patterns of usage as inputs to
decisions about planning products and provisioning for new decisions about planning products and provisioning for new
deployments. This measurement information can also be correlated deployments. This measurement information can also be correlated
with billing information when this is also collected by an operator. with billing information when this is also collected by an operator.
A network operator supporting traffic that uses transport header A network operator supporting traffic that uses transport header
encryption might not have access to per-flow measurement data. encryption might not have access to per-flow measurement data.
Trends in aggregate traffic can be observed and can be related to the Trends in aggregate traffic can be observed and can be related to the
endpoint addresses being used, but it may be impossible to correlate endpoint addresses being used, but it may be impossible to correlate
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flows from acquiring excessive network capacity. Operators can flows from acquiring excessive network capacity. Operators can
implement operational practices to manage traffic flows (e.g., to implement operational practices to manage traffic flows (e.g., to
prevent flows from acquiring excessive network capacity under severe prevent flows from acquiring excessive network capacity under severe
congestion) by deploying rate-limiters, traffic shaping or network congestion) by deploying rate-limiters, traffic shaping or network
transport circuit breakers [RFC8084]. transport circuit breakers [RFC8084].
Congestion Control Compliance of Traffic: Congestion control is a Congestion Control Compliance of Traffic: Congestion control is a
key transport function [RFC2914]. Many network operators key transport function [RFC2914]. Many network operators
implicitly accept that TCP traffic complies with a behaviour that implicitly accept that TCP traffic complies with a behaviour that
is acceptable for use in the shared Internet. TCP algorithms have is acceptable for use in the shared Internet. TCP algorithms have
been continuously improved over decades, and they have reached a been continuously improved over decades and they have reached a
level of efficiency and correctness that custom application-layer level of efficiency and correctness that custom application-layer
mechanisms will struggle to easily duplicate [RFC8085]. mechanisms will struggle to easily duplicate [RFC8085].
A standards-compliant TCP stack provides congestion control that A standards-compliant TCP stack provides congestion control that
may therefore be judged safe for use across the Internet. may therefore be judged safe for use across the Internet.
Applications developed on top of well-designed transports can be Applications developed on top of well-designed transports can be
expected to appropriately control their network usage, reacting expected to appropriately control their network usage, reacting
when the network experiences congestion, by back-off and reduce when the network experiences congestion, by back-off and reduce
the load placed on the network. This is the normal expected the load placed on the network. This is the normal expected
behaviour for IETF-specified transport (e.g., TCP and SCTP). behaviour for IETF-specified transport (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the However, when anomalies are detected, tools can interpret the
transport protocol header information to help understand the transport protocol header information to help understand the
impact of specific transport protocols (or protocol mechanisms) on impact of specific transport protocols (or protocol mechanisms) on
the other traffic that shares a network. An observation in the the other traffic that shares a network. An observation in the
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Applications developed on top of well-designed transports can be Applications developed on top of well-designed transports can be
expected to appropriately control their network usage, reacting expected to appropriately control their network usage, reacting
when the network experiences congestion, by back-off and reduce when the network experiences congestion, by back-off and reduce
the load placed on the network. This is the normal expected the load placed on the network. This is the normal expected
behaviour for IETF-specified transport (e.g., TCP and SCTP). behaviour for IETF-specified transport (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the However, when anomalies are detected, tools can interpret the
transport protocol header information to help understand the transport protocol header information to help understand the
impact of specific transport protocols (or protocol mechanisms) on impact of specific transport protocols (or protocol mechanisms) on
the other traffic that shares a network. An observation in the the other traffic that shares a network. An observation in the
network can gain understanding of the dynamics of a flow and its network can gain an understanding of the dynamics of a flow and
congestion control behaviour. Analysing observed flows can help its congestion control behaviour. Analysing observed flows can
to build confidence that an application flow backs-off its share help to build confidence that an application flow backs-off its
of the network load in the face of persistent congestion, and share of the network load in the face of persistent congestion,
hence to understand whether the behaviour is appropriate for and hence to understand whether the behaviour is appropriate for
sharing limited network capacity. For example, it is common to sharing limited network capacity. For example, it is common to
visualise plots of TCP sequence numbers versus time for a flow to visualise plots of TCP sequence numbers versus time for a flow to
understand how a flow shares available capacity, deduce its understand how a flow shares available capacity, deduce its
dynamics in response to congestion, etc. The ability to identify dynamics in response to congestion, etc. The ability to identify
sources that contribute excessive congestion is important to safe sources that contribute to persistent congestion is important to
operation of network infrastructure, and mechanisms can inform safe operation of network infrastructure, and mechanisms can
configuration of network devices to complement the endpoint inform configuration of network devices to complement the endpoint
congestion avoidance mechanisms [RFC7567] [RFC8084] to avoid a congestion avoidance mechanisms [RFC7567] [RFC8084] to avoid a
portion of the network being driven into congestion collapse portion of the network being driven into congestion collapse
[RFC2914]. [RFC2914].
Congestion Control Compliance for UDP traffic: UDP provides a Congestion Control Compliance for UDP traffic: UDP provides a
minimal message-passing datagram transport that has no inherent minimal message-passing datagram transport that has no inherent
congestion control mechanisms. Because congestion control is congestion control mechanisms. Because congestion control is
critical to the stable operation of the Internet, applications and critical to the stable operation of the Internet, applications and
other protocols that choose to use UDP as a transport are required other protocols that choose to use UDP as a transport are required
to employ mechanisms to prevent congestion collapse, avoid to employ mechanisms to prevent congestion collapse, avoid
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performance, capacity planning, management of security threats performance, capacity planning, management of security threats
(including denial of service), and responding to user performance (including denial of service), and responding to user performance
questions. Sections 3.1.2 and 5 of [RFC8404] provide further questions. Sections 3.1.2 and 5 of [RFC8404] provide further
examples. These tasks seldom involve the need to determine the examples. These tasks seldom involve the need to determine the
contents of the transport payload, or other application details. contents of the transport payload, or other application details.
A network operator supporting traffic that uses transport header A network operator supporting traffic that uses transport header
encryption can see only encrypted transport headers. This prevents encryption can see only encrypted transport headers. This prevents
deployment of performance measurement tools that rely on transport deployment of performance measurement tools that rely on transport
protocol information. Choosing to encrypt all the information protocol information. Choosing to encrypt all the information
reduces the ability of an operator to observe transport performance, reduces the ability of an operator to observe transport performance
and could limit the ability of network operators to trace problems, and could limit the ability of network operators to trace problems,
make appropriate QoS decisions, or response to other queries about make appropriate QoS decisions, or response to other queries about
the network service. For some this will be blessing, for others it the network service. For some this will be blessing, for others it
may be a curse. For example, operational performance data about may be a curse. For example, operational performance data about
encrypted flows needs to be determined by traffic pattern analysis, encrypted flows needs to be determined by traffic pattern analysis,
rather than relying on traditional tools. This can impact the rather than relying on traditional tools. This can impact the
ability of the operator to respond to faults, it could require ability of the operator to respond to faults, it could require
reliance on endpoint diagnostic tools or user involvement in reliance on endpoint diagnostic tools or user involvement in
diagnosing and troubleshooting unusual use cases or non-trivial diagnosing and troubleshooting unusual use cases or non-trivial
problems. A key need here is for tools to provide useful information problems. A key need here is for tools to provide useful information
during network anomalies (e.g., significant reordering, high or during network anomalies (e.g., significant reordering, high or
intermittent loss). intermittent loss).
Measurements can be used to monitor the health of a portion of the Measurements can be used to monitor the health of a portion of the
Internet, to provide early warning of the need to take action. They Internet, to provide early warning of the need to take action. They
can assist in debugging and diagnosing the root causes of faults that can assist in debugging and diagnosing the root causes of faults that
concern a particular user's traffic. They can also be used to concern a particular user's traffic. They can also be used to
support post-mortem investigation after an anomaly to determine the support post-mortem investigation after an anomaly to determine the
root cause of a problem. root cause of a problem.
In some case, measurements may involve active injection of test In some case, measurements could involve active injection of test
traffic to perform a measurement. However, most operators do not traffic to perform a measurement. However, most operators do not
have access to user equipment, therefore the point of test is have access to user equipment, therefore the point of test is
normally different from the transport endpoint. Injection of test normally different from the transport endpoint. Injection of test
traffic can incur an additional costs in running such tests (e.g., traffic can incur an additional cost in running such tests (e.g., the
the implications of capacity tests in a mobile network are obvious). implications of capacity tests in a mobile network are obvious).
Some active measurements (e.g., response under load or particular Some active measurements (e.g., response under load or particular
workloads) perturb other traffic, and could require dedicated access workloads) perturb other traffic, and could require dedicated access
to the network segment. An alternative approach is to use in-network to the network segment. An alternative approach is to use in-network
techniques that observe transport packet headers added while traffic techniques that observe transport packet headers added while traffic
traverses an operational networks to make the measurements. These traverses an operational network to make the measurements. These
measurements do not require the cooperation of an endpoint. measurements do not require the cooperation of an endpoint.
In other cases, measurement involves dissecting network traffic In other cases, measurement involves dissecting network traffic
flows. The observed transport layer information can help identify flows. The observed transport layer information can help identify
whether the link/network tuning is effective and alert to potential whether the link/network tuning is effective and alert to potential
problems that can be hard to derive from link or device measurements problems that can be hard to derive from link or device measurements
alone. The design trade-offs for radio networks are often very alone. The design trade-offs for radio networks are often very
different to those of wired networks. A radio-based network (e.g., different from those of wired networks. A radio-based network (e.g.,
cellular mobile, enterprise WiFi, satellite access/back-haul, point- cellular mobile, enterprise WiFi, satellite access/back-haul, point-
to-point radio) has the complexity of a subsystem that performs radio to-point radio) has the complexity of a subsystem that performs radio
resource management,s with direct impact on the available capacity, resource management,s with direct impact on the available capacity,
and potentially loss/reordering of packets. The impact of the and potentially loss/reordering of packets. The impact of the
pattern of loss and congestion, differs for different traffic types, pattern of loss and congestion, differs for different traffic types,
correlation with propagation and interference can all have correlation with propagation and interference can all have
significant impact on the cost and performance of a provided service. significant impact on the cost and performance of a provided service.
The need for this type of information is expected to increase as The need for this type of information is expected to increase as
operators bring together heterogeneous types of network equipment and operators bring together heterogeneous types of network equipment and
seek to deploy opportunistic methods to access radio spectrum. seek to deploy opportunistic methods to access radio spectrum.
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The following briefly reviews some security design options for The following briefly reviews some security design options for
transport protocols. A Survey of Transport Security Protocols transport protocols. A Survey of Transport Security Protocols
[I-D.ietf-taps-transport-security] provides more details concerning [I-D.ietf-taps-transport-security] provides more details concerning
commonly used encryption methods at the transport layer. commonly used encryption methods at the transport layer.
Authenticating the Transport Protocol Header: Transport layer header Authenticating the Transport Protocol Header: Transport layer header
information can be authenticated. An integrity check that information can be authenticated. An integrity check that
protects the immutable transport header fields, but can still protects the immutable transport header fields, but can still
expose the transport protocol header information in the clear, expose the transport protocol header information in the clear,
allowing in-network devices to observe these fields. An integrity allowing in-network devices to observe these fields. An integrity
check can not prevent in-network modification, but can prevent a check is not able to prevent in-network modification, but can
receiving from accepting changes and avoid impact on the transport prevent a receiving from accepting changes and avoid impact on the
protocol operation. transport protocol operation.
An example transport authentication mechanism is TCP- An example transport authentication mechanism is TCP-
Authentication (TCP-AO) [RFC5925]. This TCP option authenticates Authentication (TCP-AO) [RFC5925]. This TCP option authenticates
the IP pseudo header, TCP header, and TCP data. TCP-AO protects the IP pseudo header, TCP header, and TCP data. TCP-AO protects
the transport layer, preventing attacks from disabling the TCP the transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. TCP-AO may connection itself and provides replay protection. TCP-AO may
interact with middleboxes, depending on their behaviour [RFC3234]. interact with middleboxes, depending on their behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This work at the network layer and authenticate the IP payload. This
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use a field for another purpose. When the specification is later use a field for another purpose. When the specification is later
updated, it is impossible to deploy the new use of the field, and updated, it is impossible to deploy the new use of the field, and
forwarding of the protocol could even become conditional on a forwarding of the protocol could even become conditional on a
specific header field value. specific header field value.
A protocol can intentionally vary the value, format, and/or A protocol can intentionally vary the value, format, and/or
presence of observable transport header fields. This behaviour, presence of observable transport header fields. This behaviour,
known as GREASE (Generate Random Extensions And Sustain known as GREASE (Generate Random Extensions And Sustain
Extensibility), is designed to avoid a network device ossifying Extensibility), is designed to avoid a network device ossifying
the use of a specific observable field. Greasing seeks to ease the use of a specific observable field. Greasing seeks to ease
deployment of new methods. It can be designed to prevent in- deployment of new methods. It can also prevent in-network devices
network devices utilising the information in a transport header, utilising the information in a transport header, or can make an
or can make an observation robust to a set of changing values, observation robust to a set of changing values, rather than a
rather than a specific set of values. specific set of values.
Encrypting the Transport Payload: The transport layer payload can be Encrypting the Transport Payload: The transport layer payload can be
encrypted to protect the content of transport segments. This encrypted to protect the content of transport segments. This
leaves transport protocol header information in the clear. The leaves transport protocol header information in the clear. The
integrity of immutable transport header fields could be protected integrity of immutable transport header fields could be protected
by combining this with an integrity check. by combining this with an integrity check.
Examples of encrypting the payload include Transport Layer Examples of encrypting the payload include Transport Layer
Security (TLS) over TCP [RFC8446] [RFC7525], Datagram TLS (DTLS) Security (TLS) over TCP [RFC8446] [RFC7525], Datagram TLS (DTLS)
over UDP [RFC6347] [RFC7525], Secure RTP [RFC3711], and TCPcrypt over UDP [RFC6347] [RFC7525], Secure RTP [RFC3711], and TCPcrypt
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sender and visible at the receiving endpoint, although methods to do sender and visible at the receiving endpoint, although methods to do
this have not currently been proposed. This method needs to be this have not currently been proposed. This method needs to be
explicitly enabled at the sender. explicitly enabled at the sender.
Current measurements suggest it can be undesirable to rely on methods Current measurements suggest it can be undesirable to rely on methods
requiring the presence of network options or extension headers. IPv4 requiring the presence of network options or extension headers. IPv4
network options are often not supported (or are carried on a slower network options are often not supported (or are carried on a slower
processing path) and some IPv6 networks are also known to drop processing path) and some IPv6 networks are also known to drop
packets that set an IPv6 header extension (e.g., [RFC7872]). Another packets that set an IPv6 header extension (e.g., [RFC7872]). Another
disadvantage is that protocols that separately expose header disadvantage is that protocols that separately expose header
information do not necessarily have an advantage to expose the information do not necessarily have an incentive to expose the
information that is utilised by the protocol itself, and could information that is utilised by the protocol itself, and could
manipulate this header information to gain an advantage from the manipulate the exposed header information to gain an advantage from
network. the network.
6. Implications of Protecting the Transport Headers 6. Implications of Protecting the Transport Headers
The choice of which fields to expose and which to encrypt is a design The choice of which fields to expose and which to encrypt is a design
choice for the transport protocol. Any selective encryption method choice for the transport protocol. Any selective encryption method
requires trading two conflicting goals for a transport protocol requires trading two conflicting goals for a transport protocol
designer to decide which header fields to encrypt. Security work designer to decide which header fields to encrypt. Security work
typically employs a design technique that seeks to expose only what typically employs a design technique that seeks to expose only what
is needed. This approach provides incentives to not reveal any is needed. This approach provides incentives to not reveal any
information that is not necessary for the end-to-end communication. information that is not necessary for the end-to-end communication.
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independent observation and evaluation of performance data, which in independent observation and evaluation of performance data, which in
turn suggests control over where and when measurement samples are turn suggests control over where and when measurement samples are
collected. This requires consideration of the appropriate balance collected. This requires consideration of the appropriate balance
between encrypting all and no transport information. between encrypting all and no transport information.
7. Conclusions 7. Conclusions
Confidentiality and strong integrity checks have properties that are Confidentiality and strong integrity checks have properties that are
being incorporated into new protocols and that have important being incorporated into new protocols and that have important
benefits. The pace of development of transports using the WebRTC benefits. The pace of development of transports using the WebRTC
data channel and the rapid deployment of QUIC transport protocol can data channel and the rapid deployment of the QUIC transport protocol
both be attributed to using the combination of UDP as a substrate can both be attributed to using the combination of UDP as a substrate
while providing confidentiality and authentication of the while providing confidentiality and authentication of the
encapsulated transport headers and payload. encapsulated transport headers and payload.
The traffic that can be observed by on-path network devices is a The traffic that can be observed by on-path network devices is a
function of transport protocol design/options, network use, function of transport protocol design/options, network use,
applications, and user characteristics. In general, when only a applications, and user characteristics. In general, when only a
small proportion of the traffic has a specific (different) small proportion of the traffic has a specific (different)
characteristic, such traffic seldom leads to operational concern, characteristic, such traffic seldom leads to operational concern,
although the ability to measure and monitor it is less. The desire although the ability to measure and monitor it is less. The desire
to understand the traffic and protocol interactions typically grows to understand the traffic and protocol interactions typically grows
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An increased pace of evolution therefore needs to be accompanied by An increased pace of evolution therefore needs to be accompanied by
methods that can be successfully deployed and used across operational methods that can be successfully deployed and used across operational
networks. This leads to a need for network operators (at various networks. This leads to a need for network operators (at various
level (ISPs, enterprises, firewall maintainer, etc) to identify level (ISPs, enterprises, firewall maintainer, etc) to identify
appropriate operational support functions and procedures. appropriate operational support functions and procedures.
Protocols that change their transport header format (wire format) or Protocols that change their transport header format (wire format) or
their behaviour (e.g., algorithms that are needed to classify and their behaviour (e.g., algorithms that are needed to classify and
characterise the protocol), will require new tooling to be developed characterise the protocol), will require new tooling to be developed
to catch-up with the changes. If the currently deployed tools and to catch-up with the change. If the currently deployed tools and
methods are no longer relevant then it may no longer be possible to methods are no longer relevant then it may no longer be possible to
correctly measure performance. This can increase the response-time correctly measure performance. This can increase the response-time
after faults, and can impact the ability to manage the network after faults, and can impact the ability to manage the network
resulting in traffic causing traffic to be treated inappropriately resulting in traffic causing traffic to be treated inappropriately
(e.g., rate limiting because of being incorrectly classified/ (e.g., rate limiting because of being incorrectly classified/
monitored). monitored).
There are benefits in exposing consistent information to the network There are benefits in exposing consistent information to the network
that avoids traffic being mis-classified and then receiving a default that avoids traffic being inappropriately classified and then
treatment by the network. The flow label and DSCP fields provide receiving a default treatment by the network. The flow label and
examples of how transport information can be made available for DSCP fields provide examples of how transport information can be made
network-layer decisions. Extension headers could also be used to available for network-layer decisions. Extension headers could also
carry transport information that can inform network-layer decisions. be used to carry transport information that can inform network-layer
decisions.
As a part of its design a new protocol specification therefore needs As a part of its design, a new protocol specification therefore needs
to weigh the benefits of ossifying common headers, versus the to weigh the benefits of ossifying common headers, versus the
potential demerits of exposing specific information that could be potential demerits of exposing specific information that could be
observed along the network path, to provide tools to manage new observed along the network path, to provide tools to manage new
variants of protocols. This can be done for the entire transport variants of protocols. This can be done for the entire transport
header, or by dividing header fields between those that are header, or by dividing header fields between those that are
observable and mutable; those that are observable, but immutable; and observable and mutable; those that are observable, but immutable; and
those that are hidden/obfusticated. those that are hidden/obfusticated.
Several scenarios to illustrate different ways this could evolve are Several scenarios to illustrate different ways this could evolve are
provided below: provided below:
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measurement and any necessary functions appropriate to the class measurement and any necessary functions appropriate to the class
of traffic (priority, retransmission, reordering, circuit of traffic (priority, retransmission, reordering, circuit
breakers, etc). breakers, etc).
o An alternative scenario adopts different design goals, with a o An alternative scenario adopts different design goals, with a
different outcome. A protocol that encrypts all header different outcome. A protocol that encrypts all header
information forces network operators to act independently from information forces network operators to act independently from
apps/transport developments to extract the information they need apps/transport developments to extract the information they need
to manage their network. A range of approaches could proliferate, to manage their network. A range of approaches could proliferate,
as in current networks. Some operators can add a shim header to as in current networks. Some operators can add a shim header to
each packet as a flow as it crosses the network; other operators/ each packet in a flow as the flow crosses a network segment; other
managers could develop heuristics and pattern recognition to operators/managers could develop heuristics and pattern
derive information that classifies flows and estimates quality recognition to derive information that classifies flows and
metrics for the service being used; some could decide to rate- estimates quality metrics for the service being used; some could
limit or block traffic until new tooling is in place. In many decide to rate-limit or block traffic until new tooling is in
cases, the derived information can be used by operators to provide place. In many cases, the derived information can be used by
necessary functions appropriate to the class of traffic (priority, operators to provide necessary functions appropriate to the class
retransmission, reordering, circuit breakers, etc). of traffic (priority, retransmission, reordering, circuit
Troubleshooting, and measurement becomes more difficult, and more breakers, etc). Troubleshooting, and measurement becomes more
diverse. This could require additional information beyond that difficult, and more diverse. This could require additional
visible in the packet header and when this information is used to information beyond that visible in the packet header and when this
inform decisions by on-path devices it can lead to dependency on information is used to inform decisions by on-path devices it can
other characteristics of the flow. In some cases, operators might lead to dependency on other characteristics of the flow. In some
need access to keying information to interpret encrypted data that cases, operators might need access to keying information to
they observe. Some use cases could demand use of transports that interpret encrypted data that they observe. Some use cases could
do not use encryption. demand use of transports that do not use encryption.
The direction in which this evolves could have significant The direction in which this evolves could have significant
implications on the way the Internet architecture develops. It implications on the way the Internet architecture develops. It
exposes a risk that significant actors (e.g., developers and exposes a risk that significant actors (e.g., developers and
transport designers) achieve more control of the way in which the transport designers) achieve more control of the way in which the
Internet architecture develops.In particular, there is a possibility Internet architecture develops.In particular, there is a possibility
that designs could evolve to significantly benefit of customers for a that designs could evolve to significantly benefit of customers for a
specific vendor, and that communities with very different network, specific vendor, and that communities with very different network,
applications or platforms could then suffer at the expense of applications or platforms could then suffer at the expense of
benefits to their vendors own customer base. In such a scenario, benefits to their vendors own customer base. In such a scenario,
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decryption of packets that cannot be successfully decrypted (forcing decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then a receiver to commit decryption resources, or to update and then
restore protocol state). restore protocol state).
One mitigation to off-path attack is to deny knowledge of what header One mitigation to off-path attack is to deny knowledge of what header
information is accepted by a receiver or obfuscate the accepted information is accepted by a receiver or obfuscate the accepted
header information, e.g., setting a non-predictable initial value for header information, e.g., setting a non-predictable initial value for
a sequence number during a protocol handshake, as in [RFC3550] and 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 [RFC6056], or a port value that can not be predicted (see section 5.1
of [RFC8085]). A receiver could also require additional information of [RFC8085]). A receiver could also require additional information
to be used as a part of check before accepting packets at the to be used as a part of a validation check before accepting packets
transport layer (e.g., utilising a part of the sequence number space at the transport layer (e.g., utilising a part of the sequence number
that is encrypted; or by verifying an encrypted token not visible to space that is encrypted; or by verifying an encrypted token not
an attacker). This would also mitigate on-path attacks. An visible to an attacker). This would also mitigate on-path attacks.
additional processing cost can be incurred when decryption needs to An additional processing cost can be incurred when decryption needs
be attempted before a receiver is able to discard injected packets. to be attempted before a receiver is able to discard injected
packets.
Open standards motivate a desire for this evaluation to include Open standards motivate a desire for this evaluation to include
independent observation and evaluation of performance data, which in independent observation and evaluation of performance data, which in
turn suggests control over where and when measurement samples are turn suggests control over where and when measurement samples are
collected. This requires consideration of the appropriate balance collected. This requires consideration of the appropriate balance
between encrypting all and no transport information. Open data, and between encrypting all and no transport information. Open data, and
accessibility to tools that can help understand trends in application accessibility to tools that can help understand trends in application
deployment, network traffic and usage patterns can all contribute to deployment, network traffic and usage patterns can all contribute to
understanding security challenges. understanding security challenges.
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referring to RTP transport. This version contains author editorial referring to RTP transport. This version contains author editorial
work and removed duplicate section. work and removed duplicate section.
-04 Revised following SecDir Review -04 Revised following SecDir Review
o Added some text on TLS story (additional input sought on relevant o Added some text on TLS story (additional input sought on relevant
considerations). considerations).
o Section 2, paragraph 8 - changed to be clearer, in particular, o Section 2, paragraph 8 - changed to be clearer, in particular,
added "Encryption with secure key distribution prevents" added "Encryption with secure key distribution prevents"
o Flow label description rewritten based on PDS/BCP RFCs. o Flow label description rewritten based on PS/BCP RFCs.
o Clarify requirements from RFCs concerning the IPv6 flow label and o Clarify requirements from RFCs concerning the IPv6 flow label and
highlight ways it can be used with encryption. (section 3.1.3) 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 o Add text on the explicit spin-bit work in the QUIC DT. Added
greasing of spin-bit. (Section 6.1) greasing of spin-bit. (Section 6.1)
o Updated section 6 and added more explanation of impact on o Updated section 6 and added more explanation of impact on
operators. operators.
o Other comments addressed. o Other comments addressed.
-05 Editorial pass and minor corrections noted on TSVWG list.
Authors' Addresses Authors' Addresses
Godred Fairhurst Godred Fairhurst
University of Aberdeen University of Aberdeen
Department of Engineering Department of Engineering
Fraser Noble Building Fraser Noble Building
Aberdeen AB24 3UE Aberdeen AB24 3UE
Scotland Scotland
EMail: gorry@erg.abdn.ac.uk EMail: gorry@erg.abdn.ac.uk
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