Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-10
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| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Gorry Fairhurst , Colin Perkins | ||
| Last updated | 2020-01-09 | ||
| Replaces | draft-fairhurst-tsvwg-transport-encrypt | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Send notices to | David Black <david.black@dell.com> |
draft-ietf-tsvwg-transport-encrypt-10
TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins
Expires: July 12, 2020 University of Glasgow
January 9, 2020
Considerations around Transport Header Confidentiality, Network
Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-10
Abstract
To protect user data and privacy, Internet transport protocols have
supported payload encryption and authentication for some time. Such
encryption and authentication is now also starting to be applied to
the transport protocol headers. This helps avoid transport protocol
ossification by middleboxes, while also protecting metadata about the
communication. Current operational practice in some networks inspect
transport header information within the network, but this is no
longer possible when those transport headers are encrypted. This
document discusses the possible impact when network traffic uses a
protocol with an encrypted transport header. It suggests issues to
consider when designing new transport protocols, to account for
network operations, prevent network ossification, and enable
transport evolution, while still respecting user privacy.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 12, 2020.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Context and Rationale . . . . . . . . . . . . . . . . . . . . 4
2.1. Use of Transport Header Information in the Network . . . 5
2.2. Authentication of Transport Header Information . . . . . 7
2.3. Observable Transport Header Fields . . . . . . . . . . . 7
3. Current uses of Transport Headers within the Network . . . . 10
3.1. Observing Transport Information in the Network . . . . . 11
3.2. Transport Measurement . . . . . . . . . . . . . . . . . . 18
3.3. Use for Network Diagnostics and Troubleshooting . . . . . 21
3.4. Header Compression . . . . . . . . . . . . . . . . . . . 23
4. Encryption and Authentication of Transport Headers . . . . . 23
4.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 23
4.2. Approaches to Transport Header Protection . . . . . . . . 24
5. Addition of Transport Information to Network-Layer Headers . 26
5.1. Use of OAM within a Maintenance Domain . . . . . . . . . 26
5.2. Use of OAM across Multiple Maintenance Domains . . . . . 26
6. Implications of Protecting the Transport Headers . . . . . . 27
6.1. Independent Measurement . . . . . . . . . . . . . . . . . 28
6.2. Characterising "Unknown" Network Traffic . . . . . . . . 30
6.3. Accountability and Internet Transport Protocols . . . . . 30
6.4. Impact on Operational Cost . . . . . . . . . . . . . . . 31
6.5. Impact on Research, Development and Deployment . . . . . 31
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
11. Informative References . . . . . . . . . . . . . . . . . . . 38
Appendix A. Revision information . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
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1. Introduction
Transport protocols have supported end-to-end encryption of payload
data for many years. Examples include Transport Layer Security (TLS)
over TCP [RFC8446], Datagram TLS (DTLS) over UDP [RFC6347], Secure
RTP [RFC3711], and TCPcrypt [RFC8548] which permits opportunistic
encryption of the TCP transport payload. Some of these also provide
integrity protection of all or part of the transport header.
This end-to-end transport payload encryption brings many benefits in
terms of providing confidentiality and protecting user privacy. The
benefits have been widely discussed, for example in [RFC7258] and
[RFC7624]. This document strongly supports and encourages increased
use of end-to-end payload encryption in transport protocols. The
implications of protecting the transport payload data are therefore
not further discussed in this document.
A further level of protection can be achieved by encrypting the
entire network layer payload, including both the transport headers
and the payload. This does not expose any transport information to
devices in the network, and therefore also prevents modification
along a network path. An example of encryption at the network layer
is the IPsec Encapsulating Security Payload (ESP) [RFC4303] in tunnel
mode. Virtual Private Networks (VPNs) typically also operate in this
way. This form of encryption is not further discussed in this
document.
There is also a middle ground, comprising transport protocols that
encrypt some, or all, of the transport layer header information, in
addition to encrypting the payload. An example of such a protocol,
that is now seeing widespread interest and deployment, is the QUIC
transport protocol [I-D.ietf-quic-transport]. The encryption and
authentication of transport header information can prevent unwanted
modification of transport headers by middleboxes, reducing the risk
of protocol ossification. It also reduces the amount of metadata
about the progress of the transport connection that is visible to the
network.
As discussed in [RFC7258], Pervasive Monitoring (PM) is a technical
attack that needs to be mitigated in the design of IETF protocols.
This document supports that conclusion and the use of transport
header encryption to protect against pervasive monitoring. RFC 7258
also notes, though, that "Making networks unmanageable to mitigate PM
is not an acceptable outcome, but ignoring PM would go against the
consensus documented here. An appropriate balance will emerge over
time as real instances of this tension are considered".
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The transport protocols developed for the Internet are used across a
wide range of paths across network segments with many different
regulatory, commercial, and engineering considerations. This
document considers some of the costs and changes to network
management and research that are implied by widespread use of
transport protocols that encrypt their transport header information.
It reviews the implications of developing transport protocols that
use end-to-end encryption to provide confidentiality of their
transport layer headers, and considers the effect of such changes on
transport protocol design and network operations. It also considers
some anticipated implications on transport and application evolution.
This provides considerations relating to the design of transport
protocols that protect their header information and respect user
privacy.
2. Context and Rationale
The transport layer provides end-to-end interactions between
endpoints (processes) using an Internet path. Transport protocols
layer over the network-layer service, and are usually sent in the
payload of network-layer packets. They support end-to-end
communication between applications, using higher-layer protocols
running on the end systems (transport endpoints).
This simple architectural view does not present one of the core
functions of an Internet transport: to discover and adapt to the
network path that is currently being used. The design of Internet
transport protocols is as much about trying to avoid the unwanted
side effects of congestion on a flow and other capacity-sharing
flows, avoiding congestion collapse, adapting to changes in the path
characteristics, etc., as it is about end-to-end feature negotiation,
flow control, and optimising for performance of a specific
application.
Transport headers have end-to-end meaning, but have often been
observed by equipment within the network. Transport protocol
specifications have not tended to consider this, and have failed to
indicate what parts of the transport header are intended to be
invariant across protocol versions and visible to the network; what
parts of the header can be modified by the network to signal to the
transport, and in what way; and what parts of the header are private
and/or expected to change in future, and need to be protected for
privacy or to prevent protocol ossification.
Increasing concern about pervasive network monitoring
[RFC7258][RFC7624], and growing awareness of the problem of protocol
ossification caused by middlebox interference with Internet traffic,
has motivated a shift in transport protocol design. Recent transport
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protocols, such as QUIC [I-D.ietf-quic-transport], encrypt the
majority of their transport headers to prevent observation and
protect against modification by the network, and to make explicit
their invariants and what is intended to be visible to the network.
Transport header encryption is expected to form a core part of future
transport protocol designs. It can help to protect against pervasive
monitoring, improve privacy, and reduce protocol ossification.
Transport protocols that use header encryption with secure key
distribution can provide confidentiality and protection for some, or
all, of the transport header information, controlling what is visible
to, and can be modified by, the network.
The increased use of transport header encryption has benefits, but
also has implications for the broader ecosystem. The transport
community has, to date, relied heavily on measurements and insights
from the network operations community to understand protocol
behaviour, and to inform the selection of appropriate mechanisms to
ensure a safe, reliable, and robust Internet. In turn, network
operators and access providers have relied upon being able to observe
traffic patterns and requirements, both in aggregate and at the flow
level, to help understand and optimise the behaviour of their
networks. Widespread use of transport header encryption could limit
such observations in future. It is important to understand how
transport header information is used in the network, to allow future
protocol designs to make an informed choice on what, if any, headers
to expose to the network.
2.1. Use of Transport Header Information in the Network
In-network measurement of transport flow characteristics can be used
to enhance performance, and control cost and service reliability. To
support network operations and enhance performance, some operators
have deployed functionality that utilises on-path observations of the
transport headers of packets passing through their network.
When network devices rely on the presence of a header field or the
semantics of specific header information, this can lead to
ossification where an endpoint has to supply a specific header to
receive the network service that it desires.
In some cases, network-layer use of transport header information can
be benign or advantageous to the protocol (e.g., recognising the
start of a TCP connection, providing header compression for a Secure
RTP flow, or explicitly using exposed protocol information to provide
consistent decisions by on-path devices). However, in other cases,
this can have unwanted outcomes, e.g., privacy impacts and
ossification.
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Ossification can frustrate the evolution of a transport protocol. A
mechanism implemented in a network device, such as a firewall, that
requires a header field to have only a specific known set of values
can prevent the device from forwarding packets using a different
version of the protocol that introduces a feature that changes to a
new value for the observed field.
An example of ossification was observed in the development of
Transport Layer Security (TLS) 1.3 [RFC8446], where the design needed
to function in the presence of deployed middleboxes that relied on
the presence of certain header fields exposed in TLS 1.2.
The design of MPTCP also had to be revised to account for middleboxes
(known as "TCP Normalizers") that monitor the evolution of the window
advertised in the TCP header and then reset connections when the
window did not grow as expected. Similarly, issues have been
reported using TCP. For example, TCP Fast Open can experience
middleboxes that modify the transport header of packets by removing
"unknown" TCP options, segments with unrecognised TCP options can be
dropped, segments that contain data and set the SYN bit can be
dropped, or middleboxes that disrupt connections which send data
before completion of the three-way handshake. Other examples of
ossification have included middleboxes that rewrite TCP sequence and
acknowledgement numbers, but are unaware of the (newer) TCP selective
acknowledgement (SACK) Option and therefore fail to correctly rewrite
the selective acknowledgement header information to match the changes
that were made to the fixed TCP header, preventing SACK from
operating correctly.
In all these cases, middleboxes with a hard-coded understanding of
transport behaviour, interacted poorly with transport protocols after
the transport behaviour was changed.
In contrast, transport header encryption prevents an on-path device
from observing the transport headers, and therefore stops mechanisms
being built that directly rely on or infer semantics of the transport
header information. Encryption is normally combined with
authentication of the protected information. RFC 8546 summarises
this approach, stating that it is "The wire image, not the protocol's
specification, determines how third parties on the network paths
among protocol participants will interact with that protocol"
[RFC8546].
While encryption can reduce ossification of the transport protocol,
it does not itself prevent ossification of the network service.
People seeking to understand network traffic could still come to rely
on pattern inferences and other heuristics or machine learning to
derive measurement data and as the basis for network forwarding
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decisions. This can also create dependencies on the transport
protocol, or the patterns of traffic it can generate.
2.2. Authentication of Transport Header Information
The designers of a transport protocol decide whether to encrypt all,
or a part of, the transport header information. Section 4 of RFC8558
states: "Anything exposed to the path should be done with the intent
that it be used by the network elements on the path" [RFC8558]. New
protocol designs can decide not to encrypt certain transport header
fields, making those fields observable in the network. Where fields
are intended to immutable (i.e., observable but not modifiable by the
network), the endpoints are encouraged to use authentication to
provide a cryptographic integrity check that includes these immutable
fields to detect any manipulation by network devices.
Making part of a transport header observable can lead to ossification
of that part of a header, as middleboxes come to rely on observations
of the exposed fields. A protocol design that provides an observable
field might want to avoid inspection restricting the choice of usable
values in the field by intentionally varying the format and/or value
of the field to reduce the chance of ossification (see Section 4).
2.3. Observable Transport Header Fields
Transport headers fields have been observed within the network for a
variety of purposes. Some of these are related to network management
and operations. The lists below, and in the following section, seek
to identify some of these uses and the implications of the increased
use of transport header encryption. This analysis does not judge
whether specific practises are necessary, or endorse the use of any
specific approach.
Network Operations: Observable transport headers enable explicit
measurement and analysis of protocol performance,
network anomalies, and failure pathologies at any
point along the Internet path. In many cases, it
is important to relate observations to specific
equipment/configurations, to a specific network
segment, or sometimes to a specific protocol or
application.
When transport header information is not
observable, it cannot be used by network
operators. Operators might work without that
information, or they might turn to more ambitious
ways to collect, estimate, or infer this data.
(Operational practises aimed at guessing
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transport parameters are out of scope for this
document, and are only mentioned here to
recognize that encryption does not stop operators
from attempting to apply practises that have been
used with unencrypted transport headers.)
See also Sections 3, 5, and 6.4.
Traffic Analysis: Observable transport headers have been used to
determine which transport protocols and features
are being used across a network segment, and to
measure trends in the pattern of usage. For some
use cases, end-to-end measurements/traces are
sufficient and can assist in developing and
debugging new transports and analysing their
deployment. In other uses, it is important to
relate observations to specific equipment/
configurations or particular network segments.
This information can help anticipate the demand
for network upgrades and roll-out, or affect on-
going traffic engineering activities performed by
operators such as determining which parts of the
path contribute delay, jitter, or loss.
Tools that rely upon observing headers, could
fail to produce useful data when those headers
are encrypted. While this impact could, in many
cases, be small, there are scenarios where
operators have actively monitored and supported
particular services, e.g., to explore issues
relating to Quality of Service (QoS), to perform
fast re-routing of critical traffic, to mitigate
the characteristics of specific radio links, and
so on.
See also Sections 3.1-3.2, and 5.
Troubleshooting: Observable transport headers have been utilised
by operators as a part of network troubleshooting
and diagnostics. Metrics can help localise the
network segment introducing the loss or latency.
Effective troubleshooting often requires
understanding of transport behaviour. Flows
experiencing packet loss or jitter are hard to
distinguish from unaffected flows when only
observing network layer headers.
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Observable transport feedback information (e.g.,
RTP Control Protocol (RTCP) reception reports
[RFC3550]) can explicitly make loss metrics
visible to operators. Loss metrics can also be
deduced with more complexity from other header
information (e.g., by observing TCP SACK blocks).
When the transport header information is
encrypted, explicit observable fields could also
be made available at the network or transport
layers to provide these functions.
See also Section 3.3 and 5.
Network Protection: Observable transport headers currently provide
useful input to classify and detect anomalous
events, such as changes in application behaviour
or distributed denial of service attacks.
Operators often seek to uniquely disambiguate
unwanted traffic.
Where flows cannot be disambiguated based on
transport information, this could result in less-
efficient identification of unwanted traffic, the
introduction of rate limits for uncharacterised
traffic, or the use of heuristics to identify
anomalous flows.
See also Sections 6.2 and 6.3.
Verifiable Data: Observable transport headers can provide open and
verifiable measurements to support operations,
research, and protocol development. The ability
of multiple stake holders to review transport
header traces helps develop insight into
performance and traffic contribution of specific
variants of a protocol. Independently observed
data is important to help ensure the health of
the research and development communities.
When transport header information can not be
observed, this can reduce the range of actors
that can observe data. This limits the
information sources available to the Internet
community to understand the operation of new
transport protocols, reducing information to
inform design decisions and standardisation of
the new protocols and related operational
practises
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See also Section 6.
SLA Compliance: Observable transport headers coupled with
published transport specifications allow
operators and regulators to explore the
compliance with Service Level Agreements (SLAs).
When transport header information can not be
observed, other methods have to be found to
confirm that the traffic produced conforms to the
expectations of the operator or developer.
Independently verifiable performance metrics can
be utilised to demonstrate regulatory compliance
in some jurisdictions, and as a basis for
informing design decisions. This can bring
assurance to those operating networks, often
avoiding deployment of complex techniques that
routinely monitor and manage Internet traffic
flows (e.g., avoiding the capital and operational
costs of deploying flow rate-limiting and network
circuit-breaker methods [RFC8084]).
See also Sections 5 and 6.1-6.3.
Note, again, that this lists uses that have been made of transport
header information, and does not necessarily endorse any particular
approach.
3. Current uses of Transport Headers within the Network
In response to pervasive monitoring [RFC7624] revelations and the
IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
efforts are underway to increase encryption of Internet traffic.
Applying confidentiality to transport header fields affects how
protocol information is used [RFC8404], requiring consideration of
the trade-offs discussed in Section 2.3.
There are architectural challenges and considerations in the way
transport protocols are designed, and the ability to characterise and
compare different transport solutions [Measure]. The decision about
which transport headers fields are made observable offers trade-offs
around header confidentiality versus header observability (including
non-encrypted but authenticated header fields) for network operations
and management, and the implications for ossification and user
privacy. Different parties will view the relative importance of
these differently. For some, the benefits of encrypting all
transport headers outweigh the impact of doing so; others might
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analyse the security, privacy and ossification impacts and arrive at
a different trade-off.
To understand the implications, it is necessary to understand how
transport layer headers are currently observed and/or modified by
middleboxes within the network. This section therefore reviews
examples of current usage. It does not consider the intentional
modification of transport headers by middleboxes (such as in Network
Address Translation, NAT, or Firewalls). Common issues concerning IP
address sharing are described in [RFC6269].
3.1. Observing Transport Information in the Network
In-network observation of transport protocol headers requires
knowledge of the format of the transport header:
o Flows have to be identified at the level where observation is
performed. This implies visibility of the protocol and version of
the header, e.g., by defining the wire image [RFC8546]. As
protocols evolve over time, new transport headers could be
introduced. Detecting this could require interpretation of
protocol version information or connection setup information;
o Observing transport information depends on knowing the location
and syntax of the observed transport headers. IETF transport
protocols can specify this information.
The following subsections describe various ways that observable
transport information has been utilised.
3.1.1. Flow Identification Using Transport Layer Headers
Flow/Session identification [RFC8558] is a common function. For
example, performed by measurement activities, QoS classification,
firewalls, Denial of Service, DOS, prevention.
Observable transport header information, together with information in
the network header, has been used to identify flows and their
connection state, together with the set of protocol options being
used. Transport protocols, such as TCP and the Stream Control
Transport Protocol (SCTP), specify a standard base header that
includes sequence number information and other data. They also have
the possibility to negotiate additional headers at connection setup,
identified by an option number in the transport header.
In some uses, a low-numbered (well-known) transport port number can
identify the protocol. However, port information alone is not
sufficient to guarantee identification. Applications can use
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arbitrary ports, multiple sessions can be multiplexed on a single
port, and ports can be re-used by subsequent sessions. UDP-based
protocols often do not use well-known port numbers. Some flows can
be identified by observing signalling protocol data (e.g., [RFC3261],
[I-D.ietf-rtcweb-overview]) or through the use of magic numbers
placed in the first byte(s) of the datagram payload [RFC7983].
When transport header information can not be observed, this removes
information that could be used to classify flows by passive observers
along the path. More ambitious ways could be used to collect,
estimate, or infer flow information, including heuristics based on
the analysis of traffic patterns. For example, an operator that
cannot access the Session Description Protocol (SDP) session
descriptions to classify a flow as audio traffic, might instead use
(possibly less-reliable) heuristics to infer that short UDP packets
with regular spacing carry audio traffic. Operational practises
aimed at inferring transport parameters are out of scope for this
document, and are only mentioned here to recognize that encryption
does not prevent operators from attempting to apply practises that
were used with unencrypted transport headers.
3.1.2. Metrics derived from Transport Layer Headers
Observable transport headers enable explicit measurement and analysis
of protocol performance, network anomalies, and failure pathologies
at any point along the Internet path. Some operators use passive
monitoring to manage their portion of the Internet by characterizing
the performance of link/network segments. Inferences from transport
headers are used to derive performance metrics. A variety of open
source and commercial tools have been deployed that utilise transport
header information in this way to derive the following metrics:
Traffic Rate and Volume: Protocol sequence number and packet size
could be used to derive volume measures per-application, to
characterise the traffic that uses a network segment or the
pattern of network usage. Measurements can be per endpoint or for
an endpoint aggregate (e.g., to assess subscriber usage).
Measurements can also be used to trigger traffic shaping, and to
associate QoS support within the network and lower layers. Volume
measures can also be valuable for capacity planning and providing
detail of trends in usage. The traffic rate and volume can be
determined providing that the packets belonging to individual
flows can be identified, but there might be no additional
information about a flow when the transport headers cannot be
observed.
Loss Rate and Loss Pattern: Flow loss rate can be derived (e.g.,
from transport sequence numbers or inferred from observing
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transport protocol interactions) and has been used as a metric for
performance assessment and to characterise transport behaviour.
Understanding the location and root cause of loss can help an
operator determine whether this requires corrective action.
Network operators have used the variation in patterns of loss as a
key performance metric, utilising this to detect changes in the
offered service.
There are various causes of loss, including: corruption of link
frames (e.g., due to interference on a radio link), buffering loss
(e.g., overflow due to congestion, Active Queue Management, AQM
[RFC7567], or inadequate provision following traffic pre-emption),
and policing (traffic management). Understanding flow loss rates
requires either observing sequence numbers in network or transport
headers, or maintaining per-flow packet counters (flow
identification often requires transport header information). Per-
hop loss can also sometimes be monitored at the interface level by
devices in the network.
Losses can often occur as bursts, randomly-timed events, etc. The
pattern of loss can provide insight into the cause of loss. It
can also be valuable to understand the conditions under which loss
occurs, which usually requires relating loss to the traffic
flowing on the network node/segment at the time of loss. This can
also help identify cases where loss could have been wrongly
identified, or where the transport did not require transmission of
a lost packet.
Throughput and Goodput: Throughput is the amount of payload data
sent by a flow per time interval. Goodput [RFC7928] is a measure
of useful data exchanged (the ratio of useful data to total volume
of traffic sent by a flow). The throughput of a flow can be
determined in the absence of transport header information,
providing that the individual flow can be identified, and the
overhead known. Goodput requires ability to differentiate loss
and retransmission of packets, for example by observing packet
sequence numbers in the TCP or the Real-time Transport Protocol
(RTP) headers [RFC3550].
Latency: Latency is a key performance metric that impacts
application and user-perceived response times. It often
indirectly impacts throughput and flow completion time. This
determines the reaction time of the transport protocol itself,
impacting flow setup, congestion control, loss recovery, and other
transport mechanisms. The observed latency can have many
components [Latency]. Of these, unnecessary/unwanted queuing in
network buffers has often been observed as a significant factor
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[bufferbloat]. Once the cause of unwanted latency has been
identified, this can often be eliminated.
To measure latency across a part of a path, an observation point
[RFC7799] can measure the experienced round trip time (RTT) using
packet sequence numbers and acknowledgements, or by observing
header timestamp information. Such information allows an
observation point in the network to determine not only the path
RTT, but also allows measurement of the upstream and downstream
contribution to the RTT. This could be used to locate a source of
latency, e.g., by observing cases where the median RTT is much
greater than the minimum RTT for a part of a path.
The service offered by network operators can benefit from latency
information to understand the impact of configuration changes and
to tune deployed services. Latency metrics are key to evaluating
and deploying AQM [RFC7567], DiffServ [RFC2474], and Explicit
Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements
could identify excessively large buffers, indicating where to
deploy or configure AQM. An AQM method is often deployed in
combination with other techniques, such as scheduling [RFC7567]
[RFC8290] and although parameter-less methods are desired
[RFC7567], current methods often require tuning [RFC8290]
[RFC8289] [RFC8033] because they cannot scale across all possible
deployment scenarios.
Variation in delay: Some network applications are sensitive to
(small) changes in packet timing (jitter). Short and long-term
delay variation can impact on the latency of a flow and hence the
perceived quality of applications using the network. For example,
jitter metrics are often cited when characterising paths
supporting real-time traffic. The expected performance of such
applications, can be inferred from a measure the variation in
delay observed along a portion of the path [RFC3393] [RFC5481].
The requirements resemble those for the measurement of latency.
Flow Reordering: Significant packet reordering within a flow can
impact time-critical applications and can be interpreted as loss
by reliable transports. Many transport protocol techniques are
impacted by reordering (e.g., triggering TCP retransmission or re-
buffering of real-time applications). Packet reordering can occur
for many reasons, from equipment design to misconfiguration of
forwarding rules. Network tools can detect and measure unwanted/
excessive reordering, and the impact on transport performance.
There have been initiatives in the IETF transport area to reduce
the impact of reordering within a transport flow, possibly leading
to a reduction in the requirements for preserving ordering. These
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have potential to simplify network equipment design as well as the
potential to improve robustness of the transport service.
Measurements of reordering can help understand the present level
of reordering within deployed infrastructure, and inform decisions
about how to progress such mechanisms. Key performance indicators
are retransmission rate, packet drop rate, sector utilisation
level, a measure of reordering, peak rate, the ECN congestion
experienced (CE) marking rate, etc.
Metrics have been defined that evaluate whether a network has
maintained packet order on a packet-by-packet basis [RFC4737]
[RFC5236].
Techniques for measuring reordering typically observe packet
sequence numbers. Some protocols provide in-built monitoring and
reporting functions. Transport fields in the RTP header [RFC3550]
[RFC4585] can be observed to derive traffic volume measurements
and provide information on the progress and quality of a session
using RTP. As with other measurement, metadata assist in
understanding the context under which the data was collected,
including the time, observation point [RFC7799], and way in which
metrics were accumulated. The RTCP protocol directly reports some
of this information in a form that can be directly visible in the
network. A user of summary measurement data has to trust the
source of this data and the method used to generate the summary
information.
These metrics can support network operations, inform capacity
planning, and assist in determining the demand for equipment and/or
configuration changes by network operators. They can also inform
Internet engineering activities by informing the development of new
protocols, methodologies, and procedures.
In some cases, measurements could involve active injection of test
traffic to perform a measurement (see section 3.4 of [RFC7799]).
However, most operators do not have access to user equipment,
therefore the point of test is normally different from the transport
endpoint. Injection of test traffic can incur an additional cost in
running such tests (e.g., the implications of capacity tests in a
mobile network are obvious). Some active measurements [RFC7799]
(e.g., response under load or particular workloads) perturb other
traffic, and could require dedicated access to the network segment.
Passive measurements (see section 3.6 of [RFC7799]) can have
advantages in terms of eliminating unproductive test traffic,
reducing the influence of test traffic on the overall traffic mix,
and the ability to choose the point of observation (see
Section 3.2.1). Measurements can rely on observing packet headers,
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which is not possible if those headers are encrypted, but could
utilise information about traffic volumes or patterns of interaction
to deduce metrics.
An alternative approach is to use in-network techniques add and
observe packet headers to facilitate measurements while traffic
traverses an operational network. This approach does not require the
cooperation of an endpoint.
3.1.3. Transport use of Network Layer Header Fields
Information from the transport protocol is used by a multi-field
classifier as a part of policy framework. Policies are commonly used
for management of the QoS or Quality of Experience (QoE) in resource-
constrained networks, and by firewalls to implement access rules (see
also section 2.2.2 of [RFC8404]). Network-layer classification
methods that rely on a multi-field classifier (e.g., inferring QoS
from the 5-tuple or choice of application protocol) are incompatible
with transport protocols that encrypt the transport information.
Traffic that cannot be classified typically receives a default
treatment.
Transport information can also be explicitly set in network-layer
header fields that are not encrypted, serving as a replacement/
addition to the exposed transport information [RFC8558]. This
information can enable a different forwarding treatment by the
network, even when a transport employs encryption to protect other
header information.
The user of a transport that multiplexes multiple sub-flows might
want to obscure the presence and characteristics of these sub-flows.
On the other hand, an encrypted transport could set the network-layer
information to indicate the presence of sub-flows, and to reflect the
service requirements of individual sub-flows. There are several ways
this could be done:
IP Address: Applications normally expose the addresses used by
endpoints, and this is used in the forwarding decisions in network
devices. Address and other protocol information can be used by a
Multi-Field (MF) classifier to determine how traffic is treated
[RFC2475], and hence the quality of experience for a flow.
Using the IPv6 Network-Layer Flow Label: A number of Standards Track
and Best Current Practice RFCs (e.g., [RFC8085], [RFC6437],
[RFC6438]) encourage endpoints to set the IPv6 Flow label field of
the network-layer header. IPv6 "source nodes SHOULD assign each
unrelated transport connection and application data stream to a
new flow" [RFC6437]. A multiplexing transport could choose to use
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multiple Flow labels to allow the network to independently forward
sub-flows. RFC6437 provides further guidance on choosing a flow
label value, stating these "should be chosen such that their bits
exhibit a high degree of variability", and chosen so that "third
parties should be unlikely to be able to guess the next value that
a source of flow labels will choose".
Once set, a flow label can provide information that can help
inform network-layer queuing and forwarding [RFC6438], for example
with Equal Cost Multi-Path routing and Link Aggregation [RFC6294].
Considerations when using IPsec are further described in
[RFC6438].
The choice of how to assign a Flow Label needs to avoid
introducing linkability that a network device could observe.
Inappropriate use by the transport can have privacy implications
(e.g., assigning the same label to two independent flows that
ought not to be classified the same).
Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to the network
by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications
identify different forwarding treatments for individual sub-flows
(audio vs. video) based on the value of the DSCP field
[I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information
to inform network-layer queuing and forwarding, rather than an
operator inferring traffic requirements from transport and
application headers via a multi-field classifier. Inappropriate
use by the transport can have privacy implications (e.g.,
assigning a different DSCP to a subflow could assist in a network
device discovering the traffic pattern used by an application,
assigning the same label to two independent flows that ought not
to be classified the same). The field is mutable, i.e., some
network devices can be expected to change this field (use of each
DSCP value is defined by an RFC).
Since the DSCP value can impact the quality of experience for a
flow, observations of service performance has to consider this
field when a network path supports differentiated service
treatment.
Using Explicit Congestion Marking: ECN [RFC3168] is a transport
mechanism that utilises the ECN field in the network-layer header.
Use of ECN explicitly informs the network-layer that a transport
is ECN-capable, and requests ECN treatment of the flow. An ECN-
capable transport can offer benefits when used over a path with
equipment that implements an AQM method with CE marking of IP
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packets [RFC8087], since it can react to congestion without also
having to recover from lost packets.
ECN exposes the presence of congestion. The reception of CE-
marked packets can be used to estimate the level of incipient
congestion on the upstream portion of the path from the point of
observation (Section 2.5 of [RFC8087]). Interpreting the marking
behaviour (i.e., assessing congestion and diagnosing faults)
requires context from the transport layer, such as path RTT.
AQM and ECN offer a range of algorithms and configuration options.
Tools therefore have to be available to network operators and
researchers to understand the implication of configuration choices
and transport behaviour as the use of ECN increases and new
methods emerge [RFC7567].
When transport headers cannot be observed, operators are unable to
use this information directly. Careful use of the network layer
features can help provide similar information in the case where the
network is unable to inspect transport protocol headers.
Section Section 5 describes use of network extension headers.
3.2. Transport Measurement
The common language between network operators and application/content
providers/users is packet transfer performance at a layer that all
can view and analyse. For most packets, this has been the transport
layer, until the emergence of transport protocols performing header
encryption, with the obvious exception of VPNs and IPsec.
When encryption hides more layers in each packet, people seeking
understanding of the network operation rely more on pattern inference
and other heuristics. It remains to be seen whether more complex
inferences can be mastered to produce the same monitoring accuracy
(see section 2.1.1 of [RFC8404]).
When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network, is
often necessary to interpret this data to answer questions about
network performance or understand a pathology. Collecting and
coordinating such metadata is more difficult when the observation
point is at a different location to the bottleneck/device under
evaluation [RFC7799].
Packet sampling techniques are used to scale the processing involved
in observing packets on high rate links. This exports only the
packet header information of (randomly) selected packets. The
utility of these measurements depends on the type of bearer and
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number of mechanisms used by network devices. Simple routers are
relatively easy to manage, a device with more complexity demands
understanding of the choice of many system parameters. This level of
complexity exists when several network methods are combined.
This section discusses topics concerning observation of transport
flows, with a focus on transport measurement.
3.2.1. Point of Observation
On-path measurements are particularly useful for locating the source
of problems, or to assess the performance of a network segment or a
particular device configuration. Often issues can only be understood
in the context of the other flows that share a particular path,
common network device, interface port, etc. A simple example is
monitoring of a network device that uses a scheduler or active queue
management technique [RFC7567], where it could be desirable to
understand whether the algorithms are correctly controlling latency,
or if overload protection is working. This understanding implies
knowledge of how traffic is assigned to any sub-queues used for flow
scheduling, but can also require information about how the traffic
dynamics impact active queue management, starvation prevention
mechanisms, and circuit-breakers.
Sometimes multiple on-path observation points have to be used. By
correlating observations of headers at multiple points along the path
(e.g., at the ingress and egress of a network segment), an observer
can determine the contribution of a portion of the path to an
observed metric, to locate a source of delay, jitter, loss,
reordering, congestion marking, etc.
3.2.2. Use by Operators to Plan and Provision Networks
Traffic rate and volume measurements are used by operators to help
plan deployment of new equipment and configuration in their networks.
Data is also valuable to equipment vendors who want to understand
traffic trends and patterns of usage as inputs to decisions about
planning products and provisioning for new deployments. This
measurement information can also be correlated with billing
information when this is also collected by an operator.
Trends in aggregate traffic can be observed and can be related to the
endpoint addresses being used, but when transport information is not
observable, it might be impossible to correlate patterns in
measurements with changes in transport protocols. This increases the
dependency on other indirect sources of information to inform
planning and provisioning.
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3.2.3. Service Performance Measurement
Performance measurements (e.g., throughput, loss, latency) can be
used by various actors to analyse the service offered to the users of
a network segment, and to inform operational practice.
3.2.4. Measuring Transport to Support Network Operations
The traffic that can be observed by on-path network devices (the
"wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational
concern, although the ability to measure and monitor it is less. The
desire to understand the traffic and protocol interactions typically
grows as the proportion of traffic increases in volume. The
challenges increase when multiple instances of an evolving protocol
contribute to the traffic that share network capacity.
Operators can manage traffic load (e.g., when the network is severely
overloaded) by deploying rate-limiters, traffic shaping, or network
transport circuit breakers [RFC8084]. The information provided by
observing transport headers is a source of data that can help to
inform such mechanisms.
Congestion Control Compliance of Traffic: Congestion control is a
key transport function [RFC2914]. Many network operators
implicitly accept that TCP traffic complies with a behaviour that
is acceptable for the shared Internet. TCP algorithms have been
continuously improved over decades, and have reached a level of
efficiency and correctness that is difficult to match in custom
application-layer mechanisms [RFC8085].
A standards-compliant TCP stack provides congestion control that
is judged safe for use across the Internet. Applications
developed on top of well-designed transports can be expected to
appropriately control their network usage, reacting when the
network experiences congestion, by back-off and reduce the load
placed on the network. This is the normal expected behaviour for
IETF-specified transports (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the
transport protocol header information to help understand the
impact of specific transport protocols (or protocol mechanisms) on
the other traffic that shares a network. An observation in the
network can gain an understanding of the dynamics of a flow and
its congestion control behaviour. Analysing observed flows can
help to build confidence that an application flow backs-off its
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share of the network load under persistent congestion, and hence
to understand whether the behaviour is appropriate for sharing
limited network capacity. For example, it is common to visualise
plots of TCP sequence numbers versus time for a flow to understand
how a flow shares available capacity, deduce its dynamics in
response to congestion, etc.
The ability to identify sources that contribute to persistent
congestion is important to the safe operation of network
infrastructure, and can inform configuration of network devices to
complement the endpoint congestion avoidance mechanisms [RFC7567]
[RFC8084] to avoid a portion of the network being driven into
congestion collapse [RFC2914].
Congestion Control Compliance for UDP traffic: UDP provides a
minimal message-passing datagram transport that has no inherent
congestion control mechanisms. Because congestion control is
critical to the stable operation of the Internet, applications and
other protocols that choose to use UDP as a transport have to
employ mechanisms to prevent collapse, avoid unacceptable
contributions to jitter/latency, and to establish an acceptable
share of capacity with concurrent traffic [RFC8085].
A network operator can observe the headers of transport protocols
layered above UDP to understand if the datagram flows comply with
congestion control expectations. This can help inform a decision
on whether it might be appropriate to deploy methods such as rate-
limiters to enforce acceptable usage.
UDP flows that expose a well-known header can be observed to gain
understanding of the dynamics of a flow and its congestion control
behaviour. For example, tools exist to monitor various aspects of
RTP header information and RTCP reports for real-time flows (see
Section 3.1.2). The Secure RTP and RTCP extensions [RFC3711] were
explicitly designed to expose some header information to enable
such observation, while protecting the payload data.
3.3. Use for Network Diagnostics and Troubleshooting
Transport header information can be utilised for a variety of
operational tasks [RFC8404]: to diagnose network problems, assess
network provider performance, evaluate equipment or protocol
performance, capacity planning, management of security threats
(including denial of service), and responding to user performance
questions. Section 3.1.2 and Section 5 of [RFC8404] provide further
examples.
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Operators can monitor the health of a portion of the Internet, to
provide early warning and trigger action. Traffic and performance
measurements can assist in setting buffer sizes, debugging and
diagnosing the root causes of faults that concern a particular user's
traffic. They can also be used to support post-mortem investigation
after an anomaly to determine the root cause of a problem.
In other cases, measurement involves dissecting network traffic
flows. Observed transport header information can help identify
whether link/network tuning is effective and alert to potential
problems that can be hard to derive from link or device measurements
alone.
An alternative could rely on access to endpoint diagnostic tools or
user involvement in diagnosing and troubleshooting unusual use cases
or to troubleshoot non-trivial problems.
Another approach is to use traffic pattern analysis. Such tools can
provide useful information during network anomalies (e.g., detecting
significant reordering, high or intermittent loss), however indirect
measurements would need to be carefully designed to provide reliable
signals for diagnostics and troubleshooting.
The design trade-offs for radio networks are often very different
from those of wired networks. A radio-based network (e.g., cellular
mobile, enterprise WiFi, satellite access/back-haul, point-to-point
radio) has the complexity of a subsystem that performs radio resource
management, with direct impact on the available capacity, and
potentially loss/reordering of packets. The impact of the pattern of
loss and congestion, differs for different traffic types, correlation
with propagation and interference can all have significant impact on
the cost and performance of a provided service. For radio links, the
use for this type of information is expected to increase as operators
bring together heterogeneous types of network equipment and seek to
deploy opportunistic methods to access radio spectrum.
Lack of tools and resulting information can reduce the ability of an
operator to observe transport performance and could limit the ability
of network operators to trace problems, make appropriate QoS
decisions, or respond to other queries about the network service.
A network operator supporting traffic that uses transport header
encryption is unable to use tools that rely on transport protocol
information. However, the use of encryption has the desirable effect
of preventing unintended observation of the payload data and these
tools seldom seek to observe the payload, or other application
details. A flow that hides its transport header information could
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imply "don't touch" to some operators. This might limit a trouble-
shooting response to "can't help, no trouble found".
3.4. Header Compression
Header compression saves link capacity by compressing network and
transport protocol headers on a per-hop basis. It was widely used
with low bandwidth dial-up access links, and still finds application
on wireless links that are subject to capacity constraints. Header
compression has been specified for use with TCP/IP and RTP/UDP/IP
flows [RFC2507], [RFC2508], [RFC4995].
While it is possible to compress only the network layer headers,
significant savings can be made if both the network and transport
layer headers are compressed together as a single unit. The Secure
RTP extensions [RFC3711] were explicitly designed to leave the
transport protocol headers unencrypted, but authenticated, since
support for header compression was considered important. Encrypting
the transport protocol headers does not break such header
compression, but does cause a fall back to compressing only the
network layer headers, with a significant reduction in efficiency.
4. Encryption and Authentication of Transport Headers
End-to-end encryption can be applied at various protocol layers. It
can be applied above the transport to encrypt the transport payload
(e.g., using TLS). This can hide information from an eavesdropper in
the network. It can also help protect the privacy of a user, by
hiding data relating to user/device identity or location.
4.1. Motivation
There are several motivations for encryption:
o One motive to encrypt transport headers is in response to
perceptions that the network has become ossified, since traffic
inspecting middleboxes prevent new protocols and mechanisms from
being deployed. This has lead to a 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. One benefit of encrypting transport headers is that it
can help improve the pace of transport development by eliminating
interference by deployed middleboxes.
o Another motivation stems from increased concerns about privacy and
surveillance. Users value the ability to protect their identity
and location, and defend against traffic analysis. Revelations
about the use of pervasive surveillance [RFC7624] have, to some
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extent, eroded trust in the service offered by network operators
and have led to an increased use of encryption to avoid unwanted
eavesdropping on communications. Concerns have also been voiced
about the addition of information to packets by third parties to
provide analytics, customization, advertising, cross-site tracking
of users, to bill the customer, or to selectively allow or block
content. Whatever the reasons, the IETF is designing protocols
that include transport header encryption (e.g., QUIC
[I-D.ietf-quic-transport]) to supplement the already widespread
payload encryption, and to further limit exposure of transport
metadata to the network.
The use of transport header authentication and encryption exposes a
tussle between middlebox vendors, operators, applications developers
and users:
o On the one hand, future Internet protocols that support transport
header encryption assist in the restoration of the end-to-end
nature of the Internet by returning complex processing to the
endpoints, since middleboxes cannot modify what they cannot see,
and can improve privacy by reducing leakage of transport metadata.
o On the other hand, encryption of transport layer header
information has implications for people who are responsible for
operating networks, and researchers and analysts seeking to
understand the dynamics of protocols and traffic patterns.
A decision to use transport header encryption can improve user
privacy, and can reduce protocol ossification and help the evolution
of the transport protocol stack, but is also has implications for
network operations and management.
4.2. Approaches to Transport Header Protection
The following briefly reviews some security design options for
transport protocols. A Survey of Transport Security Protocols
[I-D.ietf-taps-transport-security] provides more details concerning
commonly used encryption methods at the transport layer.
Authenticating the Transport Protocol Header: Transport layer header
information can be authenticated. An integrity check that
protects the immutable transport header fields, but can still
expose the transport protocol header information in the clear,
allows in-network devices to observe these fields. An integrity
check is not able to prevent in-network modification, but can
prevent a receiving from accepting changes and avoid impact on the
transport protocol operation.
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An example transport authentication mechanism is TCP-
Authentication (TCP-AO) [RFC5925]. This TCP option authenticates
the IP pseudo header, TCP header, and TCP data. TCP-AO protects
the transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. TCP-AO might
interact with middleboxes, depending on their behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This
approach authenticates all transport headers, and verifies their
integrity at the receiver, preventing in-network modification.
Secure RTP [RFC3711] is another example of a transport protocol
that allows header authentication.
Greasing: Protocols often provide extensibility features, reserving
fields or values for use by future versions of a specification.
The specification of receivers has traditionally ignored
unspecified values, however in-network devices have emerged that
ossify to require a certain value in a field, or re-use a field
for another purpose. When the specification is later updated, it
is impossible to deploy the new use of the field, and forwarding
of the protocol could even become conditional on a specific header
field value.
A protocol can intentionally vary the value, format, and/or
presence of observable transport header fields. This behaviour,
known as GREASE (Generate Random Extensions And Sustain
Extensibility) is designed to avoid a network device ossifying the
use of a specific observable field. Greasing seeks to ease
deployment of new methods. It can also prevent in-network devices
utilising the information in a transport header, or can make an
observation robust to a set of changing values, rather than a
specific set of values
Selectively Encrypting Transport Headers and Payload: A transport
protocol design can encrypt selected header fields, while also
choosing to authenticate the entire transport header. This allows
specific transport header fields to be made observable by network
devices. End-to end integrity checks can prevent an endpoint from
undetected modification of the immutable transport headers.
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 can be authenticated End-to-End across a path.
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An example of a method that encrypts some, but not all, transport
information is GRE-in-UDP [RFC8086] when used with GRE encryption.
Optional Encryption of Header Information: There are implications to
the use of optional header encryption in the design of a transport
protocol, where support of optional mechanisms can increase the
complexity of the protocol and its implementation, and in the
management decisions that are have to be made to use variable
format fields. Instead, fields of a specific type ought to always
be sent with the same level of confidentiality or integrity
protection.
As seen, different transports use encryption to protect their header
information to varying degrees. The trend is towards increased
protection.
5. Addition of Transport Information to Network-Layer Headers
An on-path device can make measurements by utilising additional
protocol headers carrying operations, administration and management
(OAM) information in an additional packet header. Using network-
layer approaches to reveal information has the potential that the
same method (and hence same observation and analysis tools) can be
consistently used by multiple transport protocols [RFC8558]. There
could also be less desirable implications of separating the operation
of the transport protocol from the measurement framework.
5.1. Use of OAM within a Maintenance Domain
OAM information can be added at the ingress to a maintenance domain
(e.g., an Ethernet protocol header with timestamps and sequence
number information using a method such as 802.11ag or in-situ OAM
[I-D.ietf-ippm-ioam-data], or as a part of encapsulation protocol).
The additional header information is typically removed the at the
egress of the maintenance domain.
Although some types of measurements are supported, this approach does
not cover the entire range of measurements described in this
document. In some cases, it can be difficult to position measurement
tools at the appropriate segments/nodes and there can be challenges
in correlating the downstream/upstream information when in-band OAM
data is inserted by an on-path device.
5.2. Use of OAM across Multiple Maintenance Domains
OAM information can also be added at the network layer as an IPv6
extension header or an IPv4 option. This information can be used
across multiple network segments, or between the transport endpoints.
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One example is the IPv6 Performance and Diagnostic Metrics (PDM)
Destination Option [RFC8250]. This allows a sender to optionally
include a destination option that caries header fields that can be
used to observe timestamps and packet sequence numbers. This
information could be authenticated by receiving transport endpoints
when the information is added at the sender and visible at the
receiving endpoint, although methods to do this have not currently
been proposed. This method has to be explicitly enabled at the
sender.
Current measurement results suggest that it could currently be
undesirable to rely on methods requiring end-to-end support of
network options or extension headers across the Internet. IPv4
network options are often not supported (or are carried on a slower
processing path) and some IPv6 networks have been observed to drop
packets that set an IPv6 header extension (e.g., results from 2016 in
[RFC7872]).
Protocols can be designed to expose header information separately to
the (hidden) fields used by the protocol state machine. On the one
hand, such approaches can simplify tools by exposing the relevant
metrics (loss, latency, etc), rather having to derive this from other
fields. This also permits the protocol to evolve independently of
the ossified observable header [RFC8558]. On the other hand,
protocols do not necessarily have an incentive to expose the actual
information that is utilised by the protocol itself and could
therefore manipulate the exposed header information to gain an
advantage from the network. Where the information is provided by an
endpoint, the incentive to reflect actual transport information has
to be considered when proposing a method.
6. Implications of Protecting the Transport Headers
The choice of which transport header fields to expose and which to
encrypt is a design decision for the transport protocol. Selective
encryption requires trading conflicting goals of observability and
network support, privacy, and risk of ossification, to decide what
header fields to protect and which to make visible.
Security work typically employs a design technique that seeks to
expose only what is needed. This approach provides incentives to not
reveal any information that is not necessary for the end-to-end
communication. However, there can be performance and operational
benefits in exposing selected information to network tools.
This section explores key implications of working with encrypted
transport protocols.
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6.1. Independent Measurement
Independent observation by multiple actors is important if the
transport community is to maintain an accurate understanding of the
network. Encrypting transport header encryption changes the ability
to collect and independently analyse data. Internet transport
protocols employ a set of mechanisms. Some of these have to work in
cooperation with the network layer for loss detection and recovery,
congestion detection and control. Others have to work only end-to-
end (e.g., parameter negotiation, flow-control).
The majority of present Internet applications use two well-known
transport protocols, TCP and UDP. Although TCP represents the
majority of current traffic, many real-time applications use UDP, and
much of this traffic utilises RTP format headers in the payload of
the UDP datagram. Since these protocol headers have been fixed for
decades, a range of tools and analysis methods have became common and
well-understood.
Protocols that expose the state information used by the transport
protocol in their header information (e.g., timestamps used to
calculate the RTT, packet numbers used to asses congestion and
requests for retransmission) provide an incentive for the sending
endpoint to provide correct information, since the protocol will not
work otherwise. This increases confidence that the observer
understands the transport interaction with the network. For example,
when TCP is used over an unencrypted network path (i.e., one that
does not use IPsec or other encryption below the transport), it
implicitly exposes header information that can be used for
measurement at any point along the path. This information is
necessary for the protocol's correct operation, therefore there is no
incentive for a TCP or RTP implementation to put incorrect
information in this transport header. A network device can have
confidence that the well-known (and ossified) transport information
represents the actual state of the endpoints.
When encryption is used to hide some or all of the transport headers,
the transport protocol chooses which information to reveal to the
network about its internal state, what information to leave
encrypted, and what fields to grease to protect against future
ossification. Such a transport could be designed, for example, to
provide summary data regarding its performance, congestion control
state, etc., or to make an explicit measurement signal available.
For example, a QUIC endpoint can optionally set the spin bit to
reflect to explicitly reveal the RTT of an encrypted transport
session to the on-path network devices [I-D.ietf-quic-transport]).
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When providing or using such information, it is important to consider
the privacy of the user and their incentive for providing accurate
and detailed information. Protocols that selectively reveal some
transport state or measurement signals are choosing to establish a
trust relationship with the network operators. There is no protocol
mechanism that can guarantee that the information provided represents
the actual transport state of the endpoints, since those endpoints
can always send additional information in the encrypted part of the
header, to update or replace whatever they reveal. This reduces the
ability to independently measure and verify that a protocol is
behaving as expected. For some operational uses, the information has
to contain sufficient detail to understand, and possibly reconstruct,
the network traffic pattern for further testing. In this case,
operators have to gain the trust of transport protocol implementers
if the transport headers are to correctly reveal such information.
Operations, Administration, and Maintenance (OAM) data records
[I-D.ietf-ippm-ioam-data] could be embedded into a variety of
encapsulation methods at different layers to support the goals of a
specific operational domain. OAM-related metadata can support
functions such as performance evaluation, path-tracing, path
verification information, classification and a diversity of other
uses. When encryption is used to hide some or all of the transport
headers, analysis requires coordination between actors at different
layers to successfully characterise flows and correlate the
performance or behaviour of a specific mechanism with the
configuration and traffic using operational equipment (e.g.,
combining transport and network measurements to explore congestion
control dynamics, the implications of designs for active queue
management or circuit breakers).
Some measurements could be completed by utilising endpoint-based
logging (e.g., based on Quic-Trace [Quic-Trace]). Such information
has a diversity of uses, including developers wishing to debug/
understand the transport/application protocols with which they work,
researchers seeking to spot trends and anomalies, and to characterise
variants of protocols. A standard format for endpoint logging could
allow these to be shared (after appropriate anonymisation) to
understand performance and pathologies. Measurements based on
logging have to establish the validity and provenance of the logged
information to establish how and when traces were captured.
Despite being applicable in some scenarios, endpoint logs do not
provide equivalent information to in-network measurements. In
particular, endpoint logs contain only a part of the information to
understand the operation of network devices and identify issues such
as link performance or capacity sharing between multiple flows.
Additional information has to be combined to determine which
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equipment/links are used and the configuration of equipment along the
network paths being measured.
6.2. Characterising "Unknown" Network Traffic
The patterns and types of traffic that share Internet capacity change
over time as networked applications, usage patterns and protocols
continue to evolve.
If "unknown" or "uncharacterised" traffic patterns form a small part
of the traffic aggregate passing through a network device or segment
of the network the path, the dynamics of the uncharacterised traffic
might not have a significant collateral impact on the performance of
other traffic that shares this network segment. Once the proportion
of this traffic increases, monitoring the traffic can determine if
appropriate safety measures have to be put in place.
Tracking the impact of new mechanisms and protocols requires traffic
volume to be measured and new transport behaviours to be identified.
This is especially true of protocols operating over a UDP substrate.
The level and style of encryption has to be considered in determining
how this activity is performed. On a shorter timescale, information
could also have to be collected to manage denial of service attacks
against the infrastructure.
6.3. Accountability and Internet Transport Protocols
Information provided by tools observing transport headers can be used
to classify traffic, and to limit the network capacity used by
certain flows, as discussed in Section 3.2.4). Equally, operators
could use analysis of transport headers and transport flow state to
demonstrate that they are not providing differential treatment to
certain flows. Obfuscating or hiding this information using
encryption could lead operators and maintainers of middleboxes
(firewalls, etc.) to seek other methods to classify, and potentially
other mechanisms to condition, network traffic.
A lack of data that reduces the level of precision with which flows
can be classified also reduces the design space for conditioning
mechanisms (e.g., rate limiting, circuit breaker techniques
[RFC8084], or blocking of uncharacterised traffic), and this has to
be considered when evaluating the impact of designs for transport
encryption [RFC5218].
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6.4. Impact on Operational Cost
Some network operators currently use observed transport header
information as a part of their operational practice, and have
developed tools and techniques that use information observed in
currently deployed transports and their applications. A variety of
open source and proprietary tools have been deployed that use this
information for a variety of short and long term measurements.
Encryption of the transport information prevents tooling from
observing the header information, limiting its utility.
Alternative diagnostic and troubleshooting tools would have to be
developed and deployed is transport header encryption is widely
deployed. Introducing a new protocol or application might then
require these tool chains and practises to be updated, and could in
turn impact operational mechanisms, and policies. Each change can
introduce associated costs, including the cost of collecting data,
and the tooling to handle multiple formats (possibly as these co-
exist in the network, when measurements span time periods during
which changes are deployed, or to compare with historical data).
These costs are incurred by an operator to manage the service and
debug network issues.
At the time of writing, the additional operational cost of using
encrypted transports is not yet well understood. Design trade-offs
could mitigate these costs by explicitly choosing to expose selected
information (e.g., header invariants and the spin-bit in QUIC
[I-D.ietf-quic-transport]), the specification of common log formats,
and development of alternative approaches.
6.5. Impact on Research, Development and Deployment
Transport protocol evolution, and the ability to measure and
understand the impact of protocol changes, have to proceed hand-in-
hand. Observable transport headers can provide open and verifiable
measurement data. Observation of pathologies has a critical role in
the design of transport protocol mechanisms and development of new
mechanisms and protocols. This helps understanding the interactions
between cooperating protocols and network mechanism, the implications
of sharing capacity with other traffic and the impact of different
patterns of usage. The ability of other stake holders to review
transport header traces helps develop insight into performance and
traffic contribution of specific variants of a protocol.
Development of new transport protocol mechanisms has to consider the
scale of deployment and the range of environments in which the
transport is used. Experience has shown that it is often difficult
to correctly implement new mechanisms [RFC8085], and that mechanisms
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often evolve as a protocol matures, or in response to changes in
network conditions, changes in network traffic, or changes to
application usage. Analysis is especially valuable when based on the
behaviour experienced across a range of topologies, vendor equipment,
and traffic patterns.
New transport protocol formats are expected to facilitate an
increased pace of transport evolution, and with it the possibility to
experiment with and deploy a wide range of protocol mechanisms.
There has been recent interest in a wide range of new transport
methods, e.g., Larger Initial Window, Proportional Rate Reduction
(PRR), congestion control methods based on measuring bottleneck
bandwidth and round-trip propagation time, the introduction of AQM
techniques and new forms of ECN response (e.g., Data Centre TCP,
DCTP, and methods proposed for L4S).The growth and diversity of
applications and protocols using the Internet also continues to
expand. For each new method or application it is desirable to build
a body of data reflecting its behaviour under a wide range of
deployment scenarios, traffic load, and interactions with other
deployed/candidate methods.
Encryption of transport header information could reduce the range of
actors that can observe useful data. This would limit the
information sources available to the Internet community to understand
the operation of new transport protocols, reducing information to
inform design decisions and standardisation of the new protocols and
related operational practises. The cooperating dependence of
network, application, and host to provide communication performance
on the Internet is uncertain when only endpoints (i.e., at user
devices and within service platforms) can observe performance, and
when performance cannot be independently verified by all parties.
Independently observed data is also important to ensure the health of
the research and development communities and can help promote
acceptance of proposed specifications by the wider community (e.g.,
as a method to judge the safety for Internet deployment) and provides
valuable input during standardisation. Open standards motivate a
desire to include independent observation and evaluation of
performance data, which in turn demands control over where and when
measurement samples are collected. This requires consideration of
the methods used to observe data and the appropriate balance between
encrypting all and no transport information.
7. Conclusions
Header encryption and strong integrity checks are being incorporated
into new transport protocols and have important benefits. The pace
of development of transports using the WebRTC data channel, and the
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rapid deployment of the QUIC transport protocol, can both be
attributed to using the combination of UDP as a substrate while
providing confidentiality and authentication of the encapsulated
transport headers and payload.
This document has described some current practises, and the
implications for some stakeholders, when transport layer header
encryption is used. It does not judge whether these practises are
necessary, or endorse the use of any specific practise. Rather, the
intent is to highlight operational tools and practises to consider
when designing transport protocols, so protocol designers can make
informed choice about what transport header fields to encrypt, and
whether it might be beneficial to make an explicit choice to expose
certain fields to the network. In making such a decision, it is
important to balance:
o User Privacy: The less transport header information that is
exposed to the network, the lower the risk of leaking metadata
that might have privacy implications for the users. Transports
that chose to expose some header fields need to make a privacy
assessment to understand the privacy cost versus benefit trade-off
in making that information available. The process used to define
and expose the QUIC spin bit to the network is an example of such
an analysis.
o Protocol Ossification: Unencrypted transport header fields are
likely to ossify rapidly, as middleboxes come to rely on their
presence, making it difficult to change the transport in future.
This argues that the choice to expose information to the network
is made deliberately and with care, since it is essentially
defining a stable interface between the transport and the network.
Some protocols will want to make that interface as limited as
possible; other protocols might find value in exposing certain
information to signal to the network, or in allowing the network
to change certain header fields as signals to the transport. The
visible wire image of a protocol should be explicitly designed.
o Impact on Operational Practice: The network operations community
has long relied on being able to understand Internet traffic
patterns, both in aggregate and at the flow level, to support
network management, traffic engineering, and troubleshooting.
Operational practice has developed based on the information
available from unencrypted transport headers. The IETF has
supported this practice by developing operations and management
specifications, interface specifications, and associated Best
Current Practises. Widespread deployment of transport protocols
that encrypt their header information might impact network
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operations, unless operators can develop alternative practises
that work without access to the transport header information.
o Pace of Evolution: Removing obstacles to change can enable an
increased pace of evolution. If a protocol changes its transport
header format (wire image) or their transport behaviour, this can
result in the currently deployed tools and methods becoming no
longer relevant. Where this needs to be accompanied by
development of appropriate operational support functions and
procedures, it can incur a cost in new tooling to catch-up with
each change. Protocols that consistently expose observable data
do not require such development, but can suffer from ossification
and need to consider if the exposed protocol metadata has privacy
implications, There is no single deployment context, and therefore
designers need to consider the diversity of operational networks
(ISPs, enterprises, DDoS mitigation and firewall maintainers,
etc.).
o Supporting Common Specifications: Common, open, specifications can
stimulate engagement by developers, users, researchers, and the
broader community. Increased protocol diversity can be beneficial
in meeting new requirements, but the ability to innovate without
public scrutiny risks point solutions that optimise for specific
cases, but that can accidentally disrupt operations of/in
different parts of the network. The social contract that
maintains the stability of the Internet relies on accepting common
interworking specifications, and on it being possible to detect
violations. It is important to find new ways of maintaining that
community trust as increased use of transport header encryption
limits visibility into transport behaviour.
o Impact on Benchmarking and Understanding Feature Interactions: An
appropriate vantage point for observation, coupled with timing
information about traffic flows, provides a valuable tool for
benchmarking network devices, endpoint stacks, functions, and/or
configurations. This can also help with understanding complex
feature interactions. An inability to observe transport layer
header information can make it harder to diagnose and explore
interactions between features at different protocol layers, a
side-effect of not allowing a choice of vantage point from which
this information is observed. New approaches might have to be
developed.
o Impact on Research and Development: Hiding transport information
can impede independent research into new mechanisms, measurement
of behaviour, and development initiatives. Experience shows that
transport protocols are complicated to design and complex to
deploy, and that individual mechanisms have to be evaluated while
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considering other mechanisms, across a broad range of network
topologies and with attention to the impact on traffic sharing the
capacity. If increased use of transport header encryption results
in reduced availability of open data, it could eliminate the
independent self-checks to the standardisation process that have
previously been in place from research and academic contributors
(e.g., the role of the IRTF Internet Congestion Control Research
Group (ICCRG) and research publications in reviewing new transport
mechanisms and assessing the impact of their deployment).
Observable transport information information might be useful to
various stakeholders. Other stakeholders have incentives to limit
what can be observed. This document does not make recommendations
about what information ought to be exposed, to whom it ought to be
observable, or how this will be achieved. There are also design
choices about where observable fields are placed. For example, one
location could be a part of the transport header outside of the
encryption envelope, another alternative is to carry the information
in a network-layer extension header. New transport protocol designs
ought to explicitly identify any fields that are intended to be
observed, consider if there are alternative ways of providing the
information, and reflect on the implications of observable fields
being used by in-network devices, and how this might impact user
privacy and protocol evolution when these fields become ossified.
As [RFC7258] notes, "Making networks unmanageable to mitigate
(pervasive monitoring) is not an acceptable outcome, but ignoring
(pervasive monitoring) would go against the consensus documented
here." Providing explicit information can help avoid traffic being
inappropriately classified, impacting application performance. An
appropriate balance will emerge over time as real instances of this
tension are analysed [RFC7258]. This balance between information
exposed and information hidden ought to be carefully considered when
specifying new transport protocols.
8. Security Considerations
This document is about design and deployment considerations for
transport protocols. Issues relating to security are discussed
throughout this document.
Authentication, confidentiality protection, and integrity protection
are identified as Transport Features by [RFC8095]. As currently
deployed in the Internet, these features are generally provided by a
protocol or layer on top of the transport protocol
[I-D.ietf-taps-transport-security].
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Confidentiality and strong integrity checks have properties that can
also be incorporated into the design of a transport protocol.
Integrity checks can protect an endpoint from undetected modification
of protocol fields by network devices, whereas encryption and
obfuscation or greasing can further prevent these headers being
utilised by network devices. Preventing observation of headers
provides an opportunity for greater freedom to update the protocols
and can ease experimentation with new techniques and their final
deployment in endpoints. A protocol specification needs to weigh the
costs of ossifying common headers, versus the potential benefits of
exposing specific information that could be observed along the
network path to provide tools to manage new variants of protocols.
A protocol design that uses header encryption can provide
confidentiality of some or all of the protocol header information.
This prevents an on-path device from knowledge of the header field.
It therefore prevents mechanisms being built that directly rely on
the information or seeks to infer semantics of an exposed header
field. Reduces visibility into transport metadata can limit the
ability to measure and characterise traffic. It can also provide
privacy benefits in some cases.
Extending the transport payload security context to also include the
transport protocol header protects both information with the same
key. A privacy concern would arise if this key was shared with a
third party, e.g., providing access to transport header information
to debug a performance issue, would also result in exposing the
transport payload data to the same third party. A layered security
design that separates network data from payload data would avoid such
risks.
Exposed transport headers are sometimes utilised as a part of the
information to detect anomalies in network traffic. "While PM is an
attack, other forms of monitoring that might fit the definition of PM
can be beneficial and not part of any attack, e.g., network
management functions monitor packets or flows and anti-spam
mechanisms need to see mail message content." [RFC7258]. This can
be used as the first line of defence to identify potential threats
from DOS or malware and redirect suspect traffic to dedicated nodes
responsible for DOS analysis, malware detection, or to perform packet
"scrubbing" (the normalization of packets so that there are no
ambiguities in interpretation by the ultimate destination of the
packet). These techniques are currently used by some operators to
also defend from distributed DOS attacks.
Exposed transport header fields are sometimes also utilised as a part
of the information used by the receiver of a transport protocol to
protect the transport layer from data injection by an attacker. In
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evaluating this use of exposed header information, it is important to
consider whether it introduces a significant DOS threat. For
example, an attacker could construct a DOS attack by sending packets
with a sequence number that falls within the currently accepted range
of sequence numbers at the receiving endpoint, this would then
introduce additional work at the receiving endpoint, even though the
data in the attacking packet might not finally be delivered by the
transport layer. This is sometimes known as a "shadowing attack".
An attack can, for example, disrupt receiver processing, trigger loss
and retransmission, or make a receiving endpoint perform unproductive
decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then
restore protocol state).
One mitigation to off-path attack is to deny knowledge of what header
information is accepted by a receiver or obfuscate the accepted
header information, e.g., setting a non-predictable initial value for
a sequence number during a protocol handshake, as in [RFC3550] and
[RFC6056], or a port value that can not be predicted (see section 5.1
of [RFC8085]). A receiver could also require additional information
to be used as a part of a validation check before accepting packets
at the transport layer (e.g., utilising a part of the sequence number
space that is encrypted; or by verifying an encrypted token not
visible to an attacker). This would also mitigate against on-path
attacks. An additional processing cost can be incurred when
decryption has to be attempted before a receiver is able to discard
injected packets.
Open standards motivate a desire for this evaluation to include
independent observation and evaluation of performance data, which in
turn suggests control over where and when measurement samples are
collected. This requires consideration of the appropriate balance
between encrypting all and no transport information. Open data, and
accessibility to tools that can help understand trends in application
deployment, network traffic and usage patterns can all contribute to
understanding security challenges.
The Security and Privacy Considerations in the Framework for Large-
Scale Measurement of Broadband Performance (LMAP) [RFC7594] contain
considerations for Active and Passive measurement techniques and
supporting material on measurement context.
9. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
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10. Acknowledgements
The authors would like to thank Mohamed Boucadair, Spencer Dawkins,
Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen
Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris
Wood, Thomas Fossati, and other members of the TSVWG for their
comments and feedback.
This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No 688421,
and the EU Stand ICT Call 4. The opinions expressed and arguments
employed reflect only the authors' view. The European Commission is
not responsible for any use that might be made of that information.
This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
11. Informative References
[bufferbloat]
Gettys, J. and K. Nichols, "Bufferbloat: dark buffers in
the Internet. Communications of the ACM, 55(1):57-65",
January 2012.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
"Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
data-06 (work in progress), July 2019.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-22 (work
in progress), July 2019.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-19
(work in progress), November 2017.
[I-D.ietf-taps-transport-security]
Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K.
Rose, "A Survey of Transport Security Protocols", draft-
ietf-taps-transport-security-08 (work in progress), August
2019.
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[I-D.ietf-tsvwg-rtcweb-qos]
Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
qos-18 (work in progress), August 2016.
[I-D.trammell-plus-abstract-mech]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", draft-trammell-plus-
abstract-mech-00 (work in progress), September 2016.
[Latency] Briscoe, B., "Reducing Internet Latency: A Survey of
Techniques and Their Merits, IEEE Comm. Surveys &
Tutorials. 26;18(3) p2149-2196", November 2014.
[Measure] Fairhurst, G., Kuehlewind, M., and D. Lopez, "Measurement-
based Protocol Design, Eur. Conf. on Networks and
Communications, Oulu, Finland.", June 2017.
[Quic-Trace]
"https:QUIC trace utilities //github.com/google/quic-
trace".
[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,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2507, DOI 10.17487/RFC2507, February
1999, <https://www.rfc-editor.org/info/rfc2507>.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508,
DOI 10.17487/RFC2508, February 1999,
<https://www.rfc-editor.org/info/rfc2508>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
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[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,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
<https://www.rfc-editor.org/info/rfc3234>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.
[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, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc4737>.
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[RFC4995] Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
Header Compression (ROHC) Framework", RFC 4995,
DOI 10.17487/RFC4995, July 2007,
<https://www.rfc-editor.org/info/rfc4995>.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R.
Whitner, "Improved Packet Reordering Metrics", RFC 5236,
DOI 10.17487/RFC5236, June 2008,
<https://www.rfc-editor.org/info/rfc5236>.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
March 2009, <https://www.rfc-editor.org/info/rfc5481>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.
[RFC6294] Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
2011, <https://www.rfc-editor.org/info/rfc6294>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://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,
<https://www.rfc-editor.org/info/rfc6437>.
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[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC7594] Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
Aitken, P., and A. Akhter, "A Framework for Large-Scale
Measurement of Broadband Performance (LMAP)", RFC 7594,
DOI 10.17487/RFC7594, September 2015,
<https://www.rfc-editor.org/info/rfc7594>.
[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,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/info/rfc7872>.
[RFC7928] Kuhn, N., Ed., Natarajan, P., Ed., Khademi, N., Ed., and
D. Ros, "Characterization Guidelines for Active Queue
Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July
2016, <https://www.rfc-editor.org/info/rfc7928>.
[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
<https://www.rfc-editor.org/info/rfc7983>.
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[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://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, <https://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, <https://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,
<https://www.rfc-editor.org/info/rfc8087>.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[RFC8250] Elkins, N., Hamilton, R., and M. Ackermann, "IPv6
Performance and Diagnostic Metrics (PDM) Destination
Option", RFC 8250, DOI 10.17487/RFC8250, September 2017,
<https://www.rfc-editor.org/info/rfc8250>.
[RFC8289] Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
Iyengar, Ed., "Controlled Delay Active Queue Management",
RFC 8289, DOI 10.17487/RFC8289, January 2018,
<https://www.rfc-editor.org/info/rfc8289>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.
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[RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of
Pervasive Encryption on Operators", RFC 8404,
DOI 10.17487/RFC8404, July 2018,
<https://www.rfc-editor.org/info/rfc8404>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[RFC8548] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic Protection of TCP Streams
(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
<https://www.rfc-editor.org/info/rfc8548>.
[RFC8558] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
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Appendix A. Revision information
-00 This is an individual draft for the IETF community.
-01 This draft was a result of walking away from the text for a few
days and then reorganising the content.
-02 This draft fixes textual errors.
-03 This draft follows feedback from people reading this draft.
-04 This adds an additional contributor and includes significant
reworking to ready this for review by the wider IETF community Colin
Perkins joined the author list.
Comments from the community are welcome on the text and
recommendations.
-05 Corrections received and helpful inputs from Mohamed Boucadair.
-06 Updated following comments from Stephen Farrell, and feedback via
email. Added a draft conclusion section to sketch some strawman
scenarios that could emerge.
-07 Updated following comments from Al Morton, Chris Seal, and other
feedback via email.
-08 Updated to address comments sent to the TSVWG mailing list by
Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on
11/05/2018, and Spencer Dawkins.
-09 Updated security considerations.
-10 Updated references, split the Introduction, and added a paragraph
giving some examples of why ossification has been an issue.
-01 This resolved some reference issues. Updated section on
observation by devices on the path.
-02 Comments received from Kyle Rose, Spencer Dawkins and Tom
Herbert. The network-layer information has also been re-organised
after comments at IETF-103.
-03 Added a section on header compression and rewriting of sections
referring to RTP transport. This version contains author editorial
work and removed duplicate section.
-04 Revised following SecDir Review
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o Added some text on TLS story (additional input sought on relevant
considerations).
o Section 2, paragraph 8 - changed to be clearer, in particular,
added "Encryption with secure key distribution prevents"
o Flow label description rewritten based on PS/BCP RFCs.
o Clarify requirements from RFCs concerning the IPv6 flow label and
highlight ways it can be used with encryption. (section 3.1.3)
o Add text on the explicit spin-bit work in the QUIC DT. Added
greasing of spin-bit. (Section 6.1)
o Updated section 6 and added more explanation of impact on
operators.
o Other comments addressed.
-05 Editorial pass and minor corrections noted on TSVWG list.
-06 Updated conclusions and minor corrections. Responded to request
to add OAM discussion to Section 6.1.
-07 Addressed feedback from Ruediger and Thomas.
Section 2 deserved some work to make it easier to read and avoid
repetition. This edit finally gets to this, and eliminates some
duplication. This also moves some of the material from section 2 to
reform a clearer conclusion. The scope remains focussed on the usage
of transport headers and the implications of encryption - not on
proposals for new techniques/specifications to be developed.
-08 Addressed feedback and completed editorial work, including
updating the text referring to RFC7872, in preparation for a WGLC.
-09 Updated following WGLC. In particular, thanks to Joe Touch
(specific comments and commentary on style and tone); Dimitri Tikonov
(editorial); Christian Huitema (various); David Black (various).
Amended privacy considerations based on SECDIR review. Emile Stephan
(inputs on operations measurement); Various others.
Added summary text and refs to key sections. Note to editors: The
section numbers are hard-linked.
-10 Updated following additional feedback from 1st WGLC. Comments
from David Black; Tommy Pauly; Ian Swett; Mirja Kuehlewind; Peter
Gutmann; Ekr; and many others via the TSVWG list. Some people
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thought that "needed" and "need" could represent requirements in the
document, etc. this has been clarified.
Authors' Addresses
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
EMail: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
Scotland
EMail: csp@csperkins.org
URI: https://csperkins.org/
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