TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins
Expires: May 6, 2021 University of Glasgow
November 2, 2020
Considerations around Transport Header Confidentiality, Network
Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-18
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, mitigate attacks against the transport
protocol, and protect metadata about the communication. Current
operational practice in some networks inspect transport header
information within the network, but this is no longer possible when
those transport headers are encrypted.
This document discusses the possible impact when network traffic uses
a protocol with an encrypted transport header. It suggests issues to
consider when designing new transport protocols or features.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2021.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Current uses of Transport Headers within the Network . . . . 4
2.1. To Identify Transport Protocols and Flows . . . . . . . . 5
2.2. To Understand Transport Protocol Performance . . . . . . 6
2.3. To Support Network Operations . . . . . . . . . . . . . . 13
2.4. To Support Network Diagnostics and Troubleshooting . . . 16
2.5. To Support Header Compression . . . . . . . . . . . . . . 17
2.6. To Verify SLA Compliance . . . . . . . . . . . . . . . . 18
3. Other Uses of Observable Transport Headers . . . . . . . . . 18
3.1. Characterising "Unknown" Network Traffic . . . . . . . . 19
3.2. Accountability and Internet Transport Protocols . . . . . 19
3.3. Impact on Tooling and Network Operations . . . . . . . . 20
3.4. Independent Measurement . . . . . . . . . . . . . . . . . 20
3.5. Impact on Research, Development and Deployment . . . . . 22
4. Encryption and Authentication of Transport Headers . . . . . 23
4.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 24
4.2. Approaches to Transport Header Protection . . . . . . . . 26
5. Intentionally Exposing Transport Information to the Network . 28
5.1. Exposing Transport Information in Extension Headers . . . 28
5.2. Common Exposed Transport Information . . . . . . . . . . 29
5.3. Considerations for Exposing Transport Information . . . . 29
6. Addition of Transport OAM Information to Network-Layer
Headers . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.1. Use of OAM within a Maintenance Domain . . . . . . . . . 30
6.2. Use of OAM across Multiple Maintenance Domains . . . . . 30
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 34
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
11. Informative References . . . . . . . . . . . . . . . . . . . 36
Appendix A. Revision information . . . . . . . . . . . . . . . . 45
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction
The transport layer supports the end-to-end flow of data across a
network path, providing features such as connection establishment,
reliability, framing, ordering, congestion control, flow control,
etc., as needed to support applications. One of the core functions
of an Internet transport: to discover and adapt to the
characteristics of the network path that is currently being used.
For some years, it has been common for the transport layer payload to
be protected by encryption and authentication, but for the transport
layer headers to be sent unprotected. Examples of protocols that
behave in this manner include Transport Layer Security (TLS) over TCP
[RFC8446], Datagram TLS [RFC6347] [I-D.ietf-tls-dtls13], the Secure
Real-time Transport Protocol [RFC3711], and tcpcrypt [RFC8548]. The
use of unencrypted transport headers has led some network operators,
researchers, and others to develop tools and processes that rely on
observations of transport headers both in aggregate and at the flow
level to infer details of the network's behaviour and inform
operational practice.
Transport protocols are now being developed that encrypt some or all
of the transport headers, in addition to the transport payload data.
The QUIC transport protocol [I-D.ietf-quic-transport] is an example
of such a protocol. Such transport header encryption makes it
difficult to observe transport protocol behaviour within the network.
This document discusses some implications of transport header
encryption for network operators, researchers, and others that have
previously observed transport headers, and highlights some issues to
consider for transport protocol designers.
As discussed in [RFC7258], the IETF has concluded that Pervasive
Monitoring (PM) is a technical attack that needs to be mitigated in
the design of IETF protocols. This document supports that
conclusion. It also recognises that RFC7258 states "Making networks
unmanageable to mitigate PM is not an acceptable outcome, but
ignoring PM would go against the consensus documented here. An
appropriate balance will emerge over time as real instances of this
tension are considered". This document is written to provide input
to the discussion around what is an appropriate balance, by
highlighting some implications of transport header encryption.
This document explains current uses of transport header information
in the network, which can be beneficial or malicious. It is written
to provide input to the discussion around what is an appropriate
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balance, by highlighting some implications of transport header
encryption.
2. Current uses of Transport Headers within the Network
In response to pervasive monitoring [RFC7624] revelations and the
IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
efforts are underway to increase encryption of Internet traffic.
Applying confidentiality to transport header fields can improve
privacy, and can help to mitigate certain attacks, but can also
affect network operations [RFC8404].
When considering what parts of the transport headers should be
encrypted to provide confidentiality, and what parts should be
visible to the network (including non-encrypted but authenticated
headers), it is necessary to consider both the impact on network
operations and management, and the implications for ossification and
user privacy [Measurement]. Different parties will view the relative
importance of these concerns differently. For some, the benefits of
encrypting all the transport headers outweigh the impact of doing so;
others might analyse the security, privacy, and ossification impacts
and arrive at a different trade-off.
This section reviews examples of the observation of transport layer
headers within the network. Unencrypted transport headers provide
information can support network operations and management, and this
section notes some ways in which this has been done. Unencrypted
transport header information also contributes metadata that can be
exploited for purposes unrelated to network transport measurement,
diagnostics or troubleshooting (e.g., to block or to throttle traffic
from a specific content provider), and this section also notes some
threats relating to unencrypted transport headers.
Exposed transport information also provides a source of information
that contributes to linked data sets, which could be exploited to
deduce private information, e.g., user patterns, user location,
tracking behaviour, etc. This might reveal information the parties
did not intend to be revealed. [RFC6973] aims to make designers,
implementers, and users of Internet protocols aware of privacy-
related design choices in IETF protocols.
This section does not consider intentional modification of transport
headers by middleboxes, such as in Network Address Translation (NAT)
or Firewalls. Common issues concerning IP address sharing are
described in [RFC6269].
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2.1. To Identify Transport Protocols and Flows
Information in exposed transport layer headers can be used by the
network to identify transport protocols and flows [RFC8558]. The
ability to identify transport protocols, flows, and sessions is a
common function performed, for example, by measurement activities,
QoS classifiers, and firewalls. These functions can be beneficial,
and performed with the consent of, and in support of, the end user.
Alternatively, a network operator could use the same mechanisms to
support practises that are adversarial to the end user, including
blocking, de-prioritising, and monitoring traffic without consent.
Observable transport header information, together with information in
the network header, has been used to identify flows and their
connection state, together with the set of protocol options being
used. Transport protocols, such as TCP and the Stream Control
Transport Protocol (SCTP), specify a standard base header that
includes sequence number information and other data. They also have
the possibility to negotiate additional headers at connection setup,
identified by an option number in the transport header.
In some uses, an assigned transport port (e.g., 0..49151) can
identify the upper-layer protocol or service [RFC7605]. However,
port information alone is not sufficient to guarantee identification.
Applications can use arbitrary ports and do not need to use assigned
port numbers. The use of an assigned port number is also not limited
to the protocol for which the port is intended. Multiple sessions
can also be multiplexed on a single port, and ports can be re-used by
subsequent sessions.
Some flows can be identified by observing signalling protocol data
(e.g., [RFC3261], [I-D.ietf-rtcweb-overview]) or through the use of
magic numbers placed in the first byte(s) of the datagram payload
[RFC7983].
When transport header information cannot be observed, this removes
information that could have been used to classify flows by passive
observers along the path. More ambitious ways could be used to
collect, estimate, or infer flow information, including heuristics
based on the analysis of traffic patterns. For example, an operator
that cannot access the Session Description Protocol (SDP) session
descriptions [RFC4566] to classify a flow as audio traffic, might
instead use (possibly less-reliable) heuristics to infer that short
UDP packets with regular spacing carry audio traffic. Operational
practises aimed at inferring transport parameters are out of scope
for this document, and are only mentioned here to recognise that
encryption does not prevent operators from attempting to apply
practises that were used with unencrypted transport headers.
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The IAB [RFC8546] have provided a summary of expected implications of
increased encryption on network functions that use the observable
headers and describe the expected benefits of designs that explicitly
declare protocol invariant header information that can be used for
this purpose.
2.2. To Understand Transport Protocol Performance
Information in exposed transport layer headers can be used by the
network to understand transport protocol performance and behaviour.
2.2.1. Using Information Derived from Transport Layer Headers
Observable transport headers enable explicit measurement and analysis
of protocol performance, network anomalies, and failure pathologies
at any point along the Internet path. Some operators use passive
monitoring to manage their portion of the Internet by characterising
the performance of link/network segments. Inferences from transport
headers are used to derive performance metrics. A variety of open
source and commercial tools have been deployed that utilise transport
header information in this way to derive the following metrics:
Traffic Rate and Volume: Volume measures per-application can be used
to characterise the traffic that uses a network segment or the
pattern of network usage. Observing the protocol sequence number
and packet size offers one way to measure this (e.g., measurements
observing counters in periodic reports such as RTCP; or
measurements observing protocol sequence numbers in statistical
samples of packet flows, or specific control packets, such as
those observed at the start and end of a flow).
Measurements can be per endpoint, or for an endpoint aggregate.
This can be used, for example, to assess subscriber usage or for
billing purposes.
Measurements can also be used to trigger traffic shaping, and to
associate QoS support within the network and lower layers. This
can be done with consent and in support of an end user, to improve
quality of service; or can be used by the network to de-prioritise
certain flows without user consent.
Volume measures can also be valuable for capacity planning and
providing detail of trends in usage.
The traffic rate and volume can be determined providing that the
packets belonging to individual flows can be identified, but there
might be no additional information about a flow when the transport
headers cannot be observed.
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Loss Rate and Loss Pattern: Flow loss rate can be derived (e.g.,
from transport sequence numbers or inferred from observing
transport protocol interactions) and has been used as a metric for
performance assessment and to characterise transport behaviour.
Understanding the location and root cause of loss can help an
operator determine whether this requires corrective action.
Network operators have used the variation in patterns of loss as a
key performance metric, utilising this to detect changes in the
offered service.
There are various causes of loss, including: corruption of link
frames (e.g., due to interference on a radio link), buffering loss
(e.g., overflow due to congestion, Active Queue Management (AQM)
[RFC7567], or inadequate provision following traffic pre-emption),
and policing (traffic management) [RFC2475]. Understanding flow
loss rates requires either observing sequence numbers in network
or transport headers, or maintaining per-flow packet counters
(flow identification often requires transport layer information).
Per-hop loss can also sometimes be monitored at the interface
level by devices in the network.
Losses can often occur as bursts, randomly-timed events, etc. The
pattern of loss can provide insight into the cause of loss. It
can also be valuable to understand the conditions under which loss
occurs, which usually requires relating loss to the traffic
flowing at a network node or segment at the time of loss. This
can also help identify cases where loss could have been wrongly
identified, or where the transport did not require transmission of
a lost packet.
Throughput and Goodput: Throughput is the amount of payload data
sent by a flow per time interval. Goodput (see Section 2.5 of
[RFC7928]) is a measure of useful data exchanged (the ratio of
useful data to total volume of traffic sent by a flow). The
throughput of a flow can be determined in the absence of transport
header information, providing that the individual flow can be
identified, and the overhead known. Goodput requires ability to
differentiate loss and retransmission of packets, for example by
observing packet sequence numbers in the TCP or RTP headers
[RFC3550].
Latency: Latency is a key performance metric that impacts
application and user-perceived response times. It often
indirectly impacts throughput and flow completion time. This
determines the reaction time of the transport protocol itself,
impacting flow setup, congestion control, loss recovery, and other
transport mechanisms. The observed latency can have many
components [Latency]. Of these, unnecessary/unwanted queueing in
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network buffers has often been observed as a significant factor
[bufferbloat]. Once the cause of unwanted latency has been
identified, this can often be eliminated.
To measure latency across a part of a path, an observation point
[RFC7799] can measure the experienced round trip time (RTT) using
packet sequence numbers and acknowledgements, or by observing
header timestamp information. Such information allows an
observation point in the network to determine not only the path
RTT, but also allows measurement of the upstream and downstream
contribution to the RTT. This could be used to locate a source of
latency, e.g., by observing cases where the median RTT is much
greater than the minimum RTT for a part of a path.
The service offered by network operators can benefit from latency
information to understand the impact of configuration changes and
to tune deployed services. Latency metrics are key to evaluating
and deploying AQM [RFC7567], DiffServ [RFC2474], and Explicit
Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements
could identify excessively large buffers, indicating where to
deploy or configure AQM. An AQM method is often deployed in
combination with other techniques, such as scheduling [RFC7567]
[RFC8290] and although parameter-less methods are desired
[RFC7567], current methods often require tuning [RFC8290]
[RFC8289] [RFC8033] because they cannot scale across all possible
deployment scenarios.
Latency and round-trip time information can potentially expose
some information useful for approximate geolocation, as discussed
in [PAM-RTT]. Encrypting transport headers can reduce the latency
information that is available.
Variation in delay: Some network applications are sensitive to
(small) changes in packet timing (jitter). Short and long-term
delay variation can impact on the latency of a flow and hence the
perceived quality of applications using the network. For example,
jitter metrics are often cited when characterising paths
supporting real-time traffic. The expected performance of such
applications, can be inferred from a measure the variation in
delay observed along a portion of the path [RFC3393] [RFC5481].
The requirements resemble those for the measurement of latency.
Flow Reordering: Significant packet reordering within a flow can
impact time-critical applications and can be interpreted as loss
by reliable transports. Many transport protocol techniques are
impacted by reordering (e.g., triggering TCP retransmission or re-
buffering of real-time applications). Packet reordering can occur
for many reasons, from equipment design to misconfiguration of
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forwarding rules. Network tools can detect and measure unwanted/
excessive reordering, and the impact on transport performance.
There have been initiatives in the IETF transport area to reduce
the impact of reordering within a transport flow, possibly leading
to a reduction in the requirements for preserving ordering. These
have potential to simplify network equipment design as well as the
potential to improve robustness of the transport service.
Measurements of reordering can help understand the present level
of reordering within deployed infrastructure, and inform decisions
about how to progress such mechanisms. Key performance indicators
are retransmission rate, packet drop rate, sector utilisation
level, a measure of reordering, peak rate, the ECN congestion
experienced (CE) marking rate, etc.
Metrics have been defined that evaluate whether a network has
maintained packet order on a packet-by-packet basis [RFC4737]
[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.
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Passive measurements (see Section 3.6 of [RFC7799]) can have
advantages in terms of eliminating unproductive test traffic,
reducing the influence of test traffic on the overall traffic mix,
and the ability to choose the point of observation (see
Section 2.3.1). Measurements can rely on observing packet headers,
which is not possible if those headers are encrypted, but could
utilise information about traffic volumes or patterns of interaction
to deduce metrics.
One alternative approach is to use in-network techniques, which does
not require the cooperation of an endpoint (see Section 6).
2.2.2. Using Information Derived from Network Layer Header Fields
Information from the transport header can be used by a multi-field
classifier as a part of policy framework. Policies are commonly used
for management of the QoS or Quality of Experience (QoE) in resource-
constrained networks, or by firewalls to implement access rules (see
also Section 2.2.2 of [RFC8404]). Operators can use such policies to
support user applications and to protect against unwanted traffic.
Such policies can also be used without user consent, to de-prioritise
certain applications or services, for example.
Network-layer classification methods that rely on a multi-field
classifier (e.g., inferring QoS from the 5-tuple or choice of
application protocol) are incompatible with transport protocols that
encrypt the transport header information. Traffic that cannot be
classified typically receives a default treatment. Some networks
block or rate traffic that cannot be classified.
Transport layer information can also be explicitly carried in
network-layer header fields that are not encrypted, serving as a
replacement/addition to the exposed transport header information
[RFC8558]. This information can enable a different forwarding
treatment by the network, even when a transport employs encryption to
protect other header information.
The user of a transport that multiplexes multiple sub-flows might
want to obscure the presence and characteristics of these sub-flows.
On the other hand, an encrypted transport could set the network-layer
information to indicate the presence of sub-flows, and to reflect the
service requirements of individual sub-flows. There are several ways
this could be done:
IP Address: Applications normally expose the addresses used by
endpoints, and this is used in the forwarding decisions in network
devices. Address and other protocol information can be used by a
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Multi-Field (MF) classifier to determine how traffic is treated
[RFC2475], and hence affect the quality of experience for a flow.
Using the IPv6 Network-Layer Flow Label: A number of Standards Track
and Best Current Practice RFCs (e.g., [RFC8085], [RFC6437],
[RFC6438]) encourage endpoints to set the IPv6 flow label field of
the network-layer header. IPv6 "source nodes SHOULD assign each
unrelated transport connection and application data stream to a
new flow" [RFC6437]. A multiplexing transport could choose to use
multiple flow labels to allow the network to independently forward
sub-flows. RFC6437 provides further guidance on choosing a flow
label value, stating these "should be chosen such that their bits
exhibit a high degree of variability", and chosen so that "third
parties should be unlikely to be able to guess the next value that
a source of flow labels will choose".
Once set, a flow label can provide information that can help
inform network-layer queueing and forwarding [RFC6438], for
example with Equal Cost Multi-Path routing and Link Aggregation
[RFC6294]. Considerations when using IPsec are further described
in [RFC6438].
The choice of how to assign a flow label needs to avoid
introducing linkability that a network device could observe.
Inappropriate use by the transport can have privacy implications
(e.g., assigning the same label to two independent flows that
ought not to be classified the same).
Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to the network
by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications
identify different forwarding treatments for individual sub-flows
(audio vs. video) based on the value of the DSCP field
[I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information
to inform network-layer queueing and forwarding, rather than an
operator inferring traffic requirements from transport and
application headers via a multi-field classifier. Inappropriate
use by the transport can have privacy implications (e.g.,
assigning a different DSCP to a subflow could assist in a network
device discovering the traffic pattern used by an application,
assigning the same label to two independent flows that ought not
to be classified the same). The field is mutable, i.e., some
network devices can be expected to change this field (use of each
DSCP value is defined by an RFC).
Since the DSCP value can impact the quality of experience for a
flow, observations of service performance has to consider this
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field when a network path supports differentiated service
treatment.
Using Explicit Congestion Marking: ECN [RFC3168] is a transport
mechanism that uses the ECN field in the network-layer header.
Use of ECN explicitly informs the network-layer that a transport
is ECN-capable, and requests ECN treatment of the flow. An ECN-
capable transport can offer benefits when used over a path with
equipment that implements an AQM method with CE marking of IP
packets [RFC8087], since it can react to congestion without also
having to recover from lost packets.
ECN exposes the presence of congestion. The reception of CE-
marked packets can be used to estimate the level of incipient
congestion on the upstream portion of the path from the point of
observation (Section 2.5 of [RFC8087]). Interpreting the marking
behaviour (i.e., assessing congestion and diagnosing faults)
requires context from the transport layer, such as path RTT.
AQM and ECN offer a range of algorithms and configuration options.
Tools therefore have to be available to network operators and
researchers to understand the implication of configuration choices
and transport behaviour as the use of ECN increases and new
methods emerge [RFC7567].
Network-Layer Options Network protocols can carry optional headers.
These can be used to explicitly expose transport header
information to on-path devices operating at the network layer (as
discussed further in Section 6).
IPv4 [RFC0791] has provision for optional header fields identified
by an option type field. IP routers can examine these headers and
are required to ignore IPv4 options that they does not recognise.
Many current paths include network devices that forward packets
that carry options on a slower processing path. Some network
devices (e.g., firewalls) can be (and are) configured to drop
these packets [RFC7126]. BCP 186 [RFC7126] provides Best Current
Practice guidance on how operators should treat IPv4 packets that
specify options.
IPv6 can encode optional network-layer information in separate
headers that may be placed between the IPv6 header and the upper-
layer header [RFC8200]. The Hop-by-Hop options header, when
present, immediately follows the IPv6 header. IPv6 permits this
header to be examined by any node along the path. While [RFC7872]
required all nodes to examine and process the Hop-by-Hop options
header, it is now expected [RFC8200] that nodes along a path only
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examine and process the Hop-by-Hop options header if explicitly
configured to do so.
Careful use of the network layer features can help provide similar
information in the case where the network is unable to inspect
transport protocol headers. Section 5 describes use of network
extension headers.
2.3. To Support Network Operations
Some network operators make use of on-path observations of transport
headers to monitor the performance of their networks, and to support
network operations. Transport protocols with observable headers
allow such operators to explicitly measurement and analyse transport
protocol performance, and in some cases this can help detect and
locate network problems. [RFC8517] gives an operator's perspective
about such use.
When encryption hides the transport headers, making it difficult to
directly observe transport behaviour and dynamics, those seeking an
understanding of network operations might learn to work without that
information. Alternatively, they might use more limited measurements
combined with pattern inference and other heuristics to infer network
behaviour (see Section 2.1.1 of [RFC8404]). Operational practises
aimed at inferring transport parameters are out of scope for this
document, and are only mentioned here to recognise that encryption
does not necessarily stop operators from attempting to apply
practises that have been used with unencrypted transport headers.
When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network, is
often necessary to interpret this data to answer questions about
network performance or understand a pathology. Collecting and
coordinating such metadata is more difficult when the observation
point is at a different location to the bottleneck or device under
evaluation [RFC7799].
Packet sampling techniques are used to scale the processing involved
in observing packets on high rate links. This exports only the
packet header information of (randomly) selected packets. The
utility of these measurements depends on the type of bearer and
number of mechanisms used by network devices. Simple routers are
relatively easy to manage, but a device with more complexity demands
understanding of the choice of many system parameters. This level of
complexity exists when several network methods are combined.
This section discusses topics concerning observation of transport
flows, with a focus on transport measurement.
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2.3.1. Problem Location
In network measurements of transport header information can be used
to locate the source of problems, or to assess the performance of a
network segment or a particular device configuration. Often issues
can only be understood in the context of the other flows that share a
particular path, common network device, interface port, etc. A
simple example is monitoring of a network device that uses a
scheduler or active queue management technique [RFC7567], where it
could be desirable to understand whether the algorithms are correctly
controlling latency, or if overload protection is working. This
understanding implies knowledge of how traffic is assigned to any
sub-queues used for flow scheduling, but can also require information
about how the traffic dynamics impact active queue management,
starvation prevention mechanisms, and circuit-breakers.
Sometimes multiple in network observation points have to be used. By
correlating observations of headers at multiple points along the path
(e.g., at the ingress and egress of a network segment), an observer
can determine the contribution of a portion of the path to an
observed metric, to locate a source of delay, jitter, loss,
reordering, congestion marking, etc.
2.3.2. Network Planning and Provisioning
Traffic rate and volume measurements are used by operators to help
plan deployment of new equipment and configuration in their networks.
Data is also valuable to equipment vendors who want to understand
traffic trends and patterns of usage as inputs to decisions about
planning products and provisioning for new deployments. This
measurement information can also be correlated with billing
information when this is also collected by an operator.
Trends in aggregate traffic can be observed and can be related to the
endpoint addresses being used, but when transport header information
is not observable, it might be impossible to correlate patterns in
measurements with changes in transport protocols. This increases the
dependency on other indirect sources of information to inform
planning and provisioning.
2.3.3. Service Performance Measurement
Performance measurements (e.g., throughput, loss, latency) can be
used by various actors to analyse the service offered to the users of
a network segment, and to inform operational practice.
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2.3.4. Compliance with Congestion Control
The traffic that can be observed by on-path network devices (the
"wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational
concern, although the ability to measure and monitor it is lower.
The desire to understand the traffic and protocol interactions
typically grows as the proportion of traffic increases in volume.
The challenges increase when multiple instances of an evolving
protocol contribute to the traffic that share network capacity.
Operators can manage traffic load (e.g., when the network is severely
overloaded) by deploying rate-limiters, traffic shaping, or network
transport circuit breakers [RFC8084]. The information provided by
observing transport headers is a source of data that can help to
inform such mechanisms.
Congestion Control Compliance of Traffic: Congestion control is a
key transport function [RFC2914]. Many network operators
implicitly accept that TCP traffic complies with a behaviour that
is acceptable for the shared Internet. TCP algorithms have been
continuously improved over decades, and have reached a level of
efficiency and correctness that is difficult to match in custom
application-layer mechanisms [RFC8085].
A standards-compliant TCP stack provides congestion control that
is judged safe for use across the Internet. Applications
developed on top of well-designed transports can be expected to
appropriately control their network usage, reacting when the
network experiences congestion, by back-off and reduce the load
placed on the network. This is the normal expected behaviour for
IETF-specified transports (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the
transport protocol header information to help understand the
impact of specific transport protocols (or protocol mechanisms) on
the other traffic that shares a network. An observation in the
network can gain an understanding of the dynamics of a flow and
its congestion control behaviour. Analysing observed flows can
help to build confidence that an application flow backs-off its
share of the network load under persistent congestion, and hence
to understand whether the behaviour is appropriate for sharing
limited network capacity. For example, it is common to visualise
plots of TCP sequence numbers versus time for a flow to understand
how a flow shares available capacity, deduce its dynamics in
response to congestion, etc.
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The ability to identify sources and flows that contribute to
persistent congestion is important to the safe operation of
network infrastructure, and can inform configuration of network
devices to complement the endpoint congestion avoidance mechanisms
[RFC7567] [RFC8084] to avoid a portion of the network being driven
into congestion collapse [RFC2914].
Congestion Control Compliance for UDP traffic: UDP provides a
minimal message-passing datagram transport that has no inherent
congestion control mechanisms. Because congestion control is
critical to the stable operation of the Internet, applications and
other protocols that choose to use UDP as a transport have to
employ mechanisms to prevent collapse, avoid unacceptable
contributions to jitter/latency, and to establish an acceptable
share of capacity with concurrent traffic [RFC8085].
A network operator can observe the headers of transport protocols
layered above UDP to understand if the datagram flows comply with
congestion control expectations. This can help inform a decision
on whether it might be appropriate to deploy methods such as rate-
limiters to enforce acceptable usage.
UDP flows that expose a well-known header can be observed to gain
understanding of the dynamics of a flow and its congestion control
behaviour. For example, tools exist to monitor various aspects of
RTP header information and RTCP reports for real-time flows (see
Section 2.2). The Secure RTP and RTCP extensions [RFC3711] were
explicitly designed to expose some header information to enable
such observation, while protecting the payload data.
2.4. To Support Network Diagnostics and Troubleshooting
Transport header information can be utilised for a variety of
operational tasks [RFC8404]: to diagnose network problems, assess
network provider performance, evaluate equipment or protocol
performance, capacity planning, management of security threats
(including DoS), and responding to user performance questions.
Section 3.1.2 and Section 5 of [RFC8404] provide further examples.
Operators can monitor the health of a portion of the Internet, to
provide early warning and trigger action. Traffic and performance
measurements can assist in setting buffer sizes, debugging and
diagnosing the root causes of faults that concern a particular user's
traffic. They can also be used to support post-mortem investigation
after an anomaly to determine the root cause of a problem. In other
cases, measurement involves dissecting network traffic flows.
Observed transport header information can help identify whether link/
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network tuning is effective and alert to potential problems that can
be hard to derive from link or device measurements alone.
An alternative could rely on access to endpoint diagnostic tools or
user involvement in diagnosing and troubleshooting unusual use cases
or to troubleshoot non-trivial problems.
Another approach is to use traffic pattern analysis. Such tools can
provide useful information during network anomalies (e.g., detecting
significant reordering, high or intermittent loss), however indirect
measurements would need to be carefully designed to provide
information for diagnostics and troubleshooting.
The design trade-offs for radio networks are often very different
from those of wired networks. A radio-based network (e.g., cellular
mobile, enterprise Wireless LAN (WLAN), satellite access/back-haul,
point-to-point radio) has the complexity of a subsystem that performs
radio resource management, with direct impact on the available
capacity, and potentially loss/reordering of packets. The impact of
the pattern of loss and congestion and differences between traffic
types, and their correlation with link propagation and interference
can all have significant impact on the cost and performance of a
provided service. For radio links, the use for this type of
information is expected to increase as operators bring together
heterogeneous types of network equipment and seek to deploy
opportunistic methods to access radio spectrum.
Lack of tools and resulting information can reduce the ability of an
operator to observe transport performance and could limit the ability
of network operators to trace problems, make appropriate QoS
decisions, or respond to other queries about the network service.
A network operator supporting traffic that uses transport header
encryption is unable to use tools that rely on transport protocol
information. However, the use of encryption has the desirable effect
of preventing unintended observation of the payload data and these
tools seldom seek to observe the payload, or other application
details. A flow that hides its transport header information could
imply "don't touch" to some operators. This might limit a trouble-
shooting response to "can't help, no trouble found".
2.5. To Support Header Compression
Header compression saves link capacity by compressing network and
transport protocol headers on a per-hop basis. It was widely used
with low bandwidth dial-up access links, and still finds application
on wireless links that are subject to capacity constraints. Examples
of header compression include use with TCP/IP and RTP/UDP/IP flows
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[RFC2507], [RFC6846], [RFC2508], [RFC5795]. Successful compression
depends on observing the transport headers and understanding of the
way header fields change packet-by-packet, and is hence incompatible
with header encryption. Devices that compress transport headers are
dependent on a stable header format, implying ossification of that
format.
Introducing a new transport protocol, or changing the format of the
transport header information, will limit the effectiveness of header
compression until the network devices are updated. Encrypting the
transport protocol headers will tend to cause the header compression
to a fall back to compressing only the network layer headers, with a
significant reduction in efficiency. This can limit connectivity if
the resulting flow exceeds the link capacity, or if the packets are
dropped because they exceed the link MTU.
The Secure RTP (SRTP) extensions [RFC3711] were explicitly designed
to leave the transport protocol headers unencrypted, but
authenticated, since support for header compression was considered
important.
2.6. To Verify SLA Compliance
Observable transport headers coupled with published transport
specifications allow operators and regulators to explore and verify
compliance with Service Level Agreements (SLAs).
When transport header information cannot be observed, other methods
have to be found to confirm that the traffic produced conforms to the
expectations of the operator or developer.
Independently verifiable performance metrics can be utilised to
demonstrate regulatory compliance in some jurisdictions, and as a
basis for informing design decisions. This can bring assurance to
those operating networks, often avoiding deployment of complex
techniques that routinely monitor and manage Internet traffic flows
(e.g., avoiding the capital and operational costs of deploying flow
rate-limiting and network circuit-breaker methods [RFC8084]).
3. Other Uses of Observable Transport Headers
The choice of which transport header fields to expose and which to
encrypt is a design decision for the transport protocol. Selective
encryption requires trading conflicting goals of observability and
network support, privacy, and risk of ossification, to decide what
header fields to protect and which to make visible.
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Security work typically employs a design technique that seeks to
expose only what is needed [RFC3552]. This approach provides
incentives to not reveal any information that is not necessary for
the end-to-end communication. The IAB has provided guidelines for
writing Security Considerations for IETF specifications [RFC3552].
Endpoint design choices impacting privacy also need to be considered
as a part of the design process [RFC6973]. The IAB has provided
guidance for analyzing and documenting privacy considerations within
IETF specifications [RFC6973].
There can also be performance and operational trade-offs in exposing
selected information to network tools. This section explores key
implications of working with encrypted transport protocols, but does
not endorse or condemn these practices.
3.1. Characterising "Unknown" Network Traffic
The patterns and types of traffic that share Internet capacity change
over time as networked applications, usage patterns and protocols
continue to evolve.
If "unknown" or "uncharacterised" traffic patterns form a small part
of the traffic aggregate passing through a network device or segment
of the network the path, the dynamics of the uncharacterised traffic
might not have a significant collateral impact on the performance of
other traffic that shares this network segment. Once the proportion
of this traffic increases, monitoring the traffic can determine if
appropriate safety measures have to be put in place.
Tracking the impact of new mechanisms and protocols requires traffic
volume to be measured and new transport behaviours to be identified.
This is especially true of protocols operating over a UDP substrate.
The level and style of encryption has to be considered in determining
how this activity is performed. On a shorter timescale, information
could also have to be collected to manage DoS attacks against the
infrastructure.
3.2. Accountability and Internet Transport Protocols
Information provided by tools observing transport headers can be used
to classify traffic, and to limit the network capacity used by
certain flows, as discussed in Section 2.3.4). Equally, operators
could use analysis of transport headers and transport flow state to
demonstrate that they are not providing differential treatment to
certain flows. Obfuscating or hiding this information using
encryption could lead operators and maintainers of middleboxes
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(firewalls, etc.) to seek other methods to classify, and potentially
other mechanisms to condition network traffic.
A lack of data that reduces the level of precision with which flows
can be classified also reduces the design space for conditioning
mechanisms (e.g., rate limiting, circuit breaker techniques
[RFC8084], or blocking of uncharacterised traffic) [RFC5218].
3.3. Impact on Tooling and Network Operations
A variety and open source and proprietary tools have been deployed to
can make use of the transport header information that's observable in
widely used protocols such as TCP or RTP/UDP/IP.
Changes to the transport, whether to protect the transport headers,
introduce a new transport protocol, protocol feature, or application
might require changes to such tools, and so could impact operational
practice and policies. Such changes have associated costs that are
incurred by the network operators that need to update their tooling
or develop alternative practises that work without access to the
changed/removed information.
If new protocols, or protocol extensions, are made to closely
resemble or match existing mechanisms, then these changes and the
associated costs can be small. Equally, more extensive changes to
the transport tend to require more extensive, and more expensive,
changes to tooling and operational practice.
Protocol designers can mitigate these costs by explicitly choosing to
expose selected information as invariants that are guaranteed not to
change for a particular protocol (e.g., the header invariants and the
spin-bit in QUIC [I-D.ietf-quic-transport]). Specification of common
log formats and development of alternative approaches can also help
mitigate the costs of transport changes.
3.4. Independent Measurement
Independent observation by multiple actors is currently used by the
transport community to maintain an accurate understanding of the
network. Encrypting transport header encryption changes the ability
to collect and independently analyse data.
Protocols that expose the state information used by the transport
protocol in their header information (e.g., timestamps used to
calculate the RTT, packet numbers used to assess congestion and
requests for retransmission) provide an incentive for the sending
endpoint to provide correct information, since the protocol will not
work otherwise. This increases confidence that the observer
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understands the transport interaction with the network. For example,
when TCP is used over an unencrypted network path (i.e., one that
does not use IPsec or other encryption below the transport), it
implicitly exposes transport header information that can be used for
measurement at any point along the path. This information is
necessary for the protocol's correct operation, therefore there is no
incentive for a TCP or RTP implementation to put incorrect
information in this transport header. A network device can have
confidence that the well-known (and ossified) transport header
information represents the actual state of the endpoints.
When encryption is used to hide some or all of the transport headers,
the transport protocol chooses which information to reveal to the
network about its internal state, what information to leave
encrypted, and what fields to grease to protect against future
ossification [RFC8701]. Such a transport could provide summary data
regarding its performance, congestion control state, etc., or to make
available explicit measurement information. For example, a QUIC
endpoint can optionally set the spin bit to reflect to explicitly
reveal the RTT of an encrypted transport session to the on-path
network devices [I-D.ietf-quic-transport]).
When providing or using such information, it is important to consider
the privacy of the user and their incentive for providing accurate
and detailed information. Protocols that selectively reveal some
transport state or measurable information are choosing to establish a
trust relationship with the network operators. There is no protocol
mechanism that can guarantee that the information provided represents
the actual transport state of the endpoints, since those endpoints
can always send additional information in the encrypted part of the
header, to update or replace whatever they reveal. This reduces the
ability to independently measure and verify that a protocol is
behaving as expected. For some operational uses, the information has
to contain sufficient detail to understand, and possibly reconstruct,
the network traffic pattern for further testing. In this case,
operators have to gain the trust of transport protocol implementers
if the transport headers are to correctly reveal such information.
OAM data records [I-D.ietf-ippm-ioam-data] could be embedded into a
variety of encapsulation methods at different layers to support the
goals of a specific operational domain. OAM-related metadata can
support functions such as performance evaluation, path-tracing, path
verification information, classification and a diversity of other
uses. When encryption is used to hide some or all of the transport
headers, analysis requires coordination between actors at different
layers to successfully characterise flows and correlate the
performance or behaviour of a specific mechanism with the
configuration and traffic using operational equipment (e.g.,
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combining transport and network measurements to explore congestion
control dynamics, the implications of designs for active queue
management or circuit breakers).
Some measurements could be completed by utilising endpoint-based
logging (e.g., based on Quic-Trace [Quic-Trace]). Such information
has a diversity of uses, including developers wishing to debug/
understand the transport/application protocols with which they work,
researchers seeking to spot trends and anomalies, and to characterise
variants of protocols. A standard format for endpoint logging could
allow these to be shared (after appropriate anonymisation) to
understand performance and pathologies. Measurements based on
logging have to establish the validity and provenance of the logged
information to establish how and when traces were captured.
Despite being applicable in some scenarios, endpoint logs do not
provide equivalent information to in-network measurements. In
particular, endpoint logs contain only a part of the information to
understand the operation of network devices and identify issues such
as link performance or capacity sharing between multiple flows.
Additional information has to be combined to determine which
equipment/links are used and the configuration of equipment along the
network paths being measured.
3.5. Impact on Research, Development and Deployment
Transport protocol evolution, and the ability to measure and
understand the impact of protocol changes, have to proceed hand-in-
hand. A transport protocol that provides observable headers can be
used to provide open and verifiable measurement data. Observation of
pathologies has a critical role in the design of transport protocol
mechanisms and development of new mechanisms and protocols. This
helps understanding of the interactions between cooperating protocols
and network mechanisms, the implications of sharing capacity with
other traffic and the impact of different patterns of usage. The
ability of other stakeholders to review transport header traces helps
develop insight into performance and traffic contribution of specific
variants of a protocol.
Development of new transport protocol mechanisms has to consider the
scale of deployment and the range of environments in which the
transport is used. Experience has shown that it is often difficult
to correctly implement new mechanisms [RFC8085], and that mechanisms
often evolve as a protocol matures, or in response to changes in
network conditions, changes in network traffic, or changes to
application usage. Analysis is especially valuable when based on the
behaviour experienced across a range of topologies, vendor equipment,
and traffic patterns.
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New transport protocol formats are expected to facilitate an
increased pace of transport evolution, and with it the possibility to
experiment with and deploy a wide range of protocol mechanisms.
There has been recent interest in a wide range of new transport
methods, e.g., Larger Initial Window, Proportional Rate Reduction
(PRR), congestion control methods based on measuring bottleneck
bandwidth and round-trip propagation time, the introduction of AQM
techniques and new forms of ECN response (e.g., Data Centre TCP,
DCTP, and methods proposed for L4S). The growth and diversity of
applications and protocols using the Internet also continues to
expand. For each new method or application, it is desirable to build
a body of data reflecting its behaviour under a wide range of
deployment scenarios, traffic load, and interactions with other
deployed/candidate methods.
Encryption of transport header information could reduce the range of
actors that can observe useful data. This would limit the
information sources available to the Internet community to understand
the operation of new transport protocols, reducing information to
inform design decisions and standardisation of the new protocols and
related operational practises. The cooperating dependence of
network, application, and host to provide communication performance
on the Internet is uncertain when only endpoints (i.e., at user
devices and within service platforms) can observe performance, and
when performance cannot be independently verified by all parties.
Independently observed data is also important to ensure the health of
the research and development communities and can help promote
acceptance of proposed specifications by the wider community (e.g.,
as a method to judge the safety for Internet deployment) and provides
valuable input during standardisation. Open standards motivate a
desire to include independent observation and evaluation of
performance data, which in turn demands control/understanding about
where and when measurement samples are collected. This requires
consideration of the methods used to observe data and the appropriate
balance between encrypting all and no transport header information.
4. Encryption and Authentication of Transport Headers
End-to-end encryption can be applied at various protocol layers. It
can be applied above the transport to encrypt the transport payload
(e.g., using TLS). This can hide information from an eavesdropper in
the network. It can also help protect the privacy of a user, by
hiding data relating to user/device identity or location. Encryption
and authentication is also increasingly used to protect the transport
headers.
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4.1. Motivation
There are several motivations for transport header encryption.
One motive to encrypt transport headers is to prevent network
ossification from network devices that inspect transport headers.
Once a network device observes a transport header and becomes reliant
upon using it, the overall use of that field can become ossified,
preventing new protocols and mechanisms from being deployed. One of
the benefits of encrypting transport headers is that it can help
improve the pace of transport development by eliminating interference
by deployed middleboxes. Examples of this include:
o During the development of TLS 1.3 [RFC8446], the design needed to
be modified to function in the presence of deployed middleboxes
that relied on the presence of certain header fields exposed in
TLS 1.2 [RFC5426].
o The design of Multipath TCP (MPTCP) [RFC8684] also had to be
revised to account for middleboxes (known as "TCP Normalizers")
that monitor the evolution of the window advertised in the TCP
header and then reset connections when the window did not grow as
expected.
o TCP Fast Open [RFC7413] can experience problems due to middleboxes
that modify the transport header of packets by removing "unknown"
TCP options, segments with unrecognised TCP options can be
dropped, segments that contain data and set the SYN bit can be
dropped, or middleboxes that disrupt connections that send data
before completion of the three-way handshake.
o Other examples of ossification have included middleboxes that
modify transport headers by rewriting TCP sequence and
acknowledgement numbers, but are unaware of the (newer) TCP
selective acknowledgement (SACK) option and therefore fail to
correctly rewrite the SACK information to match the changes that
were made to the fixed TCP header, preventing SACK from operating
correctly.
In all these cases, middleboxes with a hard-coded, but incomplete,
understanding of transport behaviour, interacted poorly with
transport protocols after the transport behaviour was changed. In
some case, the middleboxes modified or replaced information in the
transport protocol header.
Transport header encryption prevents an on-path device from observing
the transport headers, and therefore stops mechanisms being built
that directly rely on or infer semantics of the transport header
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information. Encryption is normally combined with authentication of
the protected information. RFC 8546 summarises this approach,
stating that it is "The wire image, not the protocol's specification,
determines how third parties on the network paths among protocol
participants will interact with that protocol" (Section 1 of
[RFC8546]), and it can be expected that header information that is
not encrypted will become ossified. Encryption can reduce
ossification of the transport protocol, but does not itself prevent
ossification of the network service. People seeking to understand
network traffic could still come to rely on pattern inferences and
other heuristics or machine learning to derive measurement data and
as the basis for network forwarding decisions [RFC8546]. This can
also create dependencies on the transport protocol, or the patterns
of traffic it can generate, also in time resulting in ossification of
the service.
Another motivation stems from increased concerns about privacy and
surveillance. Users value the ability to protect their identity and
location, and defend against analysis of the traffic. Revelations
about the use of pervasive surveillance [RFC7624] have, to some
extent, eroded trust in the service offered by network operators and
have led to an increased use of encryption to avoid unwanted
eavesdropping on communications. Concerns have also been voiced
about the addition of information to packets by third parties to
provide analytics, customisation, advertising, cross-site tracking of
users, to bill the customer, or to selectively allow or block
content. Whatever the reasons, the IETF is designing protocols that
include transport header encryption (e.g., QUIC
[I-D.ietf-quic-transport]) to supplement the already widespread
payload encryption, and to further limit exposure of transport
metadata to the network.
The use of transport header authentication and encryption exposes a
tussle between middlebox vendors, operators, applications developers
and users:
o On the one hand, future Internet protocols that support transport
header encryption assist in the restoration of the end-to-end
nature of the Internet by returning complex processing to the
endpoints, since middleboxes cannot modify what they cannot see,
and can improve privacy by reducing leakage of transport metadata.
o On the other hand, encryption of transport layer information has
implications for people who are responsible for operating
networks, and researchers and analysts seeking to understand the
dynamics of protocols and traffic patterns.
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A decision to use transport header encryption can improve user
privacy, and can reduce protocol ossification and help the evolution
of the transport protocol stack, but is also has implications for
network operations and management.
4.2. Approaches to Transport Header Protection
The designers of a transport protocol have to decide whether to
encrypt all, or a part of, the transport layer information.
Section 4 of [RFC8558] states: "Anything exposed to the path should
be done with the intent that it be used by the network elements on
the path".
Protocol designers can decide not to encrypt certain transport header
fields, making those fields observable in the network, or can define
new fields designed to explicitly expose observable transport layer
information to the network. Where exposed fields are intended to be
immutable (i.e., can be observed, but not modified by a network
device), the endpoints are encouraged to use authentication to
provide a cryptographic integrity check that can detect if these
immutable fields have been modified by network devices.
Authentication can also help to prevent attacks that rely on sending
packets that fake exposed control signals in transport headers (e.g.,
TCP RST spoofing). Making a part of a transport header observable or
exposing new header fields can lead to ossification of that part of a
header as network devices come to rely on observations of the exposed
fields.
The following briefly reviews some security design options for
transport protocols. A Survey of the Interaction between Security
Protocols and Transport Services [RFC8922] provides more details
concerning commonly used encryption methods at the transport layer.
Authenticating the Transport Protocol Header: Transport layer header
information can be authenticated. An integrity check that
protects the immutable transport header fields, but can still
expose the transport protocol header information in the clear,
allows in-network devices to observe these fields. An integrity
check is not able to prevent in-network modification, but can
prevent a receiving endpoint from accepting changes and avoid
impact on the transport protocol operation, including some types
of attack.
An example transport authentication mechanism is TCP-
Authentication (TCP-AO) [RFC5925]. This TCP option authenticates
the IP pseudo header, TCP header, and TCP data. TCP-AO protects
the transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. Such
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authentication might interact with middleboxes, depending on their
behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This
approach authenticates all transport headers, and verifies their
integrity at the receiver, preventing in-network modification.
The IPsec Encapsulating Security Payload (ESP) [RFC4303] can also
provide authentication and integrity without confidentiality using
the NULL encryption algorithm [RFC2410]. SRTP [RFC3711] is
another example of a transport protocol that allows header
authentication.
Selectively Encrypting Transport Headers and Payload: A transport
protocol design can encrypt selected header fields, while also
choosing to authenticate the entire transport header. This allows
specific transport header fields to be made observable by network
devices (explicitly exposed either in a transport header field or
lower layer protocol header). A design that only exposes
immutable fields can also perform end-to-end authentication of
these fields across the path to prevent undetected modification of
the immutable transport headers.
Mutable fields in the transport header provide opportunities where
network devices can modify the transport behaviour (e.g., the
extended headers described in [I-D.trammell-plus-abstract-mech]).
An example of a method that encrypts some, but not all, transport
header information is GRE-in-UDP [RFC8086] when used with GRE
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.
Greasing: Protocols often provide extensibility features, reserving
fields or values for use by future versions of a specification.
The specification of receivers has traditionally ignored
unspecified values, however in-network devices have emerged that
ossify to require a certain value in a field, or re-use a field
for another purpose. When the specification is later updated, it
is impossible to deploy the new use of the field, and forwarding
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of the protocol could even become conditional on a specific header
field value.
A protocol can intentionally vary the value, format, and/or
presence of observable transport header fields. This behaviour,
known as GREASE (Generate Random Extensions And Sustain
Extensibility) is designed to avoid a network device ossifying the
use of a specific observable field. Greasing seeks to ease
deployment of new methods. This seeks to prevent in-network
devices utilising the information in a transport header, or can
make an observation robust to a set of changing values, rather
than a specific set of values. It is not a security mechanism,
although use can be combined with an authentication mechanism.
As seen, different transports use encryption to protect their header
information to varying degrees. The trend is towards increased
protection.
5. Intentionally Exposing Transport Information to the Network
A transport protocol can choose to expose certain transport
information to on-path devices operating at the network layer by
sending observable fields. One approach is to make an explicit
choice not to encrypt certain transport header fields, making this
transport information observable by the network. Another approach is
to choose to expose transport information through the use of network-
layer extension headers (see Section 6). Both are examples of
explicit information intended to be used by network devices on the
path [RFC8558].
Whatever the mechanism used to expose the information, a decision to
only expose specific transport information, places the transport
endpoint in control of what to expose or not to expose outside of the
encrypted transport header. This decision can then be made
independently of the transport protocol functionality. This can be
done by exposing part of the transport header or as a network layer
option/extension.
5.1. Exposing Transport Information in Extension Headers
At the network-layer, packets can carry optional headers (similar to
Section 6) that may be used to explicitly expose transport header
information to the on-path devices operating at the network layer
(Section 2.2.2). For example, an endpoint that sends an IPv6 Hop-by-
Hop option [RFC8200] can provide explicit transport layer information
that can be observed and used by network devices on the path.
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Network-layer optional headers explicitly indicate the information
that is exposed, whereas use of exposed transport header information
first requires an observer to identify the transport protocol and its
format. See Section 2.1 for further discussion of transport protocol
identification.
An arbitrary path can include one or more network devices that drop
packets that include a specific header or option used for this
purpose (see [RFC7872]). This could impact the proper functioning of
the protocols using the path. Protocol methods can be designed to
probe to discover whether the specific option(s) can be used along
the current path, enabling use on arbitrary paths.
5.2. Common Exposed Transport Information
There are opportunities for multiple transport protocols to
consistently supply common observable information [RFC8558]. A
common approach can result in an open definition of the observable
fields. This has the potential that the same information can be
utilised across a range of operational and analysis tools.
5.3. Considerations for Exposing Transport Information
Considerations concerning what information, if any, it is appropriate
to expose include:
o On the one hand, explicitly exposing derived fields containing
relevant transport information (e.g., metrics for loss, latency,
etc) can avoid network devices needing to derive this information
from other header fields. This could result in development and
evolution of transport-independent tools around a common
observable header, and permit transport protocols to also evolve
independently of this ossified header [RFC8558].
o On the other hand, protocols and implementations might be designed
to avoid consistently exposing external information that reflects
the actual internal information used by the protocol itself. An
endpoint/protocol could choose to expose transport header
information to optimise the benefit it gets from the network
[RFC8558]. The value of this information would be enhanced if the
exposed information could be verified to match the protocol's
observed behavior.
The motivation to reflect actual transport header information and the
implications of network devices using this information has to be
considered when proposing such a method. RFC 8558 summarises this as
"When signals from endpoints to the path are independent from the
signals used by endpoints to manage the flow's state mechanics, they
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may be falsified by an endpoint without affecting the peer's
understanding of the flow's state. For encrypted flows, this
divergence is not detectable by on-path devices." [RFC8558].
6. Addition of Transport OAM Information to Network-Layer Headers
If the transport headers are encrypted, on-path devices can make
measurements by utilising additional protocol headers carrying
operations, administration and management (OAM) information in an
additional packet header. Using network-layer approaches to reveal
information has the potential that the same method (and hence same
observation and analysis tools) can be consistently used by multiple
transport protocols. This approach also could be applied to methods
beyond OAM (see Section 5). There can also be less desirable
implications from separating the operation of the transport protocol
from the measurement framework.
6.1. Use of OAM within a Maintenance Domain
OAM information can be added at the ingress to a maintenance domain
(e.g., an Ethernet protocol header with timestamps and sequence
number information using a method such as 802.11ag or in-situ OAM
[I-D.ietf-ippm-ioam-data], or as a part of encapsulation protocol).
The additional header information is typically removed the at the
egress of the maintenance domain.
Although some types of measurements are supported, this approach does
not cover the entire range of measurements described in this
document. In some cases, it can be difficult to position measurement
tools at the appropriate segments/nodes and there can be challenges
in correlating the downstream/upstream information when in-band OAM
data is inserted by an on-path device.
6.2. Use of OAM across Multiple Maintenance Domains
OAM information can also be added at the network layer as an IPv6
extension header or an IPv4 option. This information can be used
across multiple network segments, or between the transport endpoints.
One example is the IPv6 Performance and Diagnostic Metrics (PDM)
destination option [RFC8250]. This allows a sender to optionally
include a destination option that caries header fields that can be
used to observe timestamps and packet sequence numbers. This
information could be authenticated by receiving transport endpoints
when the information is added at the sender and visible at the
receiving endpoint, although methods to do this have not currently
been proposed. This method has to be explicitly enabled at the
sender.
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7. Conclusions
Header encryption and strong integrity checks are being incorporated
into new transport protocols and have important benefits. The pace
of development of transports using the WebRTC data channel, and the
rapid deployment of the QUIC transport protocol, can both be
attributed to using the combination of UDP as a substrate while
providing confidentiality and authentication of the encapsulated
transport headers and payload.
This document has described some current practises, and the
implications for some stakeholders, when transport layer header
encryption is used. It does not judge whether these practises are
necessary, or endorse the use of any specific practise. Rather, the
intent is to highlight operational tools and practises to consider
when designing and modifying transport protocols, so protocol
designers can make informed choice about what transport header fields
to encrypt, and whether it might be beneficial to make an explicit
choice to expose certain fields to the network. In making such a
decision, it is important to balance:
o User Privacy: The less transport header information that is
exposed to the network, the lower the risk of leaking metadata
that might have privacy implications for the users. Transports
that chose to expose some header fields need to make a privacy
assessment to understand the privacy cost versus benefit trade-off
in making that information available. The process used to define
and expose the QUIC spin bit to the network is an example of such
an analysis.
o Transport Ossification: Unencrypted transport header fields are
likely to ossify rapidly, as network devices come to rely on their
presence, making it difficult to change the transport in future.
This argues that the choice to expose information to the network
is made deliberately and with care, since it is essentially
defining a stable interface between the transport and the network.
Some protocols will want to make that interface as limited as
possible; other protocols might find value in exposing certain
information to signal to the network, or in allowing the network
to change certain header fields as signals to the transport. The
visible wire image of a protocol should be explicitly designed.
o Network Ossification: While encryption can reduce ossification of
the transport protocol, it does not itself prevent ossification of
the network service. People seeking to understand network traffic
could still come to rely on pattern inferences and other
heuristics or machine learning to derive measurement data and as
the basis for network forwarding decisions [RFC8546]. This can
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also create dependencies on the transport protocol, or the
patterns of traffic it can generate, also in time resulting in
ossification of the service.
o Impact on Operational Practice: The network operations community
has long relied on being able to understand Internet traffic
patterns, both in aggregate and at the flow level, to support
network management, traffic engineering, and troubleshooting.
Operational practice has developed based on the information
available from unencrypted transport headers. The IETF has
supported this practice by developing operations and management
specifications, interface specifications, and associated Best
Current Practises. Widespread deployment of transport protocols
that encrypt their information will impact network operations,
unless operators can develop alternative practises that work
without access to the transport header.
o Pace of Evolution: Removing obstacles to change can enable an
increased pace of evolution. If a protocol changes its transport
header format (wire image) or their transport behaviour, this can
result in the currently deployed tools and methods becoming no
longer relevant. Where this needs to be accompanied by
development of appropriate operational support functions and
procedures, it can incur a cost in new tooling to catch-up with
each change. Protocols that consistently expose observable data
do not require such development, but can suffer from ossification
and need to consider if the exposed protocol metadata has privacy
implications. There is no single deployment context, and
therefore designers need to consider the diversity of operational
networks (ISPs, enterprises, Distributed DoS (DDoS) mitigation and
firewall maintainers, etc.).
o Supporting Common Specifications: Common, open, specifications can
stimulate engagement by developers, users, researchers, and the
broader community. Increased protocol diversity can be beneficial
in meeting new requirements, but the ability to innovate without
public scrutiny risks point solutions that optimise for specific
cases, but that can accidentally disrupt operations of/in
different parts of the network. The social contract that
maintains the stability of the Internet relies on accepting common
interworking specifications, and on it being possible to detect
violations. It is important to find new ways of maintaining that
community trust as increased use of transport header encryption
limits visibility into transport behaviour.
o Impact on Benchmarking and Understanding Feature Interactions: An
appropriate vantage point for observation, coupled with timing
information about traffic flows, provides a valuable tool for
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benchmarking network devices, endpoint stacks, functions, and/or
configurations. This can also help with understanding complex
feature interactions. An inability to observe transport header
information can make it harder to diagnose and explore
interactions between features at different protocol layers, a
side-effect of not allowing a choice of vantage point from which
this information is observed. New approaches might have to be
developed.
o Impact on Research and Development: Hiding transport header
information can impede independent research into new mechanisms,
measurement of behaviour, and development initiatives. Experience
shows that transport protocols are complicated to design and
complex to deploy, and that individual mechanisms have to be
evaluated while considering other mechanisms, across a broad range
of network topologies and with attention to the impact on traffic
sharing the capacity. If increased use of transport header
encryption results in reduced availability of open data, it could
eliminate the independent checks to the standardisation process
that have previously been in place from research and academic
contributors (e.g., the role of the IRTF Internet Congestion
Control Research Group (ICCRG) and research publications in
reviewing new transport mechanisms and assessing the impact of
their deployment).
Observable transport header information might be useful to various
stakeholders. Other sets of stakeholders have incentives to limit
what can be observed. This document does not make recommendations
about what information ought to be exposed, to whom it ought to be
observable, or how this will be achieved. There are also design
choices about where observable fields are placed. For example, one
location could be a part of the transport header outside of the
encryption envelope, another alternative is to carry the information
in a network-layer option or extension header. New transport
protocol designs ought to explicitly identify any fields that are
intended to be observed, consider if there are alternative ways of
providing the information, and reflect on the implications of
observable fields being used by network devices, and how this might
impact user privacy and protocol evolution when these fields become
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
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exposed and information hidden ought to be carefully considered when
specifying new transport protocols.
8. Security Considerations
This document is about design and deployment considerations for
transport protocols. Issues relating to security are discussed
throughout this document.
Authentication, confidentiality protection, and integrity protection
are identified as Transport Features by [RFC8095]. As currently
deployed in the Internet, these features are generally provided by a
protocol or layer on top of the transport protocol [RFC8922].
Confidentiality and strong integrity checks have properties that can
also be incorporated into the design of a transport protocol or to
modify an existing transport. Integrity checks can protect an
endpoint from undetected modification of protocol fields by network
devices, whereas encryption and obfuscation or greasing can further
prevent these headers being utilised by network devices [RFC8701].
Preventing observation of headers provides an opportunity for greater
freedom to update the protocols and can ease experimentation with new
techniques and their final deployment in endpoints. A protocol
specification needs to weigh the costs of ossifying common headers,
versus the potential benefits of exposing specific information that
could be observed along the network path to provide tools to manage
new variants of protocols.
Header encryption can provide confidentiality of some or all of the
transport header information. This prevents an on-path device from
knowledge of the header field. It therefore prevents mechanisms
being built that directly rely on the information or seeks to infer
semantics of an exposed header field. Reduced visibility into
transport metadata can limit the ability to measure and characterise
traffic, and conversely can provide privacy benefits.
Extending the transport payload security context to also include the
transport protocol header protects both information with the same
key. A privacy concern would arise if this key was shared with a
third party, e.g., providing access to transport header information
to debug a performance issue, would also result in exposing the
transport payload data to the same third party. Such risks would be
mitigated using a layered security design that provides one domain of
protection and associated keys for the transport payload and
encrypted transport headers; and a separate domain of protection and
associated keys for any observable transport header fields.
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Exposed transport headers are sometimes utilised as a part of the
information to detect anomalies in network traffic. "While PM is an
attack, other forms of monitoring that might fit the definition of PM
can be beneficial and not part of any attack, e.g., network
management functions monitor packets or flows and anti-spam
mechanisms need to see mail message content." [RFC7258]. This can
be used as the first line of defence to identify potential threats
from DoS or malware and redirect suspect traffic to dedicated nodes
responsible for DoS analysis, malware detection, or to perform packet
"scrubbing" (the normalisation of packets so that there are no
ambiguities in interpretation by the ultimate destination of the
packet). These techniques are currently used by some operators to
also defend from distributed DoS attacks.
Exposed transport header fields can also form a part of the
information used by the receiver of a transport protocol to protect
the transport layer from data injection by an attacker. In
evaluating this use of exposed header information, it is important to
consider whether it introduces a significant DoS threat. For
example, an attacker could construct a DoS attack by sending packets
with a sequence number that falls within the currently accepted range
of sequence numbers at the receiving endpoint, this would then
introduce additional work at the receiving endpoint, even though the
data in the attacking packet might not finally be delivered by the
transport layer. This is sometimes known as a "shadowing attack".
An attack can, for example, disrupt receiver processing, trigger loss
and retransmission, or make a receiving endpoint perform unproductive
decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then
restore protocol state).
One mitigation to off-path attack is to deny knowledge of what header
information is accepted by a receiver or obfuscate the accepted
header information, e.g., setting a non-predictable initial value for
a sequence number during a protocol handshake, as in [RFC3550] and
[RFC6056], or a port value that cannot be predicted (see Section 5.1
of [RFC8085]). A receiver could also require additional information
to be used as a part of a validation check before accepting packets
at the transport layer (e.g., utilising a part of the sequence number
space that is encrypted; or by verifying an encrypted token not
visible to an attacker). This would also mitigate against on-path
attacks. An additional processing cost can be incurred when
decryption has to be attempted before a receiver is able to discard
injected packets.
Open standards motivate a desire for this evaluation to include
independent observation and evaluation of performance data, which in
turn suggests control over where and when measurement samples are
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collected. This requires consideration of the appropriate balance
between encrypting all and no transport header information. Open
data, and accessibility to tools that can help understand trends in
application deployment, network traffic and usage patterns can all
contribute to understanding security challenges.
The Security and Privacy Considerations in the Framework for Large-
Scale Measurement of Broadband Performance (LMAP) [RFC7594] contain
considerations for Active and Passive measurement techniques and
supporting material on measurement context.
Addition of observable transport information to the path increases
the information available to an observer and may, when this
information can be linked to a node or user, reduce the privacy of
the user. See the security considerations of [RFC8558].
9. IANA Considerations
This memo includes no request to IANA.
10. Acknowledgements
The authors would like to thank Mohamed Boucadair, Spencer Dawkins,
Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen
Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris
Wood, Thomas Fossati, Mohamed Boucadair, Martin Thomson, David Black,
Martin Duke, and other members of TSVWG for their comments and
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.
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[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", draft-ietf-ippm-ioam-data-10 (work in
progress), July 2020.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-29 (work
in progress), June 2020.
[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-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-38 (work in progress), May
2020.
[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.
[Measurement]
Fairhurst, G., Kuehlewind, M., and D. Lopez, "Measurement-
based Protocol Design, Eur. Conf. on Networks and
Communications, Oulu, Finland.", June 2017.
[PAM-RTT] Trammell, B. and M. Kuehlewind, "Revisiting the Privacy
Implications of Two-Way Internet Latency Data (in Proc.
PAM 2018)", March 2018.
[Quic-Trace]
"https:QUIC trace utilities //github.com/google/quic-
trace".
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[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, DOI 10.17487/RFC2410,
November 1998, <https://www.rfc-editor.org/info/rfc2410>.
[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>.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135,
DOI 10.17487/RFC3135, June 2001,
<https://www.rfc-editor.org/info/rfc3135>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<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>.
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[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<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>.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[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>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[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>.
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[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.
[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>.
[RFC5426] Okmianski, A., "Transmission of Syslog Messages over UDP",
RFC 5426, DOI 10.17487/RFC5426, March 2009,
<https://www.rfc-editor.org/info/rfc5426>.
[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>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[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>.
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[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[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>.
[RFC6846] Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West,
"RObust Header Compression (ROHC): A Profile for TCP/IP
(ROHC-TCP)", RFC 6846, DOI 10.17487/RFC6846, January 2013,
<https://www.rfc-editor.org/info/rfc6846>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7126] Gont, F., Atkinson, R., and C. Pignataro, "Recommendations
on Filtering of IPv4 Packets Containing IPv4 Options",
BCP 186, RFC 7126, DOI 10.17487/RFC7126, February 2014,
<https://www.rfc-editor.org/info/rfc7126>.
[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>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[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>.
[RFC7605] Touch, J., "Recommendations on Using Assigned Transport
Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
August 2015, <https://www.rfc-editor.org/info/rfc7605>.
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[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<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>.
[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>.
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[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
[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>.
[RFC8462] Rooney, N. and S. Dawkins, Ed., "Report from the IAB
Workshop on Managing Radio Networks in an Encrypted World
(MaRNEW)", RFC 8462, DOI 10.17487/RFC8462, October 2018,
<https://www.rfc-editor.org/info/rfc8462>.
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[RFC8517] Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
Jacquenet, "An Inventory of Transport-Centric Functions
Provided by Middleboxes: An Operator Perspective",
RFC 8517, DOI 10.17487/RFC8517, February 2019,
<https://www.rfc-editor.org/info/rfc8517>.
[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>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.
[RFC8922] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Wood, "A Survey of the Interaction between Security
Protocols and Transport Services", RFC 8922,
DOI 10.17487/RFC8922, October 2020,
<https://www.rfc-editor.org/info/rfc8922>.
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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.
-11 Updated following additional feedback from Martin Thomson, and
corrections from other reviewers.
-12 Updated following additional feedback from reviewers.
-13 Updated following 2nd WGLC with comments from D.L.Black; T.
Herbert; Ekr; and other reviewers.
-14 Update to resolve feedback to rev -13. This moves the general
discussion of adding fields to transport packets to section 6, and
discusses with reference to material in RFC8558.
-15 Feedback from D.L. Black, T. Herbert, J. Touch, S. Dawkins
and M. Duke. Update to add reference to RFC7605. Clarify a focus
on immutable transport fields, rather than modifying middleboxes with
Tom H. Clarified Header Compression discussion only provides a list
of examples of HC methods for transport. Clarified port usage with
Tom H/Joe T. Removed some duplicated sentences, and minor edits.
Added NULL-ESP. Improved after initial feedback from Martin Duke.
-16 Editorial comments from Mohamed Boucadair. Added DTLS 1.3.
-17 Revised to satisfy ID-NITs and updates REFs to latest rev,
updated HC REFs; cited IAB guidance on security and privacy within
IETF specs.
-18 Revised based on AD review.
Authors' Addresses
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
EMail: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
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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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