The Impact of Transport Header Confidentiality on Network Operation and Evolution of the Internet
draft-ietf-tsvwg-transport-encrypt-08
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| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Gorry Fairhurst , Colin Perkins | ||
| Last updated | 2019-08-26 (Latest revision 2019-07-04) | ||
| Replaces | draft-fairhurst-tsvwg-transport-encrypt | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Send notices to | David Black <david.black@dell.com> |
draft-ietf-tsvwg-transport-encrypt-08
TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins
Expires: February 24, 2020 University of Glasgow
August 23, 2019
The Impact of Transport Header Confidentiality on Network Operation and
Evolution of the Internet
draft-ietf-tsvwg-transport-encrypt-08
Abstract
This document describes some implications of applying end-to-end
encryption at the transport layer. It first identifies in-network
uses of transport layer header information. Then, it reviews some
implications of developing end-to-end transport protocols that use
encryption to provide confidentiality of the transport protocol
headers, or that use authentication to protect the integrity of
transport header information. Since measurement and analysis of the
impact of network characteristics on transport protocols has been
important to the design of current transports, it also considers the
impact of transport encryption on transport and application
evolution.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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 February 24, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Context and Rationale . . . . . . . . . . . . . . . . . . . . 3
2.1. Use of Transport Header Information in the Network . . . 4
2.2. Encryption of Transport Header Information . . . . . . . 5
2.3. Encryption tradeoffs . . . . . . . . . . . . . . . . . . 6
3. Current uses of Transport Headers within the Network . . . . 8
3.1. Observing Transport Information in the Network . . . . . 9
3.2. Transport Measurement . . . . . . . . . . . . . . . . . . 15
3.3. Use for Network Diagnostics and Troubleshooting . . . . . 19
3.4. Header Compression . . . . . . . . . . . . . . . . . . . 20
4. Encryption and Authentication of Transport Headers . . . . . 21
5. Addition of Transport Information to Network-Layer Protocol
Headers . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6. Implications of Protecting the Transport Headers . . . . . . 25
6.1. Independent Measurement . . . . . . . . . . . . . . . . . 26
6.2. Characterising "Unknown" Network Traffic . . . . . . . . 28
6.3. Accountability and Internet Transport Protocols . . . . . 28
6.4. Impact on Operational Cost . . . . . . . . . . . . . . . 28
6.5. Impact on Research, Development and Deployment . . . . . 29
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
11. Informative References . . . . . . . . . . . . . . . . . . . 36
Appendix A. Revision information . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
There is increased interest in, and deployment of, protocols that
employ end-to-end encryption at the transport layer, including the
transport layer headers. An example of such a transport is the QUIC
transport protocol [I-D.ietf-quic-transport], currently being
standardised in the IETF. Encryption of transport layer headers and
payload data has many benefits in terms of protecting user privacy.
These benefits have been widely discussed [RFC7258], [RFC7624], and
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this document strongly supports the increased use of encryption in
transport protocols. Encryption and authentication can also be used
to prevent unwanted modification of transport header information by
middleboxes. There are also, however, some costs, in that the
widespread use of transport encryption requires changes to network
operations, and complicates network measurement for research,
operational, and standardisation purposes. The direction in which
the use of transport header confidentiality evolves could have
significant implications on the way the Internet architecture
develops, and therefore needs to be considered as a part of protocol
design.
The remainder of this document discusses some consequences of
applying end-to-end encryption at the transport layer. It reviews
the implications of developing end-to-end transport protocols that
use encryption to provide confidentiality of the transport protocol
headers, and considers the effect of such changes on transport
protocol design and network operations. It also considers some
anticipated implications on transport and application evolution.
Transports are also increasingly encrypting and authenticating the
payload (i.e., the application data carried within the transport
connection) end-to-end. Such protection is encouraged, and the
implications of protecting the payload are not further discussed in
this document.
2. Context and Rationale
The transport layer provides end-to-end interactions between
endpoints (processes) using an Internet path. Transport protocols
layer directly over the network-layer service, and are sent in the
payload of network-layer packets. They support end-to-end
communication between applications, supported by higher-layer
protocols, running on the end systems (transport endpoints). This
simple architectural view hides one of the core functions of the
transport: to discover and adapt to the Internet path that is
currently used. The design of Internet transport protocols is as
much about trying to avoid the unwanted side effects of congestion on
a flow and other capacity-sharing flows, avoiding congestion
collapse, adapting to changes in the path characteristics, etc., as
it is about end-to-end feature negotiation, flow control, and
optimising for performance of a specific application.
To achieve stable Internet operations, the IETF transport community
has to date relied heavily on the results of measurements and the
insights of the network operations community to understand the trade-
offs, and to inform selection of appropriate mechanisms to ensure a
safe, reliable, and robust Internet (e.g., [RFC1273]). In turn, the
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network operator and access provider community has relied on being
able to understand the pattern and requirements of traffic passing
over the Internet, both in aggregate and at the flow level. The
widespread use of transport header encryption may change this.
2.1. Use of Transport Header Information in the Network
In-network measurement of transport flow characteristics can be used
to enhance performance, and control cost and service reliability.
Some operators have deployed functionality in middleboxes to both
support network operations and enhance performance. This reliance on
the presence and semantics of specific header information leads to
ossification, where an endpoint could be required to supply a
specific header to receive the network service that it desires. In
some cases, this could be benign or advantageous to the protocol
(e.g., recognising the start of a connection, or explicitly exposing
protocol information can be expected to provide more consistent
decisions by on-path devices than the use of diverse methods to infer
semantics from other flow properties). In other cases, the
ossification could frustrate the evolution of the protocol (e.g., a
mechanism implemented in a network device, such as a firewall, that
required a header field to have only a specific known set of values
would prevent the device from forwarding packets using a different
version of a protocol that introduces a feature that changes the
value of this field).
As an example of ossification, consider the experience of developing
Transport Layer Security (TLS) 1.3 [RFC8446]. This required a design
that recognised that deployed middleboxes relied on the presence of
certain header filed exposed in TLS 1.2, and failed if those headers
were changed. Other examples of the impact of ossification can be
found in the development of Multipath TCP (MPTCP) and the TCP Fast
Open option. The design of MPTCP had to be revised to account for
middleboxes, so called "TCP Normalizers", that monitor the evolution
of the window advertised in the TCP headers and that reset
connections if the window does not grow as expected. Similarly, TCP
Fast Open has had issues with middleboxes that remove unknown TCP
options, that drop segments with unknown TCP options, that drop
segments that contain data and have the SYN bit set, that drop
packets with SYN/ACK that acknowledge data, or that disrupt
connections that send data before the three-way handshake completes.
In all these cases, the issue was caused by middleboxes that had a
hard-coded understanding of transport behaviour, and that interacted
poorly with transports that tried to change that behaviour. Other
examples have included middleboxes that rewrite TCP sequence and
acknowledgement numbers but are unaware of the (newer) SACK option
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and don't correctly rewrite selective acknowledgements to match the
changes made to the fixed TCP header.
2.2. Encryption of Transport Header Information
Encryption is expected to form a basis for many future transport
protocol designs. These can be in the form of encrypted transport
protocols (i.e., transport protocols that use encryption to provide
confidentiality of some or all of the transport-layer header
information), and/or the encryption of transport payloads (i.e.,
confidentiality of the payload data). There are many motivations for
deploying such transports, and increasing public concerns about
interference with Internet traffic [RFC7624] have led to a rapidly
expanding deployment of encrypted transport protocols such as QUIC
[I-D.ietf-quic-transport]. Using encryption to provide
confidentiality of the transport layer therefore brings some well-
known privacy and security benefits.
Authentication and the introduction of cryptographic integrity checks
for header fields can prevent undetected manipulation of transport
headers by network devices. This does not prevent inspection of the
information by devices on path, and it is possible that such devices
could develop mechanisms that rely on the presence of such a field or
a known value in the field. In this context, specification of a non-
encrypted transport header field explicitly allows protocol designers
to make the certain header information observable by the network.
This supports use of this information by on-path devices, but at the
same time can be expected to lead to ossification of the transport
header, because network forwarding could evolve to depend on the
presence and/or value of these fields. To avoid unwanted inspection,
a protocol could intentionally vary the format or value of exposed
header fields [I-D.ietf-tls-grease].
A protocol design that uses header encryption with secure key
distribution can provide confidentiality for some, or all, of the
protocol header information. This prevents an on-path device from
observing the transport headers, and stops mechanisms being built
that directly rely on transport header information, or that seek to
infer semantics of exposed header fields. Transport header
encryption can therefore help reduce ossification of the transport
layer.
While encryption can hide transport header information, it does not
prevent ossification of the network service. People seeking to
understand network traffic could come to rely on pattern inferences
and other heuristics as the basis for network decision and to derive
measurement data. This can create new dependencies on the transport
protocol, or the patterns of traffic it can generate. This use of
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machine-learning methods usually demands large data sets, presenting
it own requirements for collecting and distributing the data.
2.3. Encryption tradeoffs
The are architectural challenges and considerations in the way
transport protocols are designed, and the ability to characterise and
compare different transport solutions [Measure]. The decision about
which transport headers fields are made observable offers trade-offs
around authentication and confidentiality versus observability,
network operations and management, and ossification. The impact
differs depending on the activity, for example:
Network Operations and Research: Observable transport headers enable
explicit measure and analysis protocol performance, network
anomalies, and failure pathologies at any point along the Internet
path. In many cases, it is important to relate observations to
specific equipment/configurations or network segments.
Concealing transport header information makes performance/
behaviour unavailable to passive observers along the path,
Operators will be unable to use this information directly and may
turn to more ambitious ways to collect, estimate, or infer that
data. Operational practices aimed at guessing transport
parameters are out of scope for this document, and are only
mentioned here to recognize that encryption does not prevent
operators from attempting to apply practices that were used with
unencrypted transport headers.
Confidentiality of the transport payload could be provided while
leaving some, or all, transport headers unencrypted (or providing
this information in a network-layer extension), possibly with
authentication. This provides many of the privacy and security
benefits while supporting operations and research, but at the cost
of ossifying the exposed headers.
Protection from Denial of Service: Observable transport headers
currently provide useful input to classify and detect anomalous
events, such as changes in application behaviour or distributed
denial of service attacks. For this application to be effective,
it needs to be possible for an operator to uniquely disambiguate
unwanted traffic. Concealing transport header information would
prevent disambiguation based on transport information. This could
result in less-efficient identification of unwanted traffic, the
use of heuristics to identify anomalous flows, or the introduction
of rate limits for uncharacterised traffic.
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Network Troubleshooting and Diagnostics: Observable transport
headers can be utilised by operators for network troubleshooting
and diagnostics. Flows experiencing packet loss or jitter are
hard to distinguish from unaffected flows when only observing
network layer headers. Effective troubleshooting often requires
visibility into the transport layer behaviour.
Concealing transport header information reduces the incentive for
operators to troubleshoot, since they cannot interpret the data.
It can limit understanding of transport dynamics, such as the
impact of packet loss or latency on the flows, or make it harder
to localise the network segment intoducing the packet loss or
latency. Additional mechanisms will be needed to help reconstruct
or replace transport-level metrics for troubleshooting and
diagnostics. These can add complexity and operational costs
(e.g., in deploying additional functions in equipment or adding
traffic overhead).
Network Traffic Analysis: Observable transport headers can support
network traffic analysis to determine which transport protocols
and features are being used across a network segment and to
measure trends in the pattern of usage. For some applications
end-to-end measurements/traces are sufficient, but in other
applications it is important to relate observations to specific
equipment/configurations or particular network segments.
Concealing transport header information can make analysis harder
or impossible. This could impact the ability for an operator to
anticipate the need for network upgrades and roll-out. It can
also impact the on-going traffic engineering activities performed
by operators, such as determining which parts of the path
contribute delay, jitter or loss. While this impact could, in
many cases, be small, there are scenarios where operators directly
support particular services and need visibility to explore issues
relating to Quality of Service (QoS), the ability to perform fast
re-routing of critical traffic, or to mitigate the characteristics
of specific radio links, and so on.
Open and Verifiable Network Data: Observable transport headers can
provide open and verifiable measurement data. The ability of
other stake holders to review transport header traces helps
develop insight into performance and traffic contribution of
specific variants of a protocol. Independently observed data is
important to help ensure the health of the research and
development communities.
Concealing transport header information can reduce the range of
actors that can observe useful data. This would limit the
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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 practices.
Compliance: Observable transport headers coupled with published
transport specifications allow operators and regulators to check
compliance. Independently verifiable performance metrics can also
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 the need to deploy 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]).
When transport header information is concealed, it is not possible
to observe transport header information. Methods are still needed
to confirm that the traffic produced conforms to the expectations
of the operator or developer.
Different parties will view the relative importance of these issues
differently. For some, the benefits of encrypting some, or all, of
the transport headers may outweigh the impact of doing so; others
might make a different trade-off. The purpose of highlighting the
trade-offs is to make such analysis possible.
3. Current uses of Transport Headers within the Network
Despite transport headers having end-to-end meaning, some of these
transport headers have come to be used in various ways within the
Internet. In response to pervasive monitoring [RFC7624] revelations
and the IETF consensus that "Pervasive Monitoring is an Attack"
[RFC7258], efforts are underway to increase encryption of Internet
traffic. Applying confidentiality to transport header fields affects
how protocol information is used [RFC8404], requiring consideration
of the trade-offs discussed in Section 2.3. To understand the
implications, it is necessary to understand how transport layer
headers are currently observed and/or modified by middleboxes within
the network.
We review some current uses in the following section. This does not
consider the intentional modification of transport headers by
middleboxes (such as in Network Address Translation, NAT, or
Firewalls). Common issues concerning IP address sharing are
described in [RFC6269].
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3.1. Observing Transport Information in the Network
If in-network observation of transport protocol headers is needed,
this requires knowledge of the format of the transport header:
o Flows need to be identified at the level required to perform the
observation;
o The protocol and version of the header need to be visible, e.g.,
by defining the wire image [RFC8546]. As protocols evolve over
time and there could be a need to introduce new transport headers.
This could require interpretation of protocol version information
or connection setup information;
o The location and syntax of any observed transport headers need to
be known. IETF transport protocols can specify this information.
The following subsections describe various ways that observable
transport information has been utilised.
3.1.1. Flow Identification Using Transport Layer Headers
Flow identification is a common function. For example, performed by
measurement activities, QoS classification, firewalls, Denial of
Service, DOS, prevention. This becomes more complex and less easily
achieved when multiplexing is used at or above the transport layer.
Observable transport header information, together with information in
the network header, has been used to identify flows and their
connection state, together with the protocol options being used.
Transport protocols, such as TCP and the Stream Control Transport
Protocol (SCTP), specify a standard base header that includes
sequence number information and other data. They also have the
possibility to negotiate additional headers at connection setup,
identified by an option number in the transport header.
In some uses, a low-numbered (well-known) transport port number can
be used to identify the protocol, although port information alone is
not sufficient to guarantee identification of a protocol since
applications can use arbitrary ports, multiple sessions can be
multiplexed on a single port, and ports can be re-used by subsequent
sessions.
UDP-based protocols often do not use well-known port numbers. Some
flows can instead 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].
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Concealing transport header information can remove information used
to classify flows by passive observers along the path, so operators
will be unable to use this information directly. Careful use of the
network layer features can help address provide similar information
in the case where the network is unable to inspect transport protocol
headers. Operators could also turn to more ambitious ways to
collect, estimate, or infer that data, including heuristics based on
the analysis of traffic patterns. For example, an operator that no
longer has access to Session Description Protocol (SDP) session
descriptions to classify a flow carry as audio traffic might instead
use heuristics to infer that short UDP packets with regular spacing
carry audio traffic. Operational practices aimed at inferring
transport parameters are out of scope for this document, and are only
mentioned here to recognize that encryption does not prevent
operators from attempting to apply practices that were used with
unencrypted transport headers.
Confidentiality of the transport payload could be provided while
leaving some, or all, transport headers unencrypted, or providing
this information in a network-layer extension, possibly with
authentication. This provides many of the privacy and security
benefits while supporting operations and research, but at the cost of
ossifying the exposed headers.
3.1.2. Metrics derived from Transport Layer Headers
Observable transport headers enable explicit measurement and analysis
of protocol performance, network anomalies, and failure pathologies
at any point along the Internet path. Some operators manage their
portion of the Internet by characterizing the performance of link/
network segments. Passive monitoring can observe traffic that does
not encrypt the transport header information, and make inferences
from transport headers to derive performance metrics.
A variety of open source and commercial tools have been deployed that
utilise transport header information in this way. The following
metrics can be derived:
Traffic Rate and Volume: Header information (e.g., sequence number
and packet size) allows derivation of volume measures per-
application, to characterise the traffic that uses a network
segment or the pattern of network usage. This can be measured per
endpoint or for an aggregate of endpoints (e.g., to assess
subscriber usage). It can also be used to trigger measurement-
based traffic shaping, and to implement QoS support within the
network and lower layers. Volume measures can be valuable for
capacity planning and providing detail of trends, rather than the
volume per subscriber.
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Loss Rate and Loss Pattern: Flow loss rate can be derived (e.g.,
from transport sequence numbers) 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., interference on a radio link), buffer overflow
(e.g., due to congestion), policing (traffic management), buffer
management (e.g., Active Queue Management, AQM [RFC7567]), and
inadequate provision of traffic pre-emption. Understanding flow
loss rates requires either observing sequence numbers in transport
headers, or maintaining per-flow packet counters (but note that
flow identification often requires transport header information).
Per-hop loss can be monitored at the interface level by devices in
the network. It is often valuable to understand the conditions
under which packet loss occurs. This usually requires relating
per-flow loss to the traffic flowing on the network node/segment
at the time of loss.
Observation of transport feedback information (e.g., RTP Control
Protocol (RTCP) reception reports [RFC3550], TCP SACK blocks) can
increase understanding of the impact of loss and help identify
cases where loss could have been wrongly identified, or where the
transport did not require the lost packet. It is sometimes more
helpful to understand the pattern of loss, than the loss rate,
because losses can often occur as bursts, rather than randomly-
timed events.
Throughput and Goodput: Throughput is the amount of data sent by a
flow per time interval. Goodput [RFC7928] is a measure of useful
data exchanged (the ratio of useful data to total volume of
traffic sent by a flow). The throughput achieved by a flow can be
determined even when transport header information is concealed,
providing the individual flow can be identified. Goodput requires
ability to differentiate loss and retransmission of packets, for
example by observing packet sequence numbers in the TCP or the
Real-time Transport Protocol (RTP) headers [RFC3550].
Latency: Latency is a key performance metric that impacts
application and user-perceived response times. It often
indirectly impacts throughput and flow completion time. Latency
determines the reaction time of the transport protocol itself,
impacting flow setup, congestion control, loss recovery, and other
transport mechanisms. The observed latency can have many
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components [Latency]. Of these, unnecessary/unwanted queuing in
network buffers has often been observed as a significant factor
[bufferbloat]. Once the cause of unwanted latency has been
identified, this can often be eliminated.
To measure latency across a part of a path, an observation point
[RFC7799] can measure the experienced round trip time (RTT) using
packet sequence numbers, and acknowledgements, or by observing
header timestamp information. Such information allows an
observation point in the network to determine not only the path
RTT, but also to measure 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 [RFC8290] [RFC8289] [RFC8033] often
cannot scale across all possible deployment scenarios.
Variation in delay: Some network applications are sensitive to
(small) changes in packet timing (jitter). Short and long-term
delay variation can impact on the latency of a flow and hence the
perceived quality of applications using the network. For example,
jitter metrics are often cited when characterising paths
supporting real-time traffic. To assess the performance of such
applications, it can be necessary to measure the variation in
delay observed along a portion of the path [RFC3393] [RFC5481].
The requirements for observable transport headers resemble those
for the measurement of latency.
Flow Reordering: Significant packet reordering within a flow can
impact time-critical applications and can be interpreted as loss
by reliable transports. Many transport protocol techniques are
impacted by reordering (e.g., triggering TCP retransmission or re-
buffering of real-time applications). Packet reordering can occur
for many reasons, from equipment design to misconfiguration of
forwarding rules. Since this impacts transport performance,
network tools are needed to detect and measure unwanted/excessive
reordering.
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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] and
[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 is often needed to
understand 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 needs to trust the
source of this data and the method used to generate the summary
information.
This information can support network operations, inform capacity
planning, and assist in determining the need for equipment and/or
configuration changes by network operators. It can also inform
Internet engineering activities by informing the development of new
protocols, methodologies, and procedures.
3.1.3. Transport use of Network Layer Header Fields
Information from the transport protocol 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, and by firewalls to implement access rules (see
also section 2.2.2 of [RFC8404]). Network-layer classification
methods that rely on a multi-field classifier (e.g., inferring QoS
from the 5-tuple or choice of application protocol) are incompatible
with transport protocols that encrypt the transport information.
Traffic that cannot be classified will typically receive a default
treatment.
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Transport information can also be explicitly set in network-layer
header fields that are not encrypted. This can provide information
to 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 hide 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
network needs of individual sub-flows. There are several ways this
could be done:
IP Address: Applications expose the addresses used by endpoints, and
this is used in the forwarding decisions in network devices.
Address and other protocol information can be used by a Multi-
Field (MF) classifier to determine how traffic is treated
[RFC2475], and hence the quality of experience for a flow.
Using the IPv6 Network-Layer Flow Label: A number of Standards Track
and Best Current Practice RFCs (e.g., [RFC8085], [RFC6437],
[RFC6438]) encourage endpoints to set the IPv6 Flow label field of
the network-layer header. IPv6 "source nodes SHOULD assign each
unrelated transport connection and application data stream to a
new flow" [RFC6437]. A multiplexing transport could choose to use
multiple Flow labels to allow the network to independently forward
subflows. 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". To promote privacy, the
Flow Label assignment needs to avoid introducing linkability that
a network device may observe. Once set, a flow label can provide
information that can help inform network-layer queuing and
forwarding [RFC6438], for example with Equal Cost Multi-Path
routing and Link Aggregation [RFC6294]. Considerations when using
IPsec are further described in [RFC6438].
Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to the network
by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications
identify different forwarding treatments for individual sub-flows
(audio vs. video) based on the value of the DSCP field
[I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information
to inform network-layer queuing and forwarding, rather than an
operator inferring traffic requirements from transport and
application headers via a multi-field classifier.
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Since the DSCP value can impact the quality of experience for a
flow, observations of service performance need to consider this
field when a network path has support for differentiated service
treatment.
Using Explicit Congestion Marking: ECN [RFC3168] is a transport
mechanism that utilises the ECN field in the network-layer header.
Use of ECN explicitly informs the network-layer that a transport
is ECN-capable, and requests ECN treatment of the flow. An ECN-
capable transport can offer benefits when used over a path with
equipment that implements an AQM method with CE marking of IP
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 need to be available to network operators and
researchers to understand the implication of configuration choices
and transport behaviour as the use of ECN increases and new
methods emerge [RFC7567].
When transport headers are concealed, operators will be unable to use
this information directly. Careful use of the network layer features
can help address provide similar information in the case where the
network is unable to inspect transport protocol headers.
3.2. Transport Measurement
The common language between network operators and application/content
providers/users is packet transfer performance at a layer that all
can view and analyse. For most packets, this has been the transport
layer, until the emergence of QUIC, with the obvious exception of
Virtual Private Networks (VPNs) and IPsec.
When encryption conceals more layers in each packet, people seeking
understanding of the network operation rely more on pattern inference
and other heuristics. It remains to be seen whether more complex
inferences can be mastered to produce the same monitoring accuracy
(see section 2.1.1 of [RFC8404]).
When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network, is
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often required to interpret this data to answer questions about
network performance or understand a pathology. Collecting and
coordinating such metadata is more difficult when the observation
point is at a different location to the bottleneck/device under
evaluation [RFC7799].
Packet sampling techniques are used to scale the processing involved
in observing packets on high rate links. This exports only the
packet header information of (randomly) selected packets. The
utility of these measurements depends on the type of bearer and
number of mechanisms used by network devices. Simple routers are
relatively easy to manage, a device with more complexity demands
understanding of the choice of many system parameters. This level of
complexity exists when several network methods are combined.
This section discusses topics concerning observation of transport
flows, with a focus on transport measurement.
3.2.1. Point of Observation
On-path measurements are particularly useful for locating the source
of problems, or to assess the performance of a network segment or a
particular device configuration. Often issues can only be understood
in the context of the other flows that share a particular path,
common network device, interface port, etc. A simple example is
monitoring of a network device that uses a scheduler or active queue
management technique [RFC7567], where it could be desirable to
understand whether the algorithms are correctly controlling latency,
or if overload protection is working. This understanding implies
knowledge of how traffic is assigned to any sub-queues used for flow
scheduling, but can also require information about how the traffic
dynamics impact active queue management, starvation prevention
mechanisms, and circuit-breakers.
Sometimes multiple on-path observation points are needed. By
correlating observations of headers at multiple points along the path
(e.g., at the ingress and egress of a network segment), an observer
can determine the contribution of a portion of the path to an
observed metric, to locate a source of delay, jitter, loss,
reordering, congestion marking, etc.
3.2.2. Use by Operators to Plan and Provision Networks
Traffic measurements (e.g., traffic volume, loss, latency) 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
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new deployments. This measurement information can also be correlated
with billing information when this is also collected by an operator.
A network operator supporting traffic that uses transport header
encryption might not have access to per-flow measurement data.
Trends in aggregate traffic can be observed and can be related to the
endpoint addresses being used, but it may be impossible to correlate
patterns in measurements with changes in transport protocols (e.g.,
the impact of changes in introducing a new transport protocol
mechanism). This increases the dependency on other indirect sources
of information to inform planning and provisioning.
3.2.3. Service Performance Measurement
Traffic measurements (e.g., traffic volume, loss, latency) can be
used by various actors to help analyse the performance offered to the
users of a network segment, and to inform operational practice.
While active measurements (see section 3.4 of [RFC7799]) may be used
within a network, passive measurements (see section 3.6 of [RFC7799])
can have advantages in terms of eliminating unproductive test
traffic, reducing the influence of test traffic on the overall
traffic mix, and the ability to choose the point of observation (see
Section 3.2.1). However, passive measurements can rely on observing
transport headers which is not possible if those headers are
encrypted.
3.2.4. Measuring Transport to Support Network Operations
Information provided by tools observing transport headers can help
determine whether mechanisms are needed in the network to prevent
flows from acquiring excessive network capacity. Operators can
implement operational practices to manage traffic flows (e.g., to
prevent flows from acquiring excessive network capacity under severe
congestion) by deploying rate-limiters, traffic shaping or network
transport circuit breakers [RFC8084].
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 use in the shared Internet. TCP algorithms have
been continuously improved over decades and they have reached a
level of efficiency and correctness that custom application-layer
mechanisms will struggle to easily duplicate [RFC8085].
A standards-compliant TCP stack provides congestion control that
may therefore be judged safe for use across the Internet.
Applications developed on top of well-designed transports can be
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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 transport (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 in the face of persistent congestion,
and hence to understand whether the behaviour is appropriate for
sharing limited network capacity. For example, it is common to
visualise plots of TCP sequence numbers versus time for a flow to
understand how a flow shares available capacity, deduce its
dynamics in response to congestion, etc.
The ability to identify sources that contribute to persistent
congestion is important to safe operation of network
infrastructure, and mechanisms 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 are required
to employ mechanisms to prevent congestion collapse, avoid
unacceptable contributions to jitter/latency, and to establish an
acceptable share of capacity with concurrent traffic [RFC8085].
A network operator needs tools to understand if datagram flows
(e.g., using UDP) comply with congestion control expectations and
therefore whether there is a need to deploy methods such as rate-
limiters, transport circuit breakers, or other methods to enforce
acceptable usage for the offered service.
UDP flows that expose a well-known header by specifying the format
of header fields can allow information to 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 and RTCP header information for real-time flows (see
Section 3.1.2). The Secure RTP extensions [RFC3711] were
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explicitly designed to expose some header information to enable
such observation, while protecting the payload data.
3.3. Use for Network Diagnostics and Troubleshooting
Transport header information can be useful for a variety of
operational tasks [RFC8404]: to diagnose network problems, assess
network provider performance, evaluate equipment/protocol
performance, capacity planning, management of security threats
(including denial of service), and responding to user performance
questions. Section 3.1.2 and Section 5 of [RFC8404] provide further
examples. These tasks seldom involve the need to determine the
contents of the transport payload, or other application details.
A network operator supporting traffic that uses transport header
encryption can see only encrypted transport headers. This prevents
deployment of performance measurement tools that rely on transport
protocol information. Choosing to encrypt all the information
reduces 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 response to other queries about
the network service. For some this will be blessing, for others it
may be a curse. For example, operational performance data about
encrypted flows needs to be determined by traffic pattern analysis,
rather than relying on traditional tools. This can impact the
ability of the operator to respond to faults, it could require
reliance on endpoint diagnostic tools or user involvement in
diagnosing and troubleshooting unusual use cases or non-trivial
problems. A key need here is for tools to provide useful information
during network anomalies (e.g., significant reordering, high or
intermittent loss).
Measurements can be used to monitor the health of a portion of the
Internet, to provide early warning of the need to take action. They
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 some cases, measurements could involve active injection of test
traffic to perform a measurement. 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. An alternative approach is
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to use in-network techniques that observe transport packet headers
added while traffic traverses an operational network to make the
measurements. These measurements do not require the cooperation of
an endpoint.
In other cases, measurement involves dissecting network traffic
flows. The observed transport layer information can help identify
whether the link/network tuning is effective and alert to potential
problems that can be hard to derive from link or device measurements
alone. The design trade-offs for radio networks are often very
different from those of wired networks. A radio-based network (e.g.,
cellular mobile, enterprise WiFi, satellite access/back-haul, point-
to-point radio) has the complexity of a subsystem that performs radio
resource management, with direct impact on the available capacity,
and potentially loss/reordering of packets. The impact of the
pattern of loss and congestion, differs for different traffic types,
correlation with propagation and interference can all have
significant impact on the cost and performance of a provided service.
The need for this type of information is expected to increase as
operators bring together heterogeneous types of network equipment and
seek to deploy opportunistic methods to access radio spectrum.
A flow that conceals its transport header information could imply
"don't touch" to some operators. This could limit a trouble-shooting
response to "can't help, no trouble found".
3.4. Header Compression
Header compression saves link capacity by compressing network and
transport protocol headers on a per-hop basis. It was widely used
with low bandwidth dial-up access links, and still finds application
on wireless links that are subject to capacity constraints. Header
compression has been specified for use with TCP/IP and RTP/UDP/IP
flows [RFC2507], [RFC2508], [RFC4995].
While it is possible to compress only the network layer headers,
significant savings can be made if both the network and transport
layer headers are compressed together as a single unit. The Secure
RTP extensions [RFC3711] were explicitly designed to leave the
transport protocol headers unencrypted, but authenticated, since
support for header compression was considered important. Encrypting
the transport protocol headers does not break such header
compression, but does cause it to fall back to compressing only the
network layer headers, with a significant reduction in efficiency.
This can impact the efficiency of a link/path.
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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.
Encryption methods can hide information from an eavesdropper in the
network. Encryption can also help protect the privacy of a user, by
hiding data relating to user/device identity or location. Neither an
integrity check nor encryption methods prevent traffic analysis, and
usage needs to reflect that profiling of users, identification of
location and fingerprinting of behaviour can take place even on
encrypted traffic flows. Any header information that has a clear
definition in the protocol's message format(s), or is implied by that
definition, and is not cryptographically confidentiality-protected
can be unambiguously interpreted by on-path observers [RFC8546].
There are several motivations for encryption:
o One motive to use encryption is a response to perceptions that the
network has become ossified by over-reliance on middleboxes that
prevent new protocols and mechanisms from being deployed. This
has lead to a perception that there is too much "manipulation" of
protocol headers within the network, and that designing to deploy
in such networks is preventing transport evolution. In the light
of this, a method that authenticates transport headers could help
improve the pace of transport development, by eliminating the need
to always consider deployed middleboxes
[I-D.trammell-plus-abstract-mech], or potentially to only
explicitly enable use by middleboxes for particular paths with
particular middleboxes that are deliberately deployed to realise a
useful function for the network and/or users[RFC3135].
o Another motivation stems from increased concerns about privacy and
surveillance. Some Internet users have valued the ability to
protect identity, user location, and defend against traffic
analysis, and have used methods such as IPsec Encapsulated
Security Payload (ESP), VPNs and other encrypted tunnel
technologies. Revelations about the use of pervasive surveillance
[RFC7624] have, to some extent, eroded trust in the service
offered by network operators, and following the Snowden revelation
in the USA in 2013 has led to an increased desire for people to
employ encryption to avoid unwanted "eavesdropping" on their
communications. Concerns have also been voiced about the addition
of information to packets by third parties to provide analytics,
customization, advertising, cross-site tracking of users, to bill
the customer, or to selectively allow or block content. Whatever
the reasons, there are now activities in the IETF to design new
protocols that could include some form of transport header
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encryption (e.g., QUIC [I-D.ietf-quic-transport]) to supplement
the already widespread payload encryption.
Authentication methods that provide integrity checks of protocols
fields have also been specified at the network layer, and this also
protects transport header fields. The network layer itself carries
protocol header fields that are increasingly used to help forwarding
decisions reflect the need of transport protocols, such as the IPv6
Flow Label [RFC6437], DSCP, and ECN fields.
The use of transport layer authentication and encryption exposes a
tussle between middlebox vendors, operators, applications developers
and users:
o On the one hand, future Internet protocols that enable large-scale
encryption assist in the restoration of the end-to-end nature of
the Internet by returning complex processing to the endpoints,
since middleboxes cannot modify what they cannot see.
o On the other hand, encryption of transport layer header
information has implications for people who are responsible for
operating networks and researchers and analysts seeking to
understand the dynamics of protocols and traffic patterns.
Whatever the motives, a decision to use pervasive transport header
encryption will have implications on the way in which design and
evaluation is performed. This can, in turn, impact the direction of
evolution of the transport protocol stack. While the IETF can
specify protocols, the success in actual deployment is often
determined by many factors [RFC5218] that are not always clear at the
time when protocols are being defined.
The following briefly reviews some security design options for
transport protocols. A Survey of Transport Security Protocols
[I-D.ietf-taps-transport-security] provides more details concerning
commonly used encryption methods at the transport layer.
Authenticating the Transport Protocol Header: Transport layer header
information can be authenticated. An integrity check that
protects the immutable transport header fields, but can still
expose the transport protocol header information in the clear,
allows in-network devices to observe these fields. An integrity
check is not able to prevent in-network modification, but can
prevent a receiving from accepting changes and avoid impact on the
transport protocol operation.
An example transport authentication mechanism is TCP-
Authentication (TCP-AO) [RFC5925]. This TCP option authenticates
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the IP pseudo header, TCP header, and TCP data. TCP-AO protects
the transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. TCP-AO may
interact with middleboxes, depending on their behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This
approach authenticates all transport headers, and verifies their
integrity at the receiver, preventing in-network modification.
Secure RTP [RFC3711] is another example of a transport protocol
that allows header authentication.
Greasing: Protocols often provide extensibility features, reserving
fields or values for use by future versions of a specification.
The specification of receivers has traditionally ignored
unspecified values, however in-network devices have emerged that
ossify to require a certain value in a field, or re-use a field
for another purpose. When the specification is later updated, it
is impossible to deploy the new use of the field, and forwarding
of the protocol could even become conditional on a specific header
field value.
A protocol can intentionally vary the value, format, and/or
presence of observable transport header fields. This behaviour,
known as GREASE (Generate Random Extensions And Sustain
Extensibility) is designed to avoid a network device ossifying the
use of a specific observable field. Greasing seeks to ease
deployment of new methods. It can also prevent in-network devices
utilising the information in a transport header, or can make an
observation robust to a set of changing values, rather than a
specific set of values.
Encrypting the Transport Payload: The transport layer payload can be
encrypted to protect the content of transport segments. This
leaves transport protocol header information in the clear. The
integrity of immutable transport header fields could be protected
by combining this with an integrity check.
Examples of encrypting the payload include Transport Layer
Security (TLS) over TCP [RFC8446] [RFC7525], Datagram TLS (DTLS)
over UDP [RFC6347] [RFC7525], Secure RTP [RFC3711], and TCPcrypt
[RFC8548] which permits opportunistic encryption of the TCP
transport payload.
Encrypting the Transport Headers and Payload: The network layer
payload could be encrypted (including the entire transport header
and the payload). This method provides confidentiality of the
entire transport packet. It therefore does not expose any
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transport information to devices in the network, which also
prevents modification along a network path.
One example of encryption at the network layer is the use of IPsec
Encapsulating Security Payload (ESP) [RFC4303] in tunnel mode.
This encrypts and authenticates all transport headers, preventing
visibility of the transport headers by in-network devices. Some
VPN methods also encrypt these headers.
Selectively Encrypting Transport Headers and Payload: A transport
protocol design can encrypt selected header fields, while also
choosing to authenticate the entire transport header. This allows
specific transport header fields to be made observable by network
devices. End-to end integrity checks can prevent an endpoint from
undetected modification of the immutable transport headers.
Mutable fields in the transport header provide opportunities for
middleboxes to modify the transport behaviour (e.g., the extended
headers described in [I-D.trammell-plus-abstract-mech]). This
considers only immutable fields in the transport headers, that is,
fields that can be authenticated End-to-End across a path.
An example of a method that encrypts some, but not all, transport
information is GRE-in-UDP [RFC8086] when used with GRE encryption.
Optional Encryption of Header Information: There are implications to
the use of optional header encryption in the design of a transport
protocol, where support of optional mechanisms can increase the
complexity of the protocol and its implementation and in the
management decisions that are required to use variable format
fields. Instead, fields of a specific type ought to always be
sent with the same level of confidentiality or integrity
protection.
As seen, different transports use encryption to protect their header
information to varying degrees. There is, however, a trend towards
increased protection with newer transport protocols.
5. Addition of Transport Information to Network-Layer Protocol Headers
An on-path device can make measurements by appending additional
protocol headers carrying operations, administration and management
(OAM) information to packets at the ingress to a maintenance domain
(e.g., an Ethernet protocol header with timestamps and sequence
number information using a method such as 802.11ag or in-situ OAM
[I-D.ietf-ippm-ioam-data]) and removing the additional header at the
egress of the maintenance domain. This approach enables some types
of measurements, but does not cover the entire range of measurements
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described in this document. In some cases, it can be difficult to
position measurement tools at the required segments/nodes and there
can be challenges in correlating the downsream/upstream information
when in-band OAM data is inserted by an on-path device. This has the
advantage that a single header can support all transport protocols,
but there could also be less desirable implications of separating the
operation of the transport protocol from the measurement framework.
Another example of a network-layer approach 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 needs to be
explicitly enabled at the sender.
Current measurement results suggest that it can be undesirable to
rely on methods requiring end to end support of network options or
extension headers across the Internet. IPv4 network options are
often not supported (or are carried on a slower processing path) and
some IPv6 networks have been observed to drop packets that set an
IPv6 header extension (e.g., results from 2016 in [RFC7872]).
Another possibility is that protocols that separately expose header
information do not necessarily have an incentive to expose the actual
information that is utilised by the protocol itself and could
therefore manipulate the exposed header information to gain an
advantage from the network. The incentive to reflect actual
transport information needs to be considered when proposing a method
that selectively exposes header information.
6. Implications of Protecting the Transport Headers
The choice of which fields to expose and which to encrypt is a design
choice for the transport protocol. Any selective encryption method
requires trading two conflicting goals for a transport protocol
designer to decide which header fields to encrypt. Security work
typically employs a design technique that seeks to expose only what
is needed. This approach provides incentives to not reveal any
information that is not necessary for the end-to-end communication.
However, there can be performance and operational benefits in
exposing selected information to network tools.
This section explores key implications of working with encrypted
transport protocols.
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6.1. Independent Measurement
Independent observation by multiple actors is important if the
transport community is to maintain an accurate understanding of the
network. Encrypting transport header encryption changes the ability
to collect and independently analyse data. Internet transport
protocols employ a set of mechanisms. Some of these need to work in
cooperation with the network layer for loss detection and recovery,
congestion detection and congestion control. Others need to work
only end-to-end (e.g., parameter negotiation, flow-control).
The majority of present Internet applications use two well-known
transport protocols, TCP and UDP. Although TCP represents the
majority of current traffic, many real-time applications use UDP, and
much of this traffic utilises RTP format headers in the payload of
the UDP datagram. Since these protocol headers have been fixed for
decades, a range of tools and analysis methods have became common and
well-understood.
Protocols that expose the state information used by the transport
protocol in their header information (e.g., timestamps used to
calculate the RTT, packet numbers used to asses congestion and
requests for retransmission) provide an incentive for the sending
endpoint to provide correct information, since the protocol will not
work otherwise. This increases confidence that the observer
understands the transport interaction with the network. For example,
when TCP is used over an unencrypted network path (i.e., one that
does not use IPsec or other encryption below the transport), it
implicitly exposes header information that can be used for
measurement at any point along the path. This information is
necessary for the protocol's correct operation, therefore there is no
incentive for a TCP implementation to put incorrect information in
this transport header. A network device can have confidence that the
well-known (and ossified) transport information represents the actual
state of the endpoints.
When encryption is used to conceal some or all of the transport
headers, the transport protocol choose what information to reveal to
the network about its internal state, what information to leave
encrypted, and what fields to grease to protect against future
ossification. Such a transport could be designed, for example, to
provide summary data regarding its performance, congestion control
state, etc., or to make an explicit measurement signal available.
For example, a QUIC endpoint could set the spin bit to reflect to
explicitly reveal a session's RTT [I-D.ietf-quic-spin-exp]).
When providing or using such information, it becomes important to
consider the privacy of the user and their incentive for providing
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accurate and detailed information. Protocols that selectively reveal
some transport state or measurement signals are choosing to establish
a trust relationship with the network operators. There is no
protocol mechanism that can guarantee that the information provided
represents the actual transport state of the endpoints, since those
endpoints can always send additional information in the encrypted
part of the header, to update or replace whatever they reveal. This
reduces the ability to independently measure and verify that a
protocol is behaving as expected. Some operational uses need the
information to contain sufficient detail to understand, and possibly
reconstruct, the network traffic pattern for further testing; such
operators must gain the trust of transport protocol implementers if
they 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 conceal some or all of the
transport headers, analysis will require coordination between actors
at different layers to successfully characterise flows and correlate
the performance or behaviour of a specific mechanism with the
configuration and traffic using operational equipment (e.g.,
combining transport and network measurements to explore congestion
control dynamics, the implications of designs for active queue
management or circuit breakers).
For some usage a standardised endpoint-based logging format (e.g.,
based on Quic-Trace [Quic-Trace]) could offer an alternative for some
in-network measurement. Such information will have 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.
Measurments based on logging will need to establish the validity and
provenance of the logged information to establish how and when traces
were captured.
However, endpoint logs do not provide equivalent information to in-
network measurements. In particular, endpoint logs contain only a
part of the information needed to understand the operation of network
devices and identify issues such as link performance or capacity
sharing between multiple flows. Additional information is needed to
determine which equipment/links are used and the configuration of
equipment along the network paths being measured.
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6.2. Characterising "Unknown" Network Traffic
The patterns and types of traffic that share Internet capacity change
over time as networked applications, usage patterns and protocols
continue to evolve.
If "unknown" or "uncharacterised" traffic patterns form a small part
of the traffic aggregate passing through a network device or segment
of the network the path, the dynamics of the uncharacterised traffic
may not have a significant collateral impact on the performance of
other traffic that shares this network segment. Once the proportion
of this traffic increases, the need to monitor the traffic and
determine if appropriate safety measures need to be put in place.
Tracking the impact of new mechanisms and protocols requires traffic
volume to be measured and new transport behaviours to be identified.
This is especially true of protocols operating over a UDP substrate.
The level and style of encryption needs to be considered in
determining how this activity is performed. On a shorter timescale,
information may also need to be collected to manage denial of service
attacks against the infrastructure.
6.3. Accountability and Internet Transport Protocols
Information provided by tools observing transport headers can be used
to classify traffic, and to limit the network capacity used by
certain flows, as discussed in Section 3.2.4). Equally, operators
could use analysis of transport headers and transport flow state to
demonstrate that they are not providing differential treatment to
certain flows. Obfuscating or hiding this information using
encryption may lead operators and maintainers of middleboxes
(firewalls, etc.) to seek other methods to classify, and potentially
other mechanisms to condition, network traffic.
A lack of data that reduces the level of precision with which flows
can be classified also reduces the design space for conditioning
mechanisms (e.g., rate limiting, circuit breaker techniques
[RFC8084], or blocking of uncharacterised traffic), and this needs to
be considered when evaluating the impact of designs for transport
encryption [RFC5218].
6.4. Impact on Operational Cost
Many network operators currently utilise observed transport
information as a part of their operational practice, and have
developed tools and operational practices based around currently
deployed transports and their applications. Encryption of the
transport information prevents tools from directly observing this
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information. A variety of open source and commercial tools have been
deployed that utilise this information for a variety of short and
long term measurements.
The network will not break just because transport headers are
encrypted, although alternative diagnostic and troubleshooting tools
would need to be developed and deployed. Introducing a new protocol
or application can require these tool chains and practice to be
updated, and may in turn impact operational mechanisms, and policies.
Each change can introduce associated costs, including the cost of
collecting data, and the tooling needed to handle multiple formats
(possibly as these co-exist in the network, when measurements need to
span time periods during which changes are deployed, or to compare
with historical data). These costs are incurred by an operator to
manage the service and debug network issues.
At the time of writing, the additional operational cost of using
encrypted transports is not yet well understood. Design trade-offs
could mitigate these costs by explicitly choosing to expose selected
information (e.g., header invariants and the spin-bit in QUIC
[I-D.ietf-quic-transport]), the specification of common log formats,
and development of alternative approaches.
6.5. Impact on Research, Development and Deployment
Evolution and the ability to understand (measure) the impact need to
proceed hand-in-hand. Observable transport headers can provide open
and verifiable measurement data. Observation of pathologies has a
critical role in the design of transport protocol mechanisms and
development of new mechanisms and protocols. This helps
understanding the interactions between cooperating protocols and
network mechanism, the implications of sharing capacity with other
traffic and the impact of different patterns of usage. The ability
of other stake holders to review transport header traces helps
develop insight into performance and traffic contribution of specific
variants of a protocol.
In development of new transport protocol mechanisms, attention needs
to be paid to the expected scale of deployment. Whatever the
mechanism, experience has shown that it is often difficult to
correctly implement combinations of mechanisms [RFC8085]. 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.
Concealing 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 practices. The cooperating dependence of
network, application, and host to provide communication performance
on the Internet is uncertain when only endpoints (i.e., at user
devices and within service platforms) can observe performance, and
when performance cannot be independently verified by all parties.
Independently observed data is also important to ensure the health of
the research and development communities and can help promote
acceptance of proposed specifications by the wider community (e.g.,
as a method to judge the safety for Internet deployment) and provides
valuable input during standardisation. Open standards motivate a
desire to include independent observation and evaluation of
performance data, which in turn demands control over where and when
measurement samples are collected. This requires consideration of
the methods used to observe data and the appropriate balance between
encrypting all and no transport information.
7. Conclusions
Confidentiality and strong integrity checks have properties that are
being incorporated into new protocols and that 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.
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To achieve stable Internet operations, the IETF transport community
has, to date, relied heavily on measurement and insights of the
network operations community to understand the trade-offs, and to
inform selection of appropriate mechanisms, to ensure a safe,
reliable, and robust Internet (e.g., [RFC1273],[RFC2914]).
The traffic that can be observed by on-path network devices is a
function of transport protocol design/options, network use,
applications, and user characteristics. In general, when only a
small proportion of the traffic has a specific (different)
characteristic, such traffic seldom leads to operational concern,
although the ability to measure and monitor it is less. The desire
to understand the traffic and protocol interactions typically grows
as the proportion of traffic increases in volume. The challenges
increase when multiple instances of an evolving protocol contribute
to the traffic that share network capacity.
An increased pace of evolution therefore needs to be accompanied by
methods that can be successfully deployed and used across operational
networks. This leads to a need for network operators at various
levels (ISPs, enterprises, firewall maintainer, etc.) to identify
appropriate operational support functions and procedures.
Protocols that change their transport header format (wire format) or
their behaviour (e.g., algorithms that are needed to classify and
characterise the protocol), will require new tooling to be developed
to catch-up with the change. If the currently deployed tools and
methods are no longer relevant, then it may no longer be possible to
correctly measure performance. This can increase the response-time
after faults, and can impact the ability to manage the network
resulting in traffic causing traffic to be treated inappropriately
(e.g., rate limiting because of being incorrectly classified/
monitored).
There are benefits in exposing consistent information to the network
that avoids traffic being inappropriately classified and then
receiving a default treatment by the network. The flow label and
DSCP fields provide examples of how transport information can be made
available for network-layer decisions. Extension headers could also
be used to carry transport information that can inform network-layer
decisions. Other information may also be useful to various
stakeholders, however this document does not make recommendations
about what information should be exposed, to whom it should be
observable, or how this will be achieved.
There are trade-offs and implications of increased use of encryption
when designing a protocol. Transport protocol designers have often
ignored the implications of whether the information in transport
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header fields can or will be used by in-network devices, and the
implications this places on protocol evolution. This motivates a
design that provides confidentiality of header information. This
lack of visibility of transport header information can be expected to
impact the ways that protocols are deployed, standardised, and their
operational support. The impact of hiding transport headers
therefore needs to be considered in the specification and development
of protocols and standards. This has a potential impact on the way
in which the IRTF and IETF develop new protocols, specifications, and
guidelines:
o Coexistence of Transport and Network Device Protocols/
Configuration: Transmission Control Protocol (TCP) is currently
the predominant transport protocol used over Internet paths. Its
many variants have broadly consistent approaches to avoiding
congestion collapse, and to ensuring the stability of the
Internet. Increased use of transport layer encryption can
overcome ossification, allowing deployment of new transports and
different types of congestion control. This flexibility can be
beneficial, but it could come at the cost of fragmenting the
ecosystem. There is little doubt that developers will try to
produce high quality transports for their intended target uses,
but it is not yet clear there are sufficient incentives to ensure
good practice that benefits the wide diversity of requirements for
the Internet community as a whole.
o Supporting Common Specifications: Common open specifications can
stimulate engagement by developers, users, and researchers.
Increased diversity, and the ability to innovate without public
scrutiny, risks point solutions that optimise for specific needs,
but 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.
o Benchmarking and Understanding Feature Interactions: An
appropriate vantage point for observation, coupled with timing
information about traffic flows, provides a valuable tool for
benchmarking network devices, endpoint stacks, functions, and/or
configurations. This can also help understand complex feature
interactions. An inability to observe transport protocol
information can limit the ability 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 need to be
developed.
o Operational Practice: The network operations community relies on
being able to understand the pattern and requirements of traffic
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passing over the Internet, both in aggregate and at the flow
level. These operational practices have developed based on the
information available from unencrypted transport headers. The
IETF supports this activity by developing operations and
management specifications, interface specifications, and
associated Best Current Practice (BCP) specifications. Concealing
transport header information impacts current practice and demand
new specifications.
o Research and Development: Concealing transport information can
impede independent research into new mechanisms, measurement of
behaviour, and development initiatives. Experience shows that
transport protocols are complicated to design and complex to
deploy, and that individual mechanisms need 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 this results in reduced availability of open data,
it could eliminate the independent self-checks to the
standardisation process that have previously been in place from
research and academic contributors (e.g., the role of the IRTF
Internet Congestion Control Research Groups (ICCRG) and research
publications in reviewing new transport mechanisms and assessing
the impact of their experimental deployment).
The choice of whether future transport protocols encrypt their
protocol headers needs to be taken based not solely on security and
privacy considerations, but also taking into account the impact on
operations, standards and research. 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."
As part of a protocol's design, the community therefore needs to
weigh the benefits of ossifying common headers versus the potential
demerits of exposing specific information that could be observed
along the network path, to ensure network operators, researchers and
other stakeholders have appropriate tools to manage their networks
and enable stable operation of the Internet as new protocols are
deployed. An appropriate balance will emerge over time as real
instances of this tension are analysed [RFC7258]. This balance
between information exposed and information concealed 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.
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Authentication, confidentiality protection, and integrity protection
are identified as Transport Features by [RFC8095]. As currently
deployed in the Internet, these features are generally provided by a
protocol or layer on top of the transport protocol
[I-D.ietf-taps-transport-security].
Confidentiality and strong integrity checks have properties that can
also be incorporated into the design of a transport protocol.
Integrity checks can protect an endpoint from undetected modification
of protocol fields by network devices, whereas encryption and
obfuscation or greasing can further prevent these headers being
utilised by network devices. Hiding headers can therefore provide
the 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 benefits
of ossifying common headers, versus the potential demerits of
exposing specific information that could be observed along the
network path to provide tools to manage new variants of protocols.
A protocol design that uses header encryption can provide
confidentiality of some or all of the protocol header information.
This prevents an on-path device from knowledge of the header field.
It therefore prevents mechanisms being built that directly rely on
the information or seeks to infer semantics of an exposed header
field. Hiding headers can limit the ability to measure and
characterise traffic.
Exposed transport headers are sometimes utilised as a part of the
information to detect anomalies in network traffic. This can be used
as the first line of defence to identify potential threats from DOS
or malware and redirect suspect traffic to dedicated nodes
responsible for DOS analysis, malware detection, or to perform packet
"scrubbing" (the normalization of packets so that there are no
ambiguities in interpretation by the ultimate destination of the
packet). These techniques are currently used by some operators to
also defend from distributed DOS attacks.
Exposed transport header fields are sometimes also utilised as a part
of the information used by the receiver of a transport protocol to
protect the transport layer from data injection by an attacker. In
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 may not finally be delivered by the
transport layer. This is sometimes known as a "shadowing attack".
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An attack can, for example, disrupt receiver processing, trigger loss
and retransmission, or make a receiving endpoint perform unproductive
decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then
restore protocol state).
One mitigation to off-path attack is to deny knowledge of what header
information is accepted by a receiver or obfuscate the accepted
header information, e.g., setting a non-predictable initial value for
a sequence number during a protocol handshake, as in [RFC3550] and
[RFC6056], or a port value that can not be predicted (see section 5.1
of [RFC8085]). A receiver could also require additional information
to be used as a part of a validation check before accepting packets
at the transport layer (e.g., utilising a part of the sequence number
space that is encrypted; or by verifying an encrypted token not
visible to an attacker). This would also mitigate against on-path
attacks. An additional processing cost can be incurred when
decryption needs to be attempted before a receiver is able to discard
injected packets.
Open standards motivate a desire for this evaluation to include
independent observation and evaluation of performance data, which in
turn suggests control over where and when measurement samples are
collected. This requires consideration of the appropriate balance
between encrypting all and no transport information. Open data, and
accessibility to tools that can help understand trends in application
deployment, network traffic and usage patterns can all contribute to
understanding security challenges.
The Security and Privacy Considerations in the Framework for Large-
Scale Measurement of Broadband Performance (LMAP) [RFC7594] contain
considerations for Active and Passive measurement techniques and
supporting material on measurement context.
9. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
10. Acknowledgements
The authors would like to thank Mohamed Boucadair, Spencer Dawkins,
Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen
Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris
Wood, Thomas Fossati, and other members of the TSVWG for their
comments and feedback.
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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 may be made of that information.
This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
11. Informative References
[bufferbloat]
Gettys, J. and K. Nichols, "Bufferbloat: dark buffers in
the Internet. Communications of the ACM, 55(1):57-65",
January 2012.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
"Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
data-06 (work in progress), July 2019.
[I-D.ietf-quic-spin-exp]
Trammell, B. and M. Kuehlewind, "The QUIC Latency Spin
Bit", draft-ietf-quic-spin-exp-01 (work in progress),
October 2018.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-22 (work
in progress), July 2019.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-19
(work in progress), November 2017.
[I-D.ietf-taps-transport-security]
Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K.
Rose, "A Survey of Transport Security Protocols", draft-
ietf-taps-transport-security-08 (work in progress), August
2019.
[I-D.ietf-tls-grease]
Benjamin, D., "Applying GREASE to TLS Extensibility",
draft-ietf-tls-grease-04 (work in progress), August 2019.
Fairhurst & Perkins Expires February 24, 2020 [Page 36]
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[I-D.ietf-tsvwg-rtcweb-qos]
Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
qos-18 (work in progress), August 2016.
[I-D.trammell-plus-abstract-mech]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", draft-trammell-plus-
abstract-mech-00 (work in progress), September 2016.
[Latency] Briscoe, B., "Reducing Internet Latency: A Survey of
Techniques and Their Merits, IEEE Comm. Surveys &
Tutorials. 26;18(3) p2149-2196", November 2014.
[Measure] Fairhurst, G., Kuehlewind, M., and D. Lopez, "Measurement-
based Protocol Design, Eur. Conf. on Networks and
Communications, Oulu, Finland.", June 2017.
[Quic-Trace]
"https:QUIC trace utilities //github.com/google/quic-
trace".
[RFC1273] Schwartz, M., "Measurement Study of Changes in Service-
Level Reachability in the Global TCP/IP Internet: Goals,
Experimental Design, Implementation, and Policy
Considerations", RFC 1273, DOI 10.17487/RFC1273, November
1991, <https://www.rfc-editor.org/info/rfc1273>.
[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>.
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[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>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
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[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.
[RFC4995] Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
Header Compression (ROHC) Framework", RFC 4995,
DOI 10.17487/RFC4995, July 2007,
<https://www.rfc-editor.org/info/rfc4995>.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R.
Whitner, "Improved Packet Reordering Metrics", RFC 5236,
DOI 10.17487/RFC5236, June 2008,
<https://www.rfc-editor.org/info/rfc5236>.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
March 2009, <https://www.rfc-editor.org/info/rfc5481>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.
[RFC6294] Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
2011, <https://www.rfc-editor.org/info/rfc6294>.
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[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC7594] Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
Aitken, P., and A. Akhter, "A Framework for Large-Scale
Measurement of Broadband Performance (LMAP)", RFC 7594,
DOI 10.17487/RFC7594, September 2015,
<https://www.rfc-editor.org/info/rfc7594>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
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[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>.
[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>.
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[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>.
[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>.
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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.
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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