Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-09
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Gorry Fairhurst , Colin Perkins | ||
| Last updated | 2019-11-03 (Latest revision 2019-08-26) | ||
| Replaces | draft-fairhurst-tsvwg-transport-encrypt | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Document shepherd | David L. Black | ||
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| Send notices to | David Black <david.black@dell.com> |
draft-ietf-tsvwg-transport-encrypt-09
TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins
Expires: May 6, 2020 University of Glasgow
November 3, 2019
Considerations around Transport Header Confidentiality, Network
Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-09
Abstract
To protect user data and privacy, Internet transport protocols have
supported payload encryption and authentication for some time. Such
encryption and authentication is now also starting to be applied to
the transport protocol headers. This helps avoid transport protocol
ossification by middleboxes, while also protecting metadata about the
communication. Current operational practice in some networks inspect
transport header information within the network, but this is no
longer possible when those transport headers are encrypted. This
document discusses the possible impact when network traffic uses a
protocol with an encrypted transport header. It suggests issues to
consider when designing new transport protocols, to account for
network operations, prevent network ossification, and enable
transport evolution, while still respecting user privacy.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2020.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Context and Rationale . . . . . . . . . . . . . . . . . . . . 4
2.1. Use of Transport Header Information in the Network . . . 5
2.2. Authentication of Transport Header Information . . . . . 6
2.3. Observable Transport Header Fields . . . . . . . . . . . 7
3. Current uses of Transport Headers within the Network . . . . 10
3.1. Observing Transport Information in the Network . . . . . 11
3.2. Transport Measurement . . . . . . . . . . . . . . . . . . 17
3.3. Use for Network Diagnostics and Troubleshooting . . . . . 21
3.4. Header Compression . . . . . . . . . . . . . . . . . . . 22
4. Encryption and Authentication of Transport Headers . . . . . 23
5. Addition of Transport Information to Network-Layer Headers . 26
5.1. Use of OAM within a Maintenance Domain . . . . . . . . . 26
5.2. Use of OAM across Multiple Maintenance Domains . . . . . 26
6. Implications of Protecting the Transport Headers . . . . . . 27
6.1. Independent Measurement . . . . . . . . . . . . . . . . . 27
6.2. Characterising "Unknown" Network Traffic . . . . . . . . 29
6.3. Accountability and Internet Transport Protocols . . . . . 30
6.4. Impact on Operational Cost . . . . . . . . . . . . . . . 30
6.5. Impact on Research, Development and Deployment . . . . . 31
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
11. Informative References . . . . . . . . . . . . . . . . . . . 38
Appendix A. Revision information . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
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1. Introduction
Transport protocols have supported end-to-end encryption of payload
data for many years. Examples include Transport Layer Security (TLS)
over TCP [RFC8446], Datagram TLS (DTLS) over UDP [RFC6347], and their
corresponding usage guidelines [RFC7525], Secure RTP [RFC3711], and
TCPcrypt [RFC8548] which permits opportunistic encryption of the TCP
transport payload. Some of these also provide integrity protection
of all or part of the transport header.
This end-to-end transport payload encryption brings many benefits in
terms of providing confidentiality and protecting user privacy. Such
benefits have been widely discussed [RFC7258] [RFC7624]. This
document strongly supports and encourages increased use of end-to-end
payload encryption in transport protocols. The implications of
protecting the transport payload data are therefore not further
discussed in this document.
A further level of protection can be achieved by encrypting the
entire network layer payload, including both the transport layer
headers and the payload. This method provides confidentiality for
the entire transport packet. It therefore does not expose any
transport information to devices in the network, and prevents
modification along a network path. An example of encryption at the
network layer is the IPsec Encapsulating Security Payload (ESP)
[RFC4303] in tunnel mode. Some Virtual Private Network (VPN) methods
also encrypt these headers. This form of encryption is not further
discussed in this document.
There is also a middle ground, comprising transport protocols that
encrypt some, or all, of their transport layer header information, in
addition to the payload. An example of such a protocol, that is
seeing widespread interest and deployment, is the QUIC transport
protocol [I-D.ietf-quic-transport]. Encryption and authentication of
the transport header information can prevent unwanted modification of
transport headers by middleboxes. It also reduces the amount of
metadata about the progress of the transport connection that is
visible to the network.
As discussed in [RFC7258], Pervasive Monitoring (PM) nis a technical
attack that needs to be mitigated in the design of IETF protocols.
This document supports that conclusion and the use of transport
header encryption to protect against pervasive monitoring. RFC 7258
also notes, though, that "Making networks unmanageable to mitigate PM
is not an acceptable outcome, but ignoring PM would go against the
consensus documented here. An appropriate balance will emerge over
time as real instances of this tension are considered".
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The following sections further considers some of the costs and
changes to network management and research that are implied by
widespread use of transport protocols that encrypt the transport
header information. It reviews the implications of developing
transport protocols that use end-to-end encryption to provide
confidentiality of their transport layer headers, and considers the
effect of such changes on transport protocol design and network
operations. It also considers some anticipated implications on
transport and application evolution. That is, it considers the
issues in designing transport protocols that both protect their
header information and respect user privacy.
2. Context and Rationale
The transport layer provides end-to-end interactions between
endpoints (processes) using an Internet path. Transport protocols
layer directly over the network-layer service, and are sent in the
payload of network-layer packets. They support end-to-end
communication between applications, using higher-layer protocols
running on the end systems (transport endpoints).
This simple architectural view hides one of the core functions of the
transport: to discover and adapt to the Internet path that is
currently being used. The design of Internet transport protocols is
as much about trying to avoid the unwanted side effects of congestion
on a flow and other capacity-sharing flows, avoiding congestion
collapse, adapting to changes in the path characteristics, etc., as
it is about end-to-end feature negotiation, flow control, and
optimising for performance of a specific application.
To achieve stable Internet operations, the IETF transport community
has to date relied heavily on 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
network operator and access provider communities have 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 could change this.
Encryption is expected to form a core part of future transport
protocol designs. This 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. Increasing public concerns about
interference with Internet traffic [RFC7624] have led to a rapidly
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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.
2.1. Use of Transport Header Information in the Network
In-network measurement of transport flow characteristics can be used
to enhance performance, and control cost and service reliability. To
support network operations and enhance performance, some operators
have deployed functionality that utilises on-path observations of the
transport headers of packets passing through their network. These
devices can rely on the presence and semantics of specific header
information, which leads to ossification where an endpoint has to
supply a specific header to receive the network service that it
desires.
In some cases, network-layer use of transport header information can
be benign or advantageous to the protocol (e.g., recognising the
start of a TCP connection, providing header compression for a Secure
RTP flow, or explicitly using exposed protocol information to provide
consistent decisions by on-path devices). However, in other cases,
ossification can frustrate the evolution of the transport protocol.
A mechanism implemented in a network device, such as a firewall, that
requires a header field to have only a specific known set of values
(i.e., that regards the field as invariant) can prevent the device
from forwarding packets using a different version of a protocol that
introduces a feature that changes the value of the observed field.
An example of such ossification was observed in the development of
Transport Layer Security (TLS) 1.3 [RFC8446]. This necessitated a
design that recognised that deployed middleboxes relied on the
presence of certain header fields exposed in TLS 1.2, and failed if
those headers were changed.
The design of MPTCP also had to be revised to account for middleboxes
(known as "TCP Normalizers") that monitor the evolution of the window
advertised in the TCP header and reset connections when the window
does not grow as expected. Similarly, Issues have been reported with
TCP Fast Open using middleboxes that modify the transport header of
packets by removing unknown TCP options, that drop segments with
unknown TCP options, drop segments that contain data and have the SYN
bit set, drop packets with SYN/ACK that acknowledge data, or that
disrupt connections that send data before the three-way handshake
completes. Other examples of ossification have included middleboxes
that rewrite TCP sequence and acknowledgement numbers, but are
unaware of the (newer) TCP selective acknowledgement (SACK) Option
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and therefore fail to correctly rewrite the selective acknowledgement
header information to match the changes that were made to the fixed
TCP header.
In all these cases, the issue was caused by middleboxes that had a
hard-coded understanding of transport behaviour, and that interacted
poorly with transport protocols when the transport behaviour changed.
Many protocol specifications had also failed to clearly indicate the
invariant parts of the transport header and were designed without
thought for how header information could be used within the network.
Transport header encryption can help reduce such ossification of the
transport layer. 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. This encryption is
normally combined with authentication of the protected information.
RFC 8546 summarises this, stating that it is "The wire image, not the
protocol's specification, determines how third parties on the network
paths among protocol participants will interact with that protocol"
[RFC8546].
While encryption can hide transport header information and therefore
help to reduce ossification of the transport protocol, 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
machine-learning methods usually demands large data sets, presenting
it own requirements for collecting and distributing the data.
2.2. Authentication of Transport Header Information
The design of a transport protocol needs to determine whether to
encrypt all or a part of the transport information. It is possible
that on-path devices could develop mechanisms that rely on the
presence of any non-encrypted field, or a known value in the field.
Section 4 of RFC8558 goes further, to state: "Anything exposed to the
path should be done with the intent that it be used by the network
elements on the path" [RFC8558]. 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 this can lead to ossification of the exposed
part of a transport header. That is, network forwarding could evolve
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to depend on the presence and/or value of these fields (even if the
header is not modified by the in-network device).
New protocol designs will make use of authentication to provide a
cryptographic integrity check for the transport header fields.
Transport header information that is authenticated, but not
encrypted, permits inspection of the non-encrypted header fields by
devices on the path, but does prevent undetected manipulation by
network devices.
Sometimes a protocol design employs a header field that is not
encrypted, but it is desired to avoid unwanted inspection restricting
the choice of usable values in the field (and the resulting potential
for undesirable ossification). In this case, the protocol designers
can choose to intentionally vary the format and/or value of exposed
header fields to reduce the chance of ossification (see Section 4 and
[I-D.ietf-tls-grease]).
2.3. Observable Transport Header Fields
Transport headers have end-to-end meaning, but are often observed by
equipment within the network. The decision about which transport
headers fields are made observable offers trade-offs around header
confidentiality versus header observability (including non-encrypted
but authenticated header fields) for network operations and
management, and the implications for ossification and user privacy.
The impact differs depending on the activity, as discussed below and
developed in the remainder of this document:
Network Operations: Observable transport headers enable explicit
measurement and analysis of protocol performance,
network anomalies, and failure pathologies at any
point along the Internet path. In many cases, it
is important to relate observations to specific
equipment/configurations or a specific network
segment.
Concealing transport header information makes
performance/behaviour unavailable to passive
observers along the path. Operators will then be
unable to use this information directly and could
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 stop operators
from attempting to apply practices that have been
used with unencrypted transport headers.)
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See also Sections 3, 5, and 6.4.
Traffic Analysis: Observable transport headers can be used to
determine which transport protocols and features
are being used across a network segment, and to
measure trends in the pattern of usage. For some
use cases, end-to-end measurements/traces are
sufficient and can assist in developing and
debugging new transports and analysing their
deployment. In other uses, it is important to
relate observations to specific equipment/
configurations or particular network segments.
Concealing transport header information can make
analysis harder or impossible. This could impact
the ability to anticipate the need for network
upgrades and roll-out, or affect on-going traffic
engineering activities performed by operators
such as determining which parts of the path
contribute delay, jitter, or loss. While this
impact could, in many cases, be small, there are
scenarios where operators will actively monitor
and support particular services, e.g., to explore
issues relating to Quality of Service (QoS), to
perform fast re-routing of critical traffic, to
mitigate the characteristics of specific radio
links, and so on.
See also Sections 3.1-3.2, and 5.
Troubleshooting: Observable transport headers can be utilised by
operators for network troubleshooting and
diagnostics. Effective troubleshooting often
requires visibility into the transport layer
behaviour. Flows experiencing packet loss or
jitter are hard to distinguish from unaffected
flows when only observing network layer headers.
Concealing transport header information reduces
the incentive for operators to troubleshoot,
since they cannot interpret the data. This 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 introducing the packet loss or latency.
Additional mechanisms will be needed to help
reconstruct or replace transport-level metrics
for troubleshooting and diagnostics. These can
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add complexity and operational costs (e.g., in
deploying additional functions in equipment or
adding traffic overhead).
See also Section 3.3 and 5.
Network Protection: Observable transport headers currently provide
useful input to classify and detect anomalous
events, such as changes in application behaviour
or distributed denial of service attacks. An
operator needs 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.
See also Sections 6.2 and 6.3.
SLA Compliance: Observable transport headers coupled with
published transport specifications allow
operators and regulators to explore teh
compliance with Service Level Agreements (SLAs).
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.
See also Sections 5 and 6.1-6.3.
Verifiable Data: Observable transport headers can provide open and
verifiable measurements to support operations,
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research, and protocol development. 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 limits the information sources
available to the Internet community to understand
the operation of new transport protocols,
reducing information to inform design decisions
and standardisation of the new protocols and
related operational practices
See also Section 6.
There are architectural challenges and considerations in the way
transport protocols are designed, and the ability to characterise and
compare different transport solutions [Measure]. Different parties
will view the relative importance of these differently. For some,
the benefits of encrypting the transport headers could outweigh the
impact of doing so; others might make a different trade-off.
3. Current uses of Transport Headers within the Network
In response to pervasive monitoring [RFC7624] revelations and the
IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258],
efforts are underway to increase encryption of Internet traffic.
Applying confidentiality to transport header fields affects how
protocol information is used [RFC8404], requiring consideration of
the trade-offs discussed in Section 2.3. To understand the
implications, it is necessary to understand how transport layer
headers are currently observed and/or modified by middleboxes within
the network.
This section reviews some current usage. This review 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 needed 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/Session identification [RFC8558] is a common function. For
example, performed by measurement activities, QoS classification,
firewalls, Denial of Service, DOS, prevention.
Observable transport header information, together with information in
the network header, has been used to identify flows and their
connection state, together with the protocol options being used.
Transport protocols, such as TCP and the Stream Control Transport
Protocol (SCTP), specify a standard base header that includes
sequence number information and other data. They also have the
possibility to negotiate additional headers at connection setup,
identified by an option number in the transport header.
In some uses, a low-numbered (well-known) transport port number can
identify the protocol. However, port information alone is not
sufficient to guarantee identification when 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].
Concealing transport header information can remove information used
to classify flows by passive observers along the path, so operators
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will be unable to use this information directly. Operators could
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 cannot access the Session Description
Protocol (SDP) session descriptions to classify a flow as audio
traffic, might instead use (possibly less-reliable) heuristics to
infer that short UDP packets with regular spacing carry audio
traffic. Operational 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.
3.1.2. Metrics derived from Transport Layer Headers
Observable transport headers enable explicit measurement and analysis
of protocol performance, network anomalies, and failure pathologies
at any point along the Internet path. Some operators use passive
monitoring to manage their portion of the Internet by characterizing
the performance of link/network segments. Inferences from transport
headers are used to derive performance metrics. A variety of open
source and commercial tools have been deployed that utilise transport
header information in this way to derive the following metrics:
Traffic Rate and Volume: Protocol sequence number and packet size
can be used to derive volume measures per-application, to
characterise the traffic that uses a network segment or the
pattern of network usage. Measurements can be per endpoint or for
an endpoint aggregate (e.g., to assess subscriber usage).
Measurments can also be used to trigger traffic shaping, and to
associate QoS support within the network and lower layers. Volume
measures can also be valuable for capacity planning and providing
detail of trends in usage.
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., due to interference on a radio link), buffering loss
(e.g., overflow due to congestion, Active Queue Management, AQM
[RFC7567], or inadequate provision following traffic pre-emption),
and policing (traffic management). Understanding flow loss rates
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requires either observing sequence numbers in transport headers,
or maintaining per-flow packet counters (flow identification often
requires transport header information). Per-hop loss can also
sometimes be monitored at the interface level by devices in the
network. It is often valuable to understand the conditions under
which packet loss occurs, which usually requires relating 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 transmission of 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 of 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. This
determines the reaction time of the transport protocol itself,
impacting flow setup, congestion control, loss recovery, and other
transport mechanisms. The observed latency can have many
components [Latency]. Of these, unnecessary/unwanted queuing in
network buffers has often been observed as a significant factor
[bufferbloat]. Once the cause of unwanted latency has been
identified, this can often be eliminated.
To measure latency across a part of a path, an observation point
[RFC7799] can measure the experienced round trip time (RTT) using
packet sequence numbers, and acknowledgements, or by observing
header timestamp information. Such information allows an
observation point in the network to determine not only the path
RTT, but also allows measurement of the upstream and downstream
contribution to the RTT. This could be used to locate a source of
latency, e.g., by observing cases where the median RTT is much
greater than the minimum RTT for a part of a path.
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The service offered by network operators can benefit from latency
information to understand the impact of configuration changes and
to tune deployed services. Latency metrics are key to evaluating
and deploying AQM [RFC7567], DiffServ [RFC2474], and Explicit
Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements
could identify excessively large buffers, indicating where to
deploy or configure AQM. An AQM method is often deployed in
combination with other techniques, such as scheduling [RFC7567]
[RFC8290] and although parameter-less methods are desired
[RFC7567], current methods often require tuning [RFC8290]
[RFC8289] [RFC8033] because they cannot scale across all possible
deployment scenarios.
Variation in delay: Some network applications are sensitive to
(small) changes in packet timing (jitter). Short and long-term
delay variation can impact on the latency of a flow and hence the
perceived quality of applications using the network. For example,
jitter metrics are often cited when characterising paths
supporting real-time traffic. 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.
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.
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Metrics have been defined that evaluate whether a network has
maintained packet order on a packet-by-packet basis [RFC4737]
[RFC5236].
Techniques for measuring reordering typically observe packet
sequence numbers. Some protocols provide in-built monitoring and
reporting functions. Transport fields in the RTP header [RFC3550]
[RFC4585] can be observed to derive traffic volume measurements
and provide information on the progress and quality of a session
using RTP. As with other measurement, metadata 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.
Transport information can also be explicitly set in network-layer
header fields that are not encrypted, serving as a replacement/
addition to the exposed transport information [RFC8558]. This 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
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network needs of individual sub-flows. There are several ways this
could be done:
IP Address: Applications normally expose the addresses used by
endpoints, and this is used in the forwarding decisions in network
devices. Address and other protocol information can be used by a
Multi-Field (MF) classifier to determine how traffic is treated
[RFC2475], and hence the quality of experience for a flow.
Using the IPv6 Network-Layer Flow Label: A number of Standards Track
and Best Current Practice RFCs (e.g., [RFC8085], [RFC6437],
[RFC6438]) encourage endpoints to set the IPv6 Flow label field of
the network-layer header. IPv6 "source nodes SHOULD assign each
unrelated transport connection and application data stream to a
new flow" [RFC6437]. A multiplexing transport could choose to use
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".
Once set, a flow label can provide information that can help
inform network-layer queuing and forwarding [RFC6438], for example
with Equal Cost Multi-Path routing and Link Aggregation [RFC6294].
Considerations when using IPsec are further described in
[RFC6438].
The choice of how to assign a Flow Label needs to avoid
introducing linkability that a network device could observe.
Inappropriate use by the transport can have privacy implications
(e.g., assigning the same label to two independent flows that
ought not to be classified the same).
Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to the network
by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets [RFC2474]. For example, WebRTC applications
identify different forwarding treatments for individual sub-flows
(audio vs. video) based on the value of the DSCP field
[I-D.ietf-tsvwg-rtcweb-qos]). This provides explicit information
to inform network-layer queuing and forwarding, rather than an
operator inferring traffic requirements from transport and
application headers via a multi-field classifier. Inappropriate
use can have privacy implications (e.g., assigning the same label
to two independent flows that ought not to be classified the
same). Inappropriate use by the transport can have privacy
implications (e.g., assigning a different DSCP to a subflow could
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assist in a network device discovering the traffic pattern used by
an application). The field is mutable, i.e., some network devices
can be expected to change this field (use of each DSCP value is
defined by an RFC).
Since the DSCP value can impact the quality of experience for a
flow, observations of service performance 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.
Section Section 5 describes use of network extension headers.
3.2. Transport Measurement
The common language between network operators and application/content
providers/users is packet transfer performance at a layer that all
can view and analyse. For most packets, this has been the transport
layer, until the emergence of transport protocols performing header
encryption, with the obvious exception of VPNs and IPsec.
When encryption conceals more layers in each packet, people seeking
understanding of the network operation rely more on pattern inference
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and other heuristics. It remains to be seen whether more complex
inferences can be mastered to produce the same monitoring accuracy
(see section 2.1.1 of [RFC8404]).
When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network, is
often necessary to interpret this data to answer questions about
network performance or understand a pathology. Collecting and
coordinating such metadata is more difficult when the observation
point is at a different location to the bottleneck/device under
evaluation [RFC7799].
Packet sampling techniques are used to scale the processing involved
in observing packets on high rate links. This exports only the
packet header information of (randomly) selected packets. The
utility of these measurements depends on the type of bearer and
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.
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3.2.2. Use by Operators to Plan and Provision Networks
Traffic measurements are used by operators to help plan deployment of
new equipment and configuration in their networks. Data is also
valuable to equipment vendors who want to understand traffic trends
and patterns of usage as inputs to decisions about planning products
and provisioning for new deployments. This measurement information
can also be correlated with billing information when this is also
collected by an operator.
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 might 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]) could 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). Passive measurements can rely on observing transport
headers, which is not possible if those headers are encrypted, but
could utilise information about traffic volumes or patterns of
interaction to deduce metrics.
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., 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
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been continuously improved over decades and they have reached a
level of efficiency and correctness that custom application-layer
mechanisms will struggle to easily duplicate [RFC8085].
A standards-compliant TCP stack provides congestion control that
is judged safe for use across the Internet. Applications
developed on top of well-designed transports can be expected to
appropriately control their network usage, reacting when the
network experiences congestion, by back-off and reduce the load
placed on the network. This is the normal expected behaviour for
IETF-specified transports (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the
transport protocol header information to help understand the
impact of specific transport protocols (or protocol mechanisms) on
the other traffic that shares a network. An observation in the
network can gain an understanding of the dynamics of a flow and
its congestion control behaviour. Analysing observed flows can
help to build confidence that an application flow backs-off its
share of the network load 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 the safe operation of network
infrastructure, and can inform configuration of network devices to
complement the endpoint congestion avoidance mechanisms [RFC7567]
[RFC8084] to avoid a portion of the network being driven into
congestion collapse [RFC2914].
Congestion Control Compliance for UDP traffic: UDP provides a
minimal message-passing datagram transport that has no inherent
congestion control mechanisms. Because congestion control is
critical to the stable operation of the Internet, applications and
other protocols that choose to use UDP as a transport need to
employ mechanisms to prevent collapse, avoid unacceptable
contributions to jitter/latency, and to establish an acceptable
share of capacity with concurrent traffic [RFC8085].
A network operator 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.
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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
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. The
use of payload encryption has the desirable effect of preventing
unintended observation of the user data.
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
might 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.
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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
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
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transport protocol headers unencrypted, but authenticated, since
support for header compression was considered important. Encrypting
the transport protocol headers does not break such header
compression, but does cause a fall back to compressing only the
network layer headers, with a significant reduction in efficiency.
4. Encryption and Authentication of Transport Headers
End-to-end encryption can be applied at various protocol layers. It
can be applied above the transport to encrypt the transport payload
(e.g., using TLS). This can hide information from an eavesdropper in
the network. It can also help protect the privacy of a user, by
hiding data relating to user/device identity or location.
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
revelations 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, the IETF is designing new
protocols that include transport header encryption (e.g., QUIC
[I-D.ietf-quic-transport]) to supplement the already widespread
payload encryption.
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o Any header information that has a clear definition in the protocol
message format(s), or is implied by that definition, and is not
cryptographically confidentiality-protected can be unambiguously
interpreted by on-path observers [RFC8546].
Encryption methods do not 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. 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
the IP pseudo header, TCP header, and TCP data. TCP-AO protects
the transport layer, preventing attacks from disabling the TCP
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connection itself and provides replay protection. TCP-AO might
interact with middleboxes, depending on their behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This
approach authenticates all transport headers, and verifies their
integrity at the receiver, preventing in-network modification.
Secure RTP [RFC3711] is another example of a transport protocol
that allows header authentication.
Greasing: Protocols often provide extensibility features, reserving
fields or values for use by future versions of a specification.
The specification of receivers has traditionally ignored
unspecified values, however in-network devices have emerged that
ossify to require a certain value in a field, or re-use a field
for another purpose. When the specification is later updated, it
is impossible to deploy the new use of the field, and forwarding
of the protocol could even become conditional on a specific header
field value.
A protocol can intentionally vary the value, format, and/or
presence of observable transport header fields. This behaviour,
known as GREASE (Generate Random Extensions And Sustain
Extensibility) is designed to avoid a network device ossifying the
use of a specific observable field. Greasing seeks to ease
deployment of new methods. It can also prevent in-network devices
utilising the information in a transport header, or can make an
observation robust to a set of changing values, rather than a
specific set of values
Selectively Encrypting Transport Headers and Payload: A transport
protocol design can encrypt selected header fields, while also
choosing to authenticate the entire transport header. This allows
specific transport header fields to be made observable by network
devices. End-to end integrity checks can prevent an endpoint from
undetected modification of the immutable transport headers.
Mutable fields in the transport header provide opportunities for
middleboxes to modify the transport behaviour (e.g., the extended
headers described in [I-D.trammell-plus-abstract-mech]). This
considers only immutable fields in the transport headers, that is,
fields that can be authenticated End-to-End across a path.
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
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protocol, where support of optional mechanisms can increase the
complexity of the protocol and its implementation, and in the
management decisions that are needed 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 Headers
An on-path device can make measurements by utilising additional
protocol headers carrying operations, administration and management
(OAM) information in an additional packet header. Using network-
layer approaches to reveal information has the potential that the
same method (and hence same observation and analysis tools) can be
consistently used by multiple transport protocols [RFC8558]. There
could also be less desirable implications of separating the operation
of the transport protocol from the measurement framework.
5.1. Use of OAM within a Maintenance Domain
OAM information can be added at the ingress to a maintenance domain
(e.g., an Ethernet protocol header with timestamps and sequence
number information using a method such as 802.11ag or in-situ OAM
[I-D.ietf-ippm-ioam-data], or as a part of encapsulation protocol).
The additional header information is typically removed the at the
egress of the maintenance domain.
Although some types of measurements are supported, this approach does
not cover the entire range of measurements described in this
document. In some cases, it can be difficult to position measurement
tools at the appropriate segments/nodes and there can be challenges
in correlating the downstream/upstream information when in-band OAM
data is inserted by an on-path device.
5.2. Use of OAM across Multiple Maintenance Domains
OAM information can also be added at the network layer as an IPv6
extension header or an IPv4 option. This information can be used
across multiple network segments, or between the transport endpoints.
One example is the IPv6 Performance and Diagnostic Metrics (PDM)
Destination Option [RFC8250]. This allows a sender to optionally
include a destination option that caries header fields that can be
used to observe timestamps and packet sequence numbers. This
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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 could currently be
undesirable to rely on methods requiring end to end support of
network options or extension headers across the Internet. IPv4
network options are often not supported (or are carried on a slower
processing path) and some IPv6 networks have been observed to drop
packets that set an IPv6 header extension (e.g., results from 2016 in
[RFC7872]).
Another potential issue 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. Where the information is provided by an
endpoint, the incentive to reflect actual transport information needs
to be considered when proposing a method.
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.
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 control. Others need to work only end-to-
end (e.g., parameter negotiation, flow-control).
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The majority of present Internet applications use two well-known
transport protocols, TCP and UDP. Although TCP represents the
majority of current traffic, many real-time applications use UDP, and
much of this traffic utilises RTP format headers in the payload of
the UDP datagram. Since these protocol headers have been fixed for
decades, a range of tools and analysis methods have became common and
well-understood.
Protocols that expose the state information used by the transport
protocol in their header information (e.g., timestamps used to
calculate the RTT, packet numbers used to asses congestion and
requests for retransmission) provide an incentive for the sending
endpoint to provide correct information, since the protocol will not
work otherwise. This increases confidence that the observer
understands the transport interaction with the network. For example,
when TCP is used over an unencrypted network path (i.e., one that
does not use IPsec or other encryption below the transport), it
implicitly exposes header information that can be used for
measurement at any point along the path. This information is
necessary for the protocol's correct operation, therefore there is no
incentive for a TCP or RTP implementation to put incorrect
information in this transport header. A network device can have
confidence that the well-known (and ossified) transport information
represents the actual state of the endpoints.
When encryption is used to conceal some or all of the transport
headers, the transport protocol chooses which information to reveal
to the network about its internal state, what information to leave
encrypted, and what fields to grease to protect against future
ossification. Such a transport could be designed, for example, to
provide summary data regarding its performance, congestion control
state, etc., or to make an explicit measurement signal available.
For example, a QUIC endpoint can optionally set the spin bit to
reflect to explicitly reveal the RTT of an encrypted transport
session to the on-path network devices [I-D.ietf-quic-transport]).
When providing or using such information, it becomes important to
consider the privacy of the user and their incentive for providing
accurate and detailed information. Protocols that selectively reveal
some transport state or measurement signals are choosing to establish
a trust relationship with the network operators. There is no
protocol mechanism that can guarantee that the information provided
represents the actual transport state of the endpoints, since those
endpoints can always send additional information in the encrypted
part of the header, to update or replace whatever they reveal. This
reduces the ability to independently measure and verify that a
protocol is behaving as expected. Some operational uses need the
information to contain sufficient detail to understand, and possibly
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reconstruct, the network traffic pattern for further testing; such
operators need to gain the trust of transport protocol implementers
if they are to correctly reveal such information.
Operations, Administration, and Maintenance (OAM) data records
[I-D.ietf-ippm-ioam-data] could be embedded into a variety of
encapsulation methods at different layers to support the goals of a
specific operational domain. OAM-related metadata can support
functions such as performance evaluation, path-tracing, path
verification information, classification and a diversity of other
uses. When encryption is used to 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).
Some measurements could be completed by utilising a standardised
endpoint-based logging format (e.g., based on Quic-Trace
[Quic-Trace]). 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.
Logs collected at endpoints could be shared (after appropriate
annoymisation) to help understand performance and pathologies.
Measurements 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.
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
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might not have a significant collateral impact on the performance of
other traffic that shares this network segment. Once the proportion
of this traffic increases, 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 could 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 could lead operators and maintainers of middleboxes
(firewalls, etc.) to seek other methods to classify, and potentially
other mechanisms to condition, network traffic.
A lack of data that reduces the level of precision with which flows
can be classified also reduces the design space for conditioning
mechanisms (e.g., rate limiting, circuit breaker techniques
[RFC8084], or blocking of uncharacterised traffic), and this 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
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 could in turn impact operational mechanisms, and
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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.
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
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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
Header encryption and strong integrity checks are being incorporated
into new transport protocols and have important benefits. The pace
of development of transports using the WebRTC data channel, and the
rapid deployment of the QUIC transport protocol, can both be
attributed to using the combination of UDP as a substrate while
providing confidentiality and authentication of the encapsulated
transport headers and payload.
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 (the
"wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational
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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 image) or their
behaviour (e.g., algorithms that are needed to classify and
characterise the protocol), will require new network tooling to be
developed to catch-up with each change. If a protocol changes so
that the currently deployed tools and methods are no longer relevant,
then these tools can not be used to 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 as a result of incorrect
classification or monitoring).
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 might also be useful to various
stakeholders, however this document does not make recommendations
about what information ought to be exposed, to whom it ought to be
observable, or how this will be achieved.
There are trade-offs and implications of increased use of transport
header encryption when designing a protocol. Transport protocol
designers have often ignored the implications of whether the
information in transport 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:
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o Coexistence of Transport Protocols and Configurations: 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,
and on it being possible to detect violations.
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 with understanding complex
feature interactions. An inability to observe transport layer
header information can make it harder to diagnose and explore
interactions between features at different protocol layers, a
side-effect of not allowing a choice of vantage point from which
this information is observed. New approaches will need to be
developed.
o Operational Practice: The network operations community relies on
being able to understand the pattern and requirements of traffic
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
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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 increased use of transport header encryption results
in reduced availability of open data, it could eliminate the
independent self-checks to the standardisation process that have
previously been in place from research and academic contributors
(e.g., the role of the IRTF Internet Congestion Control Research
Group (ICCRG) and research publications in reviewing new transport
mechanisms and assessing the impact of their experimental
deployment).
The design of future transport protocols needs to consider encryption
of their transport headers to satisfy security and privacy concerns.
This choice to encrypt all, or part, of the transport layer protocol
headers needs to also take 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.
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.
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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 costs of
ossifying common headers, versus the potential benefits of exposing
specific information that could be observed along the network path to
provide tools to manage new variants of protocols.
A protocol design that uses header encryption can provide
confidentiality of some or all of the protocol header information.
This prevents an on-path device from knowledge of the header field.
It therefore prevents mechanisms being built that directly rely on
the information or seeks to infer semantics of an exposed header
field. Hiding headers reduces visibility into transport metadata,
and can limit the ability to measure and characterise traffic. It
can also provide privacy benefits in some cases.
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 might not finally be delivered by the
transport layer. This is sometimes known as a "shadowing attack".
An attack can, for example, disrupt receiver processing, trigger loss
and retransmission, or make a receiving endpoint perform unproductive
decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then
restore protocol state).
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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.
This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No 688421,
and the EU Stand ICT Call 4. The opinions expressed and arguments
employed reflect only the authors' view. The European Commission is
not responsible for any use that might be made of that information.
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This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
11. Informative References
[bufferbloat]
Gettys, J. and K. Nichols, "Bufferbloat: dark buffers in
the Internet. Communications of the ACM, 55(1):57-65",
January 2012.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
"Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
data-06 (work in progress), July 2019.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-22 (work
in progress), July 2019.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-19
(work in progress), November 2017.
[I-D.ietf-taps-transport-security]
Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K.
Rose, "A Survey of Transport Security Protocols", draft-
ietf-taps-transport-security-08 (work in progress), August
2019.
[I-D.ietf-tls-grease]
Benjamin, D., "Applying GREASE to TLS Extensibility",
draft-ietf-tls-grease-04 (work in progress), August 2019.
[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.
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[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>.
[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>.
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[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
<https://www.rfc-editor.org/info/rfc3234>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.
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[RFC4995] Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
Header Compression (ROHC) Framework", RFC 4995,
DOI 10.17487/RFC4995, July 2007,
<https://www.rfc-editor.org/info/rfc4995>.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R.
Whitner, "Improved Packet Reordering Metrics", RFC 5236,
DOI 10.17487/RFC5236, June 2008,
<https://www.rfc-editor.org/info/rfc5236>.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
March 2009, <https://www.rfc-editor.org/info/rfc5481>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.
[RFC6294] Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
2011, <https://www.rfc-editor.org/info/rfc6294>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
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[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[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>.
[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>.
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[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>.
[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>.
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[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>.
[RFC8558] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
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Appendix A. Revision information
-00 This is an individual draft for the IETF community.
-01 This draft was a result of walking away from the text for a few
days and then reorganising the content.
-02 This draft fixes textual errors.
-03 This draft follows feedback from people reading this draft.
-04 This adds an additional contributor and includes significant
reworking to ready this for review by the wider IETF community Colin
Perkins joined the author list.
Comments from the community are welcome on the text and
recommendations.
-05 Corrections received and helpful inputs from Mohamed Boucadair.
-06 Updated following comments from Stephen Farrell, and feedback via
email. Added a draft conclusion section to sketch some strawman
scenarios that could emerge.
-07 Updated following comments from Al Morton, Chris Seal, and other
feedback via email.
-08 Updated to address comments sent to the TSVWG mailing list by
Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on
11/05/2018, and Spencer Dawkins.
-09 Updated security considerations.
-10 Updated references, split the Introduction, and added a paragraph
giving some examples of why ossification has been an issue.
-01 This resolved some reference issues. Updated section on
observation by devices on the path.
-02 Comments received from Kyle Rose, Spencer Dawkins and Tom
Herbert. The network-layer information has also been re-organised
after comments at IETF-103.
-03 Added a section on header compression and rewriting of sections
referring to RTP transport. This version contains author editorial
work and removed duplicate section.
-04 Revised following SecDir Review
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o Added some text on TLS story (additional input sought on relevant
considerations).
o Section 2, paragraph 8 - changed to be clearer, in particular,
added "Encryption with secure key distribution prevents"
o Flow label description rewritten based on PS/BCP RFCs.
o Clarify requirements from RFCs concerning the IPv6 flow label and
highlight ways it can be used with encryption. (section 3.1.3)
o Add text on the explicit spin-bit work in the QUIC DT. Added
greasing of spin-bit. (Section 6.1)
o Updated section 6 and added more explanation of impact on
operators.
o Other comments addressed.
-05 Editorial pass and minor corrections noted on TSVWG list.
-06 Updated conclusions and minor corrections. Responded to request
to add OAM discussion to Section 6.1.
-07 Addressed feedback from Ruediger and Thomas.
Section 2 deserved some work to make it easier to read and avoid
repetition. This edit finally gets to this, and eliminates some
duplication. This also moves some of the material from section 2 to
reform a clearer conclusion. The scope remains focussed on the usage
of transport headers and the implications of encryption - not on
proposals for new techniques/specifications to be developed.
-08 Addressed feedback and completed editorial work, including
updating the text referring to RFC7872, in preparation for a WGLC.
-09 Updated following WGLC. In particular, thanks to Joe Touch
(specific comments and commentry on style and tone); Dimitri Tikonov
(editorial); Christian Huitema (various) David Black (various).
Ammended privacy considerations based on SECDIR review. Emile
Stephan (inputs on operations measurement); Various others.
Added summary text and refs to key sections. Note to editors: The
section numbers are hard-linked.
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Authors' Addresses
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
EMail: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
Scotland
EMail: csp@csperkins.org
URI: https://csperkins.org/
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