The Impact of Transport Header Encryption on Operation and Evolution of the Internet
draft-fairhurst-tsvwg-transport-encrypt-01
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| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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| Author | Gorry Fairhurst | ||
| Last updated | 2017-06-02 (Latest revision 2017-05-28) | ||
| Replaced by | draft-ietf-tsvwg-transport-encrypt, draft-ietf-tsvwg-transport-encrypt, RFC 9065 | ||
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draft-fairhurst-tsvwg-transport-encrypt-01
TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational June 02, 2017
Expires: December 02, 2017
The Impact of Transport Header Encryption on Operation and Evolution of
the Internet
draft-fairhurst-tsvwg-transport-encrypt-01
Abstract
This document describes the implications of applying end-to-end
encryption at the transport layer. It identifies some in-network
uses of transport layer header information that can be used with
transport header authentication,. It reviews the implication of
developing encrypted end-to-end transport protocols and examines the
implication of developing and deploying encrypted end-to-end
transport protocols.
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 http://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 December 02, 2017.
Copyright Notice
Copyright (c) 2017 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 (http://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
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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. Internet Transports and Pervasive Encryption . . . . . . . . . 4
2.1. Authenticating the Transport Protocol Header . . . . . . . 5
2.2. Encrypting the Transport Payload . . . . . . . . . . . . . 6
2.3. Encrypting the Transport Header . . . . . . . . . . . . . 6
2.4. Authenticating Transport Information and Selectively
Encrypting the Transport Header . . . . . . . . . . . . . 6
2.5. Adding transport information to network-layer Protocol
Headers . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Use of Transport Headers in the Network . . . . . . . . . . . 7
3.1. Use to Identify Flows and Packet Formats . . . . . . . . . 9
3.2. Measurements derived from Transport Header Information . . 9
3.2.1. Use to Characterise Traffic Rate and Volume . . . . . 9
3.2.2. Use of the Network-Layer DSCP . . . . . . . . . . . . 10
3.2.3. Measuring Loss Rate and Loss Pattern . . . . . . . . . 10
3.2.4. Measuring Throughput and Goodput . . . . . . . . . . . 10
3.2.5. Measuring Latency (Network Transit Delay and Jitter) . 10
3.2.6. Measuring Flow Reordering . . . . . . . . . . . . . . 12
3.3. Measurements derived from Network-Transport Information . 12
3.3.1. Use of IPv6 Network-Layer Flow Label . . . . . . . . . 12
3.3.2. Use Network-Layer Differentiated Services Code Point
Point (DSCP) . . . . . . . . . . . . . . . . . . . . . 12
3.3.3. Use of Explicit Congestion Marking . . . . . . . . . . 13
4. Transport Measurement . . . . . . . . . . . . . . . . . . . . 13
4.1. Point of Measurement . . . . . . . . . . . . . . . . . . . 14
4.2. Use by Operators to Plan and Provision Networks . . . . . 14
4.3. Service Performance Measurement . . . . . . . . . . . . . 15
4.4. Use for Network Diagnostics and Troubleshooting . . . . . 15
4.5. Acceptable Response to Congestion . . . . . . . . . . . . 16
4.5.1. Measuring Compliance of UDP Traffic . . . . . . . . . 16
4.5.2. Measuring Transport to Support Network Operations . . 17
5. Observing Transport Flows with Encrypted Transport Header Fiel 17
5.1. Transport Information at the Network Layer . . . . . . . . 17
5.2. An Observable Transport Flow Identifier . . . . . . . . . 17
5.2.1. A Method to Determine Header Format . . . . . . . . . 17
5.2.2. Use of a Transport as a Substrate . . . . . . . . . . 18
5.2.3. Support for Mobility and Flow Migration . . . . . . . 18
5.2.4. Flow Start and Stop . . . . . . . . . . . . . . . . . 18
5.3. Observable Transport Sequence Number . . . . . . . . . . . 19
5.4. Observable Transport Reception . . . . . . . . . . . . . . 19
5.5. Observable Transport Timestamps . . . . . . . . . . . . . 20
5.6. Observable ECN Transport Feedback Information . . . . . . 20
5.7. Other Observable Transport Fields . . . . . . . . . . . . 21
5.8. Interpretation of Transport Header Fields . . . . . . . . 21
5.9. Requirements for Transport Measurement . . . . . . . . . . 21
6. The Effect of Encrypting Transport Header Fields . . . . . . . 22
6.1. Independent Measurement . . . . . . . . . . . . . . . . . 22
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6.2. Characterising "Unknown" Network Traffic . . . . . . . . . 23
6.3. Accountability and Internet Transport Portocols . . . . . 23
7. Implications on Evolution of the Internet Transport . . . . . 24
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11.1. Normative References . . . . . . . . . . . . . . . . . . 28
11.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Revision information . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document discusses the implications of end-to-end encryption
applied at the transport layer, and examines the impact on transport
protocol design, transport use, and network operations and
management. It also considers some anticipated implications on
transport and application evolution.
The transport layer is the first end-to-end layer in the network
stack. Despite headers having end-to-end meaning, some transport
headers have come to be used in various ways within the Internet. In
response to pervasive monitoring [RFC7624] revelations and the IETF
consensus that "Pervasive Monitoring is an Attack" [RFC7258], efforts
are underway to increase encryption of Internet traffic, which would
prevent visibility of transport headers. This have implications on
how network protocols are designed and used (e.g., [I-D.mm-wg-effect-
encrypt]).
Transport information that is sent without end-to-end authentication
could be modified by "middleboxes" - defined as any intermediary box
performing functions apart from normal, standard functions of an IP
router on the data path between a source host and destination host
[RFC3234]. When transport headers are modified by network devices on
the path, this can change the end-to-end protocol transport protocol
behaviour in a way that may have benefits (e.g., to user performance/
cost) or may hinder (e.g., disrupting application experience).
Whatever the outcome, modification of packets by a middlebox was not
usually intended when the protocol was specified and is usually not
known by the sending or receiving endpoints.
Middleboxes have been deployed for a variety of reasons [RFC3234],
including protocol enhancement, proxies such as Protocol Enhancing
Proxies (PEPs) [RFC3135], TCP acknowledgement (ACK) enhancement
[RFC3449], use by application protocol caches [I-D.mm-wg-effect-
encrypt], application layer gateways [I-D.mm-wg-effect-encrypt], etc.
[I-D.dolson-plus-middlebox-benefits] summarizes some of the functions
provided by such middleboxes, and benefits that may arise when used
in a specific deployment scenarios. Such methods, which involve in-
network modification of transport headers, are not further discussed.
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Transport protocols can be designed to encrypt or authenticate
transport header fields. Authentication methods at the transport
layer can detect any changes to an immutable header field that were
made by a network device along a path. These methods do not require
encryption of the header fields, and hence authenticated fields may
remain visible to network devices. A receiving transport endpoint
can use authentication to avoid accepting modified protocol headers.
This document therefore considers the case where transport header
fields are not altered as a packet traverses the network path.
Authentication methods have also been specified at the network layer,
and this also protects transport header fields. The network layer
itself can carry header fields that are increasingly being used to
help forwarding decisions reflect the need of transport protocols,
such the IPv6 Flow Label [RFC6437], the Differentiated Services Code
Point (DSCP) [RFC2474] and Explicit Congestion Notification (ECN)
[RFC3168].
Encryption methods can hide information from an eavesdropper in the
network. Encryption can also help protect the privacy of a user, by
hiding data relating to user/device identity or location. Neither
authentication nor encryption methods prevent traffic analysis, and
usage needs to reflect that profiling of users and fingerprinting of
behaviour can take place even on encrypted traffic flows.
This document seeks to identify the implications of various
approaches to transport protocol authentication and encryption.
2. Internet Transports and Pervasive Encryption
End-to-end encryption can be applied at various protocol layers. It
can be applied above the transport to encrypt the transport payload.
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 common 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 may 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 middlebox use for particular
paths with particular middleboxes [RFC3135].
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Another perspective stems from increased concerns about privacy and
surveillance . Some Internet users have valued the ability to protect
identity and defend against traffic analysis, and have used methods
such as IPsec ESP and Tor [Tor]. Revelations about the use of
pervasive surveillance [RFC7624] have, to some extent, eroded trust
in the service offered by network operators, and following the
Snowden revelation in the USA in 2013 has led to an increased desire
for people to employ encryption to avoid unwanted "eavesdropping" on
their communications. Whatever the reasons, there are now activities
in the IETF to design new protocols that may include some form of
transport header encryption (e.g., QUIC [I-D.ietf-quic-transport]).
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 to people 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 of transport header
encryption will have implications on the way in which design and
evaluation is performed, and which can in turn impact the direction
of evolution of the TCP/IP stack.
The next subsections briefly review some security design options for
transport protocols.
2.1. Authenticating the Transport Protocol Header
Transport layer header information can be authenticated. An
authentication method protects the integrity of immutable transport
header fields, but can still expose the transport protocol header
information in the clear, allowing in-network devices to observes
these fields. Authentication can not prevent in-network
modification, but can avoid 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 TCP segments,
including the IP pseudo header, TCP header, and TCP data. TCP-AO
protects the transport layer, preventing attacks from disabling the
TCP connection itself. TCP-AO may interact with middleboxes,
depending on their behavior [RFC3234].
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The IPSec Authentication Header (AH) [RFC4302] works at the network
layer and authenticates the IP payload. This therefore also
authenticates all transport headers, and verifies their integrity at
the receiver, preventing in-network modification.
2.2. Encrypting the Transport Payload
The transport layer payload can be encrypted to protect the content
of transport segments. This leaves transport protocol header
information in the clear. The integrity of immutable transport
header fields could be protected by combining this with
authentication methods (Section 2.1).
Examples of encrypting the payload include Transport Layer Security
(TLS) over TCP [RFC5246] [RFC7525] or Datagram TLS (DTLS) over UDP
[RFC6347] [RFC7525].
2.3. Encrypting the Transport Header
The network layer payload could be encrypted (including the entire
transport header and payload). This method does not expose any
transport information to devices in the network, which also prevents
modification along the network path.
The IPSec Encapsulating Security Payload (ESP) [RFC4303] is an
example of encryption at the network layer, it encrypts and
authenticates all transport headers, preventing visibility of the
headers by in-network devices. Some Virtual Private Network (VPN)
methods also encrypt these headers.
2.4. Authenticating Transport Information and Selectively Encrypting
the Transport Header
A transport protocol design can encrypt selected header fields, while
also choosing to authenticate some or all of other fields in the
transport header. This allows only specific transport header fields
to be observable by network devices. End-to end authentication can
prevent an endpoint from undetected modification of the immutable
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. On the one hand,
security work typically employs a design technique that seeks to
expose only what is needed. On the other hand, there may be
performance and operational benefits in exposing selected information
to network tools.
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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 may be authenticated end-to-end across a path.
An example of a method that encrypts some, but not all, transport
information is UDP-in-GRE [RFC8086] when used with GRE encryption.
2.5. Adding transport information to network-layer Protocol Headers
The transport information can be made visible in a network-layer
header. This has the advantage that this information can then be
observed by in-network devices. This has the advantage that a single
header can support all transport protocols, but there may also be
less desirable implications of separating the operation of the
transport protocol from the measurement framework.
Some measurements may be made by adding additional packet headers
carrying operations, administration and management (OAM) information
to packets at the ingress to a maintenance domain (e.g., adding an
Ethernet protocol header with timestamps and sequence number
information using a method such as 802.11ag) and removing the
additional header at the egress of the maintenance domain. This
approach enables some types of measurements, but does not cover the
entire range of measurements described in this document.
Another example of a network-layer approach is the IPv6 Performance
and Diagnostic Metrics (PDM) Destination Option [I-D.ietf-ippm-6man-
pdm-option]. 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. Transmission of the
packets with this option can be impacted by destination-options
processing by network devices. This information could be
authenticated by receiving transport endpoints when the information
is added at the sender and visible at the receiving endpoint,
although methods to do this have not currently been proposed. This
method needs to be explicitly enabled at the sender.
A drawback of using extension headers is that IPv4 network options
are often not supported (or are carried on a slower processing path)
and some IPv6 networks are also known to drop packets that set an
IPv6 header extension. Another disadvantage is that protocols that
separately expose header information do not necessarily have an
advantage to expose the information that is utilised by the protocol
itself, and could manipulate this header information to gain an
advantage from the network.
3. Use of Transport Headers in the Network
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This section identifies ways that observable (non-encrypted)
transport header fields can be used by devices in the network. There
are a number of actors who can benefit from observing this
information. The list of actors who perform measurements include:
o Protocol developers and implementors of TCP/IP stacks;
o Researchers working on new mechanisms or new applications of
existing applications;
o Analysis researching the impact of mechanisms on network equipment
or specific network topologies;
o Staff supporting operation of a network.
One approach is to use active measurement using dedicated tools to
generate and measure test traffic. To test a transport path, such
tools need to be run from an endpoint, and most operators do not have
access to user equipment. There may also be costs associated with
running such tests. (e.g., the implications of bandwidth tests in a
mobile network are obvious.) Some tests (e.g. Response under load or
particular workloads) may 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 in
operational networks to make the measurements.
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 to those of
wired networks. A radio-based network (e.g., cellular mobile,
enterprise WiFi, satellite access/backhaul, 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.
In-network observation of transport protocol headers requires
knowledge of the format of the transport header:
o Flows, identified at the level required for monitoring. In IETF
transports, this is typically identified at the IP level by the
IPv4 protocol field or IPv6 next header field and at the transport
layer by the ports field (or other information);
o The protocol and version of the header that is being used. As
protocols evolve over time and there may be a need to introduce
new transport headers. This may require interpretation of
protocol version information or connection setup information;
o The position and syntax of any transport headers that need to be
observed. IETF transport protocols specify this information.
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The following subsections describe various ways that observable
transport information may be utilised.
3.1. Use to Identify Flows and Packet Formats
Transport protocol header information can identify a flow and the
connection state of the flow, together with the protocol options
being used. In some usages, a low-numbered (well-known ) port that
can identify a protocol (although port information alone is not
sufficient to guarantee identification of a protocol). Transport
protocols, such as TCP and SCTP specify a standard base header that
includes sequence number information and other data, with the
possibility to negotiate additional headers at connection setup and
identified by an option number in the transport header. UDP-based
protocols sometimes do not use well-known ports but also can instead
be identified by signalling protocols or through the use of magic
numbers placed in the first byte(s) of the datagram payload.
3.2. Measurements derived from Transport Header Information
Various actors have a need to characterise the performance of link/
network segments. Passive monitoring uses observed traffic to makes
inferences from transport headers to derive measurements. A variety
of open source and commercial tools can utilise this information.
Transport fields in the Real Time Protocol (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. Key performance indicators are retransmission rate, packet drop
rate, sector utilization level, a measure of reordering, peak rate,
the CE-marking rate, etc. Metadata is often important to understand
the context under which the data was collected, including the time,
observation point, and way in which metrics were accumulated.
Some Internet transports report summary performance data to the
network (e.g., RTCP feedback[RFC3550]). A user of summary measurement
data needs to trust the source of this data and the method used to
generate the summary information.
When encryption conceals information in packet headers, measurements
need to rely on pattern inferences and other heuristics grows, and
accuracy suffers [I-D.mm-wg-effect-encrypt].
3.2.1. Use to Characterise Traffic Rate and Volume
Transport headers may be observed to derive volume measures per-
application, to characterise the traffic using a network segment and
pattern of network usage. This may be measured per endpoint or
aggregate of endpoint (e.g., by an operator to assess subscriber
usage). This can also be used to trigger measurement-based traffic
shaping and to implement QoS support within the network and lower
layers. Volume measures can be valuable for capacity planning
(providing detail of trends rather than the volume per subscriber).
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3.2.2. Use of the Network-Layer DSCP
Applications and transports can expose their delivery expectations to
the network. The Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets allows endpoints to provide explicit
information that can be used in place of inferring traffic
requirements (e.g., instead of using a multi-field classifier to
infer QoS requirements from port information).
3.2.3. Measuring Loss Rate and Loss Pattern
Flow loss rate is often used as a metric for performance assessment
and to characterise the transport behaviour. There are various cause
of loss, including: corruption on a link (e.g., interference on a
radio link), buffer overflow (e.g., due to congestion), policing
(traffic management), buffer management (e.g. Active Queue
Management (AQM). Loss can be monitored at the interface level by
devices in the network. It is often important to understand the
conditions under which packet loss occurs, which usually means
relating loss to the traffic flowing on the network segment at the
time of loss. Understanding flow loss rate requires either
maintaining per flow packet counters or by observing sequence numbers
in transport headers.
Observation of transport feedback information (observing loss
reports, e.g. RTCP, TCP SACK) can also increase understanding of the
impact of loss and help identify cases where loss may have been
wrongly identified, or the transport did not require the lost packet.
It is sometimes more important to understand the pattern of loss,
than the loss rate - since losses can often occur as bursts, rather
than randomly timed events. Understanding the root cause of loss can
help an operator determine whether this requires corrective action.
3.2.4. Measuring Throughput and Goodput
The throughput observed by a flow can be determined even when a flow
is encrypted, providing the individual flow can be identified.
Goodput [RFC7928] is a measure of useful data exchanged (the ratio of
useful/total volume of traffic sent by a flow), which requires
ability to differentiate loss and retransmission of packets (e.g., by
observing packet sequence numbers in TCP or RTP).
3.2.5. Measuring Latency (Network Transit Delay and Jitter)
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Latency is a key performance metric that impacts application response
time and user-perceived response time. It also often indirectly
impacts throughput and flow completion time. Latency determines the
reaction time of the transport protocol itself, impacting flow setup,
congestion control, loss recovery, and other transport mechanisms.
The observed latency can have many components [Latency]. Of these,
unnecessary/unwanted queuing in network buffers has often been
observed as a significant factor. Once the cause of unwanted latency
has been identified, this can often be eliminated, and determining
latency metrics is a key driver in the deployment of AQM [RFC7567],
DiffServ [RFC2474], and ECN [RFC3168] [RFC8087].
To measure latency across a part of the path, an observation point
can measure the experienced round trip time (RTT) using packet
sequence numbers, and acknowledgements, or by observing header
timestamp information. Such information allows an observation point
in the network to determine not only the path RTT, but also to
measure the upstream and downstream contribution to the RTT. This may
be used to locate a source of latency, e.g., by observing cases where
the ratio of median to minimum RTT is large for a part of a path.
An example usage of this method could be to identify excessive
buffers and to deploy or configure Active Queue Management (AQM)
[RFC7567] [RFC7928]. Operators deploying such tools can effectively
eliminate unnecessary queuing in routers and other devices. AQM
methods need to be deployed at the capacity bottleneck, but are often
deployed in combination with other techniques, such as scheduling
[RFC7567] [I-D.ietf-aqm-fq-codel] and although parameter-less methods
are desired [RFC7567], current methods [I-D.ietf-aqm-fq-codel] [I-D
.ietf-aqm-codel] [I-D.ietf-aqm-pie] often cannot scale across all
possible deployment scenarios. The service offered by operators can
therefore benefit from latency information to understand the impact
of deployment and tune deployed services.
Some network applications are sensitive to packet jitter, and it can
be necessary to measure the jitter observed along a portion of the
path. The requirements to measure jitter resemble those for the
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measurement of latency.
3.2.6. Measuring Flow Reordering
Significant flow reordering 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 rebuffering of real-time applications). Packet
reordering can occur for many reasons (from equipment design to
misconfiguration of forwarding rules).
As in the drive to reduce network latency, there is a need for
operational tools to be able to detect misordered packet flows and
quantify the degree or reordering. Techniques for measuring
reordering typically observe packet sequence numbers. Metrics have
been defined that evaluate whether a network has maintained packet
order on a packet-by-packet basis [RFC4737] and [RFC5236].
There has been initiatives in the IETF transport area to reduce the
impact of reordering withing a transport flow, possibly leading to
reduced the requirements for ordering. These have promise to
simplify network equipment design as well as the potential to improve
robustness of the transport service. Measurements of reordering can
help understand the level of reordering within deployed
infrastructure, and inform decisions about how to progress such
mechanisms.
3.3. Measurements derived from Network-Transport Information
This section describes transport information that is already
observable in network-layer header fields.
3.3.1. Use of IPv6 Network-Layer Flow Label
Endpoints should expose flow information in the IPv6 Flow Label field
of the network-layer header (e..g. [RFC8085]). This can be used to
inform network-layer queuing, forwarding (e.g., for equal cost multi-
path (ECMP) routing, and Link Aggregation (LAG)). This can provide
useful information to assign packets to flows in the data collected
by measurement campaigns. Although important to characterising a
path, it does not directly provide any performance data.
3.3.2. Use Network-Layer Differentiated Services Code Point Point
(DSCP)
Application should expose their delivery expectations to the network,
by setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets. This can be used to inform network-layer
queuing and forwarding, and can also provide information on the
relative importance of packet information collected by measurement
campaigns, but does not directly provide any performance data.
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This field provides explicit information that can be used in place of
inferring traffic requirements (e.g., by inferring QoS requirements
from port information via a multi-field classifier). The DSCP value
can therefore 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.
3.3.3. Use of Explicit Congestion Marking
Explicit Congestion Notification (ECN)[RFC3168] uses a codepoint in
the network-layer header. This exposes the presence of congestion on
a network path to the transport and network layer. Use of ECN can
offer gains in terms of increased throughput, reduced delay, and
other benefits when used over a path that includes equipment that
supports an AQM method that performs Congestion Experienced (CE)
marking of IP packets [RFC8087].
ECN CE-marks are visible to in-network devices on the transport path.
The reception of CE-marked packets can therefore be used to monitor
the presence and estimate the level of incipient congestion on the
upstream portion of the path from the point of observation (Section
2.5 of [RFC8087]). Because ECN marks carried in the IP protocol
header, measuring ECN can be much easier than metering packet loss.
However, interpreting the marking behaviour (i.e., assessing
congestion and diagnosing faults) requires context from the transport
layer (path RTT, visibility of loss - that could be due to queue
overflow, congestion response, etc) [RFC7567].
Some ECN-capable network devices can provide richer (more frequent
and fine-grained) indication of their congestion state. Setting
congestion marks proportional to the level of congestion (e.g., DCTP
[I-D.ietf-tcpm-dctcp], and L4S [I-D.ietf-tsvwg-l4s-arch]).
AQM and ECN offer a range of algorithms and configuration options, it
is therefore important for tools to be available to network operators
and researchers to understand the implication of configuration
choices and transport behaviour as use of ECN increases and new
methods emerge [RFC7567] [RFC8087]. ECN-monitoring is expected to
become important as AQM is deployed that supports ECN [RFC8087]
Section 5.6 describes the transport layer feedback information that
accompanies the use of ECN.
4. Transport Measurement
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The common language between network operators and application/content
providers/users is packet transfer performance at a layer that all
can view and analyze. For most packets, this has been transport
layer, until the emergence of QUIC, with the obvious exception of
VPNs and IPsec. When encryption conceals more layers in a packet,
people seeking understanding of the network operation need to rely
more on pattern inferences and other heuristics. The accuracy of
measurements therefore suffers, as does the ability to investigate
and troubleshoot interactions between different anomalies. For
example, the traffic patterns between server and browser are
dependent on browser supplier and version, even when the sessions use
the same server application (e.g., web e-mail access). Even when
measurement datasets are made available (e.g., from endpoints)
additional metadata, such as the state of the network, is often
required to interpret the data. Collecting and coordinating such
metadata is more difficult when the observation point is at a
different location to the bottleneck/device under evaluation.
Packet sampling techniques can be used to scale processing involved
in observing packets on high rate links. This only exports 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 transport measurement.
4.1. Point of Measurement
Often measurements can only be understood in the context of the other
flows that share a bottleneck. A simple example is the monitoring of
AQM. For example, FQ-CODEL [I-D.ietf-aqm-fq-codel], combines sub
queues (statistically assigned per flow), management of the queue
length (CODEL), flow-scheduling, and a starvation prevention
mechanism. Usually such algorithms are designed to be self-tuning,
but current methods typically employ heuristics that can result in
more loss under certain path conditions (e.g., large RTT, effects of
multiple bottlenecks [RFC7567]).
In-network measurements that can distinguish between upstream and
downstream metrics with respect to the measurement point. They are
particularly useful to locating the source of problems or to asses
the performance of a network segment and or particular device
configuration.
4.2. Use by Operators to Plan and Provision Networks
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Traffic measurements (e.g. Traffic volume, loss, latency) is used by
operators to help plan deployment of new equipment and configurations
in their networks. Data is also important to equipment vendors who
need to understand traffic trends traffic 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 may not have access to per-flow measurement data. Trends
in aggregate traffic can be observed and can be related this to the
endpoint addresses being used, but it may not be possible 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.
4.3. Service Performance Measurement
Traffic measurements (e.g., traffic volume, loss, latency) can be
used by various actors to help understand the performance available
to users of a network segment. While active measurements may be used
in-network passive measurements can have advantages in terms of
eliminating unproductive traffic, reducing the influence of test
traffic on the overall traffic mix, and the ability to choose the
point of measurement Section 4.1.
4.4. Use for Network Diagnostics and Troubleshooting
Transport header information is useful for a variety of operational
tasks [I-D.mm-wg-effect-encrypt]: to diagnose network problems,
assess performance, capacity planning, management of denial of
service threats, and responding to user performance questions. These
tasks seldom involve the need to determine the contents of the
transport payload, or other application details.
A network operator supporting traffic that uses transport header
encryption can see only encrypted transport headers. This prevents
deployment of performance measurement tools that rely on transport
protocol information. Choosing to encrypt all information may be
expected to reduce the ability for networks to "help" (e.g., in
response to tracing issues, making appropriate Quality of Service,
QoS, decisions). For some this will be blessing, for others it may be
a curse. For example, operational performance data about encrypted
flows needs to be determined by traffic pattern analysis, rather than
relying on traditional tools. This can impact the ability of the
operator to respond to faults, it could require reliance on endpoint
diagnostic tools or user involvement in diagnosing and
troubleshooting unusual use cases or non-trivial problems. Although
many network operators utilise transport information as a part of
their operational practice, the network will not break because
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transport headers are encrypted.
4.5. Acceptable Response to Congestion
Congestion control is a key transport function. Many network
operators implicitly accept that TCP traffic to comply with a
behaviour that is acceptable for use in the shared Internet. TCP
algorithms have been continuously improved over decades, and they
have reached a level of efficiency and correctness that custom
application-layer mechanisms will struggle to easily duplicate
[RFC8085]. A standards-compliant TCP stack provides congestion
control that is therefore 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 TCP
and other IETF-defined transports.
Tools exist that can interpret the transport protocol header
information to help understand the impact of specific transport
protocols (or protocol mechanisms) on other traffic that shares their
network. An observation in the network can gain understanding of the
dynamics of a flow and its congestion control behaviour. Analysing
observed packet sequence numbers can be used to help 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.
This has implications:
4.5.1. Measuring Compliance of UDP Traffic
The User Datagram Protocol (UDP) provides a minimal message-passing
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
an Internet transport must employ mechanisms to prevent congestion
collapse, avoid unacceptable contributions to jitter/latency, and to
establish an acceptable share of capacity with concurrent traffic
[RFC8085]. A network operator has no way of knowing the specific
methods used by a UDP application.
An operator needs tools to understand if such flows comply with their
congestion control expectations and therefore whether they need to
deploy methods such as rate-limited, transport circuit breakers or
other methods to enforce acceptable usage. UDP flows that expose a
well-known header by specifying the format of header fields. This
information can be observed to gain understanding of the dynamics of
a flow and its congestion control behaviour. For example, tools
exist to monitor various aspects of the RTP and RTCP header
information of real-time flows (see Section 3.2).
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4.5.2. Measuring Transport to Support Network Operations
By correlating observations 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).
Information provided by tools can help determine whether mechanisms
are needed in the network to prevent flows from acquiring excessive
network capacity. Operators can manage traffic flows (e.g., to
prevent flows from acquiring excessive network capacity under severe
congestion) by deploying rate-limiters, traffic shaping or network
transport circuit breakers [RFC8084].
5. Observing Transport Flows with Encrypted Transport Header Fields
This section examines implications of encrypting specific transport
header information.
5.1. Transport Information at the Network Layer
Some transport information is made visible in the network-layer
protocol header. These header fields are not encrypted and can be
used to make flow observations. Endpoints should expose flow
information in the IPv6 Flow Label Section 3.3.1 in the network-layer
header. This can be used to inform network-layer queuing, forwarding
(e.g., for equal cost multi-path (ECMP) routing, and Link Aggregation
(LAG)). For transport measurement, this can provide useful
information to assign packets to flows in the data collected by
measurement campaigns, but does not directly provide any performance
data. Similarly the Differentiated Services CodePoint (DCSP)
indicates expected forwarding treatment Section 3.3.2. The ECN field
provides observable congestion data and can help inform measurement
of flow congestion Section 3.3.3.
5.2. An Observable Transport Flow Identifier
To measure and analyse a transport protocol, a measurement tool needs
to be able to identify traffic flows. Aggregation of sessions, and
persistent use of established transport flows by multiple sessions
means that a flow at the transport layer is not necessarily the same
as a flow seen at the application layer. This is usually not a
consequence. Data is measured for the aggregate transport flow.
Some measurement methods sample traffic, rather than collecting all
packets passing through a measurement point. These methods still
require a way to determine the presence, size and position of any
observable header fields - but may need to do this without observing
a protocol exchange for a connection setup.
5.2.1. A Method to Determine Header Format
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If flow information is observed from transport headers, then there
needs to be a way to identify the format of the header Section 3.1.
Some IETF transport protocols are identified by an IP protocol number
(e.g. ,TCP, SCTP, UDP). All IETF-defined transport protocols include
a transport port field in their transport header. Higher layer
protocols (e.g., HTTP) can be sometimes be observed by a well-known
port value, which can be indicative of the protocol being
encapsulated, but there is no way to enforce this usage. This can be
used to configure decapsulation, alternatives include a "magic"
number placed at the start of each UDP datagram.
Once the protocol has been determined, the transport header can be
determined from a published specification. If multiple formats are
permitted, this may also require observing the protocol version being
used and possibly parameter negotiation at connection setup.
5.2.2. Use of a Transport as a Substrate
When a transport is used as a substrate, the transport provides an
encapsulation that allows another transport flow to be within the
payload of a transport flow. The transported protocol header may
provide additional information for multiplexing multiple flows over
the same 5-tuple. The UDP Guidelines [RFC8085] provides some
guidance on using UDP as a substrate protocol. If there is no
additional information about the protocol transported by the
substrate, this may be viewed as an opaque traffic aggregate, and
prevents transport measurement in the network. Examples include GRE-
in-UDP [RFC8086], SCTP-in-UDP. The GRE-in-UDP encapsulation may
include an encrypted payload, but does not encrypt the GRE protocol
header.
5.2.3. Support for Mobility and Flow Migration
With the proliferation of mobile connected devices, there is a stated
need for connection-oriented protocols to maintain connections after
a network migration by an endpoint. The ability and desirability of
in-network devices to track such migration depends on the context.
On the one hand, a load-balancer device in front of server may find
it useful to map a migrated connection to the same server endpoint.
On the other hand, a user performing migration to avoid detection may
prefer the network not to be able to correlate the different parts of
a migrating session. Care must then be exercised to make sure that
the information encoded by the endpoints is not sufficient to
identify unique flows and facilitate a persistent surveillance attack
vector [I-D.mm-wg-effect-encrypt].
The impact of flow migration on measurement activities depends on the
data being measured, rate of migration and level of encryption that
is employed. Requirements for load balancing and mobility can lead
to complex protocol interactions.
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5.2.4. Flow Start and Stop
Transports can expose that start and end of flows in a transport
header field (e.g., TCP SYN, FIN, RST). This can also help
measurement devices identify the start of flows, or to remove stale
flow information. This information is supplemental - flows can start
and end at any time, the Internet network layer provides only a best
effort service that allows alternate routing, reordering, loss, etc,
so a network measurement tool can not rely upon observing these
indicators. The time to complete a protocol connection and/or
session setup can be reported.
Flow information can provide in-network devices to manage their
forwarding state [I-D.trammell-plus-statefulness]. It can assist a
firewall in deciding which flows are permitted through a security
gateway, or to help maintain the network address translation (NAT)
bindings in a NAPT or application layer gateway. This information
may also find use in load balancers, where visibility of the 5-tuple
could assist in selecting a server [I-D.mm-wg-effect-encrypt].
Access to flow information and an observable start/stop indication
[I-D.trammell-plus-statefulness] can avoid UDP middleboxes relying on
timeouts to remove old state. Without this middleboxes are unaware
when a particular flow ceases to be used by an application [RFC8085].
This can lead to the state table entries not keeping state for less
time for flows that are not identifiable.
5.3. Observable Transport Sequence Number
The TCP or RTP sequence number can be observed in one direction (the
direction that carries data segments). An authenticated header
prevents this field being modified or terminated/split [RFC3135] by a
network device, but allows this still to be used to observe progress
of the network flow.
An incrementing sequence number enables detection of loss (either by
correlating ingress and egress value, or when assuming that all
packets follow a single path), duplication and reordering (with
understanding that not necessarily all packets of a flow follow the
same path, and reordering can complicate processing of observations).
Tools are widely available to interpret RTP and TCP sequence numbers,
ranging from open source tools to dedicated commercial packages. As
for TCP, use by in-network measurement devices needs to account for
the impact of load-balancing of flows, changes in forwarding
behaviour, measurement loss (rather than observed packet loss), etc.
5.4. Observable Transport Reception
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Acknowledgement (ACK) data provides information about the path from
the network device to the remote endpoint. The information can help
identify packet loss (or the point of loss), RTT, and other network-
related performance parameters (e.g., throughput, jitter,
reordering). Unless this information is correlated with other data
there is no way to disambiguate the cause of impairments (congestion
loss, link transmission loss, equipment failure).
An in-network device must not modify the flow of end-to-end ACK data
when using an authenticated protocol. That is, must not use the in-
network methods described in [RFC3449]. This can impact the
performance and/or efficiency (e.g., cost) of using paths where the
return capacity is limited or has implications on the overall design
(e.g., using TCP with cellular mobile uplinks, DOCSIS uplinks).
The TCP stream can be observed by correlating the stream of TCP ACKs
that flow from a receiver in the return direction. Although these
ACKs are cumulative, and are not necessarily sent on the same path as
the forward data, when visible, their sequence can confirm successful
transmission and the path RTT. In the case of TCP they may also
indicate packet loss (duplicate ACKs).
An RTP session can provide reception information [RFC3550] [RFC4585]
feedback using the RTCP framework. This reception information and
can be observed by in-network measurement devices and can be
interpreted to provide a variety of quality of experience information
for the related RTP flow, as well as basic network performance data
(RTT, loss, jitter, etc).
5.5. Observable Transport Timestamps
The use of timestamps for latency and jitter measurements Section
3.2.5 is discussed in other sections of the current version of the
document.
5.6. Observable ECN Transport Feedback Information
Transport protocols that use ECN Section 3.3.3 need to provide ECN
feedback information in the transport header to inform the sender
whether packets have been received with an ECN CE-mark [RFC3819].
This information can be in the form of feedback once each RTT
[RFC3819] or more frequent. The latter may involve sending a
detailed list of all ECN-marked packets (e.g., [I-D.ietf-tcpm-
accurate-ecn] and [RFC6679]). The detailed information can provide
detail about the pattern and rate of marking. The information
provided in these protocol headers can help a network operator to
understand the congestion status of the forward path and the impact
of marking algorithms on the traffic that is carried [RFC8087].
IETF specifications for Congestion Exposure (CONVEX) [RFC7713] and
Per-congestion Notification (PCB) [RFC5559] are examples of
frameworks that monitor reception reports for CE-marked packets to
support network operations.
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5.7. Other Observable Transport Fields
This section is not complete - later revision may determine other
fields or remove this section.
5.8. Interpretation of Transport Header Fields
Understanding and analysing transport protocol behaviour typically
demands tracking changes to the protocol state at the transport
endpoints. Although protocols communicate state information in their
protocol headers, a protocol implementation typically also contains
internal state that is not directly visible from observing transport
protocol headers. Effective measurement tools need to consider that
not all packets may be observed (due to drops at the capture tap or
because packets take an alternate route that does not pass the tap).
Some flows of packets may also be encapsulated within a maintenance
domain in other protocols, which further complicates analysis.
Some examples of using network measurements of transport headers to
infer internal TCP transport state information include:
o The TCP congestion window (cwnd) and slow start threshold
(ssthresh). Tools for analysing in-network performance of TCP may
observe sequence number to infer the current congestion controller
state.
o The TCP RTT estimator and TCP Retransmission Time Out (RTO) value.
This can be estimated by correlating sequence and acknowledgement
numbers, or possibly by observing TCP timestamp options.
o Use of pacing (and pacing rate) and use of methods such as
Proportional Rate Reduction (PRR) and Congestion Window validation
(CWV). This may be estimated from observing timing of segments
with TCP sequence numbers. This is important to some congestion
control mechanisms and can be important for applications that are
rate limited or send traffic bursts.
o Receiver window and flow control state. This may be inferred from
information in TCP ACK segments. It is important to applications
where the remote endpoint is resource constrained, or the path
exhibits a large RTT.
o Retransmission state and receiver buffer. This may be inferred
from information in TCP ACK segments (especially when SACK blocks
are provided), this can be important to the performance of
applications that send traffic bursts.
o Use of ACK delay and Nagle algorithm. This may be estimated from
observing timing of segments with TCP sequence numbers, and is
important to the performance of thin application flows.
5.9. Requirements for Transport Measurement
Transport measurement introduces the following requirements to
identify the observable information:
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o Observable protocol type and version information is needed to
identify the protocol being used when characterising the traffic,
and to enable further observation of the flow.
o Observable format information is needed to allow an observer to
determine the presence of any observable header fields.
o A published specification is needed to allow an observer to
determine the size and position of any observable header fields so
that these fields may be decoded by a measurement tool.
o Observable flow start/stop information can assist some forms of
measurement and has utility for middleboxes that track state.
The need for in-network transport measurement introduces the
following requirements for observable information in transport header
fields:
o Observable transport information to determine the progress of
flows for each direction of communication. This requires
observable packet numbers.
o Observable transport information to determine loss, and understand
the response to congestion for a network segment. This requires
observable reception information (e.g., packet acknowledgment
information).
o Observable transport information is needed for more advanced
measurement of latency, jitter, etc. This requires an observable
field and a method to correlating return information with the
observed field. This could utilise a packet number and/or
transmission timestamp information. This information needs to be
available in both directions of transmission.
o Exposure of Transport ECN feedback provides a powerful tool to
understand ECN-enabled AQM-based networks. (Forward ECN
information is already observable in the network header).
6. The Effect of Encrypting Transport Header Fields
This section explores key implications of working with encrypted
transport protocols.
6.1. Independent Measurement
Independent observation by multiple actors is important for
scientific analysis. Encrypting transport header encryption changes
the ability for other actors 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 - loss
detection and recovery, congestion detection and congestion control,
some of these need to work only end-to-end (e.g., parameter
negotiation, flow-control.
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When encryption conceals information in the transport header, it
could be possible for an applications to provide summary data on
performance and usage of the network. This data could be made
available to other actors. However, this data needs to contain
sufficient detail to understand (and possibly reconstruct the network
traffic pattern for further testing) and to be correlated with the
configuration of the network paths being measured. Sharing
information between actors needs also to consider the privacy of the
user and the incentives for providing accurate and detailed
information. Protocols that expose the state information used by the
transport protocol in their header information (e.g., timestamps used
to calculate RTT, packet numbers used to asses congestion and
requests for retransmission) provide an incentive for the sending
endpoint to provide correct information, increasing confidence that
the observer understands the transport interaction with the network.
This becomes important when considering changes to transport
protocols, changes in network infrastructure, or the emergence of new
traffic patterns.
6.2. Characterising "Unknown" Network Traffic
If "unknown" or "uncharacterised" traffic patterns form a small part
of the traffic aggregate passing through a network device or segment
of the network the path, the dynamics of the uncharacterised traffic
may not have a significant collateral impact on the performance of
other traffic that shares this network segment. Once the proportion
of this traffic increases, the need to monitor the traffic and
determine if appropriate safety measures need to be put in place.
Tracking the impact of new mechanisms and protocols requires traffic
volume to be measured and new transport behaviours to be identified.
This is especially true of protocols operating over a UDP substrate.
The level and style of encryption needs to be considered in
determining how this activity is performed. On a shorter timescale,
information may also need to be collected to manage denial of service
attacks against the infrastructure.
6.3. Accountability and Internet Transport Portocols
Attention therefore needs to be paid to the expected scale of
deployment of new protocols and protocol mechanisms. Whatever the
mechanism, experience has shown that it is often difficult to
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correctly implement combination of mechanisms [RFC8085]. These
mechanisms therefore typically evolve as a protocol matures, or in
response to changes in network conditions, changes in network traffic
or changes to application usage.
The growth and diversity of applications and protocols using the
Internet continues to expand - and 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 [I-D.ietf-tcpm-dctcp], and methods
proposed for Low Latency Low Loss Scalable throughput, L4S). For
each new method 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.
Measurement therefore has a critical role in the design of transport
protocol mechanisms and their acceptance by the wider community
(e.g., as a method to judge the safety for Internet deployment. Open
standards suggest that such evaluation needs to include independent
observation and evaluation of performance data.
7. Implications on Evolution of the Internet Transport
The transport layer provides the first end-to-end interactions across
the Internet. Transport protocols are layered directly over the
network service and are sent in the payload of network-layer packets.
However, this simple architectural view hides one of the core
functions of the transport - to discover and adapt to the properties
of 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 measurement and insight provided from
the wider community to understand the trade-offs and to inform
selection of select appropriate mechanisms to ensure a safe, reliable
and robust Internet since the 1990's.
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There are many motivations for deploying encrypted transports, and
encryption of transport payloads. The increasing public concerns
about the interference with Internet traffic have led to a rapidly
expanding deployment of encryption to protect end-user privacy, in
protocols like QUIC. At the same time, network operators and access
providers, especially in mobile networks, have come to rely on the
in-network functionality provided by middleboxes both to enhance
performance and support network operations. This document has
expanded upon the expected implications on operational practices when
working with encrypted transport protocols, and offers insight into
the potential benefit of authentication, encryption and techniques
that require in-network devices to interpret specific protocol header
fields. It presents a need for architectural changes and
consideration of approaches to the way network transport protocols
are designed when using encryption[Measure].
The use of encryption at the transport layer comes with implications
that need to be considered:
Troubleshooting and diagnostics. Encrypting all transport information
eliminates the incentive for operators to troubleshoot what they
cannot interpret: one flow experiencing packet loss looks like any
other. When transport header encryption prevents decoding the
transport header (if sequence numbers and flow ID are obscured),
and hence understanding the impact on a particular flow or flows
that share a common network segment. Encrypted traffic therefore
implies "don't touch", and a likely first response will be "can't
help, no trouble found", or the implication that this complexity
comes with an additional operational cost [I-D.mm-wg-effect-
encrypt].
Open verifyable data The use of transport header encryption may
reduce the range of actors who can capture useful measurement
data. This may in future restrict the information sources
available to the Internet community to understand the operation of
the network and transport protocols, necessary to inform
standardisation and design decisions for new protocols, equipment
and operational practices. There are dangers in a model where
transport information is only observable at endpoints: i.e., at
user devices and within service platforms and a need for
independently captured data to develop open standards and
stimulate research into new methods.
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Operational practice Published transport specifications allow
operators to check compliance. This can bring assurance to those
operating networks, often avoiding the need to deploy complex
techniques that routinely monitor and manage TCP/IP traffic flows
(e.g. Avoiding the capital and operational costs of deploying
flow rate-limiting and network circuit-breaker methods). This
should continue when encrypted transport headers are used, but
methods need to confirm that the traffic produced conforms to the
expectations of the operator or developer.
Traffic analysis The use of encryption could make it harder to
determine which transport methods are being used across a network
segment and the trends in usage. This could impact the ability
for an operator to anticipate the need for network upgrades and
roll-out. It can also impact on-going traffic engineering
activities. Although the impact in many case may be small, there
are cases where operators directly support services (e.g., in
radio environments, or to troubleshoot QoS-related issues) and the
more complex the underlying infrastructure the more important this
impact.
Interactions between mechanisms An appropriate vantage point, coupled
with timing information for the flow (fine-grained timestamps) is
a valuable tool in benchmarking equipment/configurations and
understanding non-trivial interactions. Encryption restricts the
ability to explore interactions between functions at different
protocol layers. This is a side-effect of not allowing a choice
of the vantage point from which this information is observed.
This can be important (e.g., in examining collateral impact of
flows sharing a bottleneck, or where the intention is to
understand the interaction between a layer 2 function (e.g., radio
resource management policy, a channel impairment, an AQM
configuration, a Per Hop Behaviour (PHB) or scheduling method) and
a transport protocol).
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Common specifications Since the introduction of congestion control,
TCP has continued to be the predominate transport, with a
consistent approach to avoiding congestion collapse. There is a
risk that the diversity of transport mechanisms could also
increase, with incentives to use a wide range of methods, this is
not in itself a problem, nor is this a direct result of
encryption. Encryption of all headers places the onus on
validation in the hands of developers. While there is little to
doubt that developers will seek to produce high quality code for
their target use, it is not clear whether there is sufficient
incentive to ensure good practice that benefits the wide diversity
of requirements from the Internet community as a whole. However,
when encryption is used, this risk needs to be weighed against the
reduced visibility of the interactions between traffic, the
network and the mechanisms. Especially, if a development cycle
focused on specific protocols/applications could offer incentives
for optimisations that could prove suboptimal for users or
operators that utilise a network segments with different
characteristics than targeted by the developer.
Restricting research and development The use of encryption may impede
independent research into new mechanisms, measurement of
behaviour, and development initiatives. Experience shows that
high quality transport protocols are complicated to design and
complex to deploy, and that individual mechanisms need to be
evaluated while considering other mechanism, across a broad range
of network topologies and with attention to the impact on traffic
sharing the capacity. Adopting pervasive encryption of transport
information could eliminate the independent self-checks that have
previously been in place from research and academic contributors
(e.g., the role of the IRTF ICCRG, and research publications in
reviewing new transport mechanisms and assessing the impact of
their experimental deployment).
Pervasive use of transport header encryption can impact the ways that
future protocols are designed and deployed. The choice of whether
candidate transport designs should encrypt their protocol headers
therefore needs to be taken based not just on security
considerations, but also on the impact on operating networks and the
constrictions this may place on evolution of Internet protocols.
While encryption of all transport information can help reduce
ossification of the transport layer, it could result in ossification
of the network service. There can be advantages in providing a level
of ossification of the header in terms of providing a set of open
specified header fields that are observable from in-network devices.
8. Acknowledgements
The author would like to thank all who have talked to him face-to-
face or via email. ...
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This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No 688421.The
opinions expressed and arguments employed reflect only the authors'
view. The European Commission is not responsible for any use that
may be made of that information.
9. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
10. Security Considerations
This document is about design and deployment considerations for
transport protocols. 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; no current full-featured standards-track transport protocol
provides these features on its own. Therefore, these features are
not considered in this document, with the exception of native
authentication capabilities of TCP and SCTP for which the security
considerations in RFC4895.
Like congestion control mechanisms, security mechanisms are difficult
to design and implement correctly. It is hence recommended that
applications employ well-known standard security mechanisms such as
DTLS, TLS or IPsec, rather than inventing their own.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997, <http://www.rfc-editor.org/info/
rfc2119>.
11.2. Informative References
[I-D.dolson-plus-middlebox-benefits]
Dolson, D., Snellman, J., Boucadair, M. and C. Jacquenet,
"Beneficial Functions of Middleboxes", Internet-Draft
draft-dolson-plus-middlebox-benefits-03, March 2017.
[I-D.ietf-aqm-codel]
Nichols, K., Jacobson, V., McGregor, A. and J. Jana,
"Controlled Delay Active Queue Management", Internet-Draft
draft-ietf-aqm-codel-00, October 2014.
[I-D.ietf-aqm-fq-codel]
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Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J. and E. Dumazet, "FlowQueue-Codel", Internet-Draft
draft-ietf-aqm-fq-codel-00, January 2015.
[I-D.ietf-aqm-pie]
Pan, R., Natarajan, P., Baker, F. and G. White, "PIE: A
Lightweight Control Scheme To Address the Bufferbloat
Problem", Internet-Draft draft-ietf-aqm-pie-00, October
2014.
[I-D.ietf-ippm-6man-pdm-option]
Elkins, N., Hamilton, R. and m. mackermann@bcbsm.com,
"IPv6 Performance and Diagnostic Metrics (PDM) Destination
Option", Internet-Draft draft-ietf-ippm-6man-pdm-
option-10, May 2017.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Internet-Draft draft-ietf-quic-
transport-03, May 2017.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M. and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-
tcpm-accurate-ecn-00, December 2015.
[I-D.ietf-tcpm-dctcp]
Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.
and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
Control for Datacenters", Internet-Draft draft-ietf-tcpm-
dctcp-06, May 2017.
[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency,
Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture", Internet-Draft draft-ietf-tsvwg-l4s-
arch-00, May 2017.
[I-D.mm-wg-effect-encrypt]
Moriarty, K. and A. Morton, "Effect of Pervasive
Encryption on Operators", Internet-Draft draft-mm-wg-
effect-encrypt-11, April 2017.
[I-D.trammell-plus-abstract-mech]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", Internet-Draft draft-
trammell-plus-abstract-mech-00, September 2016.
[I-D.trammell-plus-statefulness]
Kuehlewind, M., Trammell, B. and J. Hildebrand,
"Transport-Independent Path Layer State Management",
Internet-Draft draft-trammell-plus-statefulness-02,
December 2016.
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[Latency] Briscoe, B., "Reducing Internet Latency: A Survey of
Techniques and Their Merits", November 2014.
[Measure] Fairhurst, G., Kuehlewind, M. and D. Lopez, "Measurement-
based Protocol Design", June 2017.
[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, <http://www.rfc-
editor.org/info/rfc2474>.
[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, <http://www.rfc-editor.org/
info/rfc3135>.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
DOI 10.17487/RFC3168, September 2001, <http://www.rfc-
editor.org/info/rfc3168>.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
<http://www.rfc-editor.org/info/rfc3234>.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G. and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <http://www.rfc-editor.org/info/rfc3449>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004, <http://www
.rfc-editor.org/info/rfc3819>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, DOI
10.17487/RFC4302, December 2005, <http://www.rfc-
editor.org/info/rfc4302>.
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[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, DOI 10.17487/RFC4303, December 2005, <http://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, <http://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, <http://www.rfc-
editor.org/info/rfc4737>.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A. and R.
Whitner, "Improved Packet Reordering Metrics", RFC 5236,
DOI 10.17487/RFC5236, June 2008, <http://www.rfc-
editor.org/info/rfc5236>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008, <http://www.rfc-editor.org/info/
rfc5246>.
[RFC5559] Eardley, P., Ed., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, DOI 10.17487/RFC5559, June 2009,
<http://www.rfc-editor.org/info/rfc5559>.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <http://www.rfc-editor.org/info/rfc5925>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://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, <http://www.rfc-editor.org/info/
rfc6437>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <http://www.rfc-editor.org/info/rfc6679>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
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[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, <http://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, <http://
www.rfc-editor.org/info/rfc7567>.
[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, <http://www.rfc-editor.org/
info/rfc7624>.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015, <http://www.rfc-
editor.org/info/rfc7713>.
[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, <http://www.rfc-editor.org/info/rfc7928>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", BCP
208, RFC 8084, DOI 10.17487/RFC8084, March 2017, <http://
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, <http://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, <http://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, <http://www.rfc-editor.org/
info/rfc8087>.
[Tor] The Tor Project, ., "https://www.torproject.org", June
2017.
Appendix A. Revision information
-00 This is an individual draft for the IETF community
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-01 This draft was a result of walking away from the text for a few
days and then reorganising the content. Comments from the community
are welcome on the text and recommendations.
Author's Address
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen, AB24 3UE
Scotland
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
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