rfc9065
Internet Engineering Task Force (IETF) G. Fairhurst
Request for Comments: 9065 University of Aberdeen
Category: Informational C. Perkins
ISSN: 2070-1721 University of Glasgow
July 2021
Considerations around Transport Header Confidentiality, Network
Operations, and the Evolution of Internet Transport Protocols
Abstract
To protect user data and privacy, Internet transport protocols have
supported payload encryption and authentication for some time. Such
encryption and authentication are now also starting to be applied to
the transport protocol headers. This helps avoid transport protocol
ossification by middleboxes, mitigate attacks against the transport
protocol, and protect metadata about the communication. Current
operational practice in some networks inspect transport header
information within the network, but this is no longer possible when
those transport headers are encrypted.
This document discusses the possible impact when network traffic uses
a protocol with an encrypted transport header. It suggests issues to
consider when designing new transport protocols or features.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9065.
Copyright Notice
Copyright (c) 2021 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
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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
2. Current Uses of Transport Headers within the Network
2.1. To Separate Flows in Network Devices
2.2. To Identify Transport Protocols and Flows
2.3. To Understand Transport Protocol Performance
2.4. To Support Network Operations
2.5. To Mitigate the Effects of Constrained Networks
2.6. To Verify SLA Compliance
3. Research, Development, and Deployment
3.1. Independent Measurement
3.2. Measurable Transport Protocols
3.3. Other Sources of Information
4. Encryption and Authentication of Transport Headers
5. Intentionally Exposing Transport Information to the Network
5.1. Exposing Transport Information in Extension Headers
5.2. Common Exposed Transport Information
5.3. Considerations for Exposing Transport Information
6. Addition of Transport OAM Information to Network-Layer Headers
6.1. Use of OAM within a Maintenance Domain
6.2. Use of OAM across Multiple Maintenance Domains
7. Conclusions
8. Security Considerations
9. IANA Considerations
10. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The transport layer supports the end-to-end flow of data across a
network path, providing features such as connection establishment,
reliability, framing, ordering, congestion control, flow control,
etc., as needed to support applications. One of the core functions
of an Internet transport is to discover and adapt to the
characteristics of the network path that is currently being used.
For some years, it has been common for the transport-layer payload to
be protected by encryption and authentication but for the transport-
layer headers to be sent unprotected. Examples of protocols that
behave in this manner include Transport Layer Security (TLS) over TCP
[RFC8446], Datagram TLS [RFC6347] [DTLS], the Secure Real-time
Transport Protocol [RFC3711], and tcpcrypt [RFC8548]. The use of
unencrypted transport headers has led some network operators,
researchers, and others to develop tools and processes that rely on
observations of transport headers both in aggregate and at the flow
level to infer details of the network's behaviour and inform
operational practice.
Transport protocols are now being developed that encrypt some or all
of the transport headers, in addition to the transport payload data.
The QUIC transport protocol [RFC9000] is an example of such a
protocol. Such transport header encryption makes it difficult to
observe transport protocol behaviour from the vantage point of the
network. This document discusses some implications of transport
header encryption for network operators and researchers that have
previously observed transport headers, and it highlights some issues
to consider for transport protocol designers.
As discussed in [RFC7258], the IETF has concluded that Pervasive
Monitoring (PM) is a technical attack that needs to be mitigated in
the design of IETF protocols. This document supports that
conclusion. It also recognises that [RFC7258] states, "Making
networks unmanageable to mitigate PM is not an acceptable outcome,
but ignoring PM would go against the consensus documented here. An
appropriate balance will emerge over time as real instances of this
tension are considered." This document is written to provide input
to the discussion around what is an appropriate balance by
highlighting some implications of transport header encryption.
Current uses of transport header information by network devices on
the Internet path are explained. These uses can be beneficial or
malicious. This is written to provide input to the discussion around
what is an appropriate balance by highlighting some implications of
transport header encryption.
2. Current Uses of Transport Headers within the Network
In response to pervasive surveillance [RFC7624] revelations and the
IETF consensus that "Pervasive Monitoring Is an Attack" [RFC7258],
efforts are underway to increase encryption of Internet traffic.
Applying confidentiality to transport header fields can improve
privacy and can help to mitigate certain attacks or manipulation of
packets by devices on the network path, but it can also affect
network operations and measurement [RFC8404].
When considering what parts of the transport headers should be
encrypted to provide confidentiality and what parts should be visible
to network devices (including unencrypted but authenticated headers),
it is necessary to consider both the impact on network operations and
management and the implications for ossification and user privacy
[Measurement]. Different parties will view the relative importance
of these concerns differently. For some, the benefits of encrypting
all the transport headers outweigh the impact of doing so; others
might analyse the security, privacy, and ossification impacts and
arrive at a different trade-off.
This section reviews examples of the observation of transport-layer
headers within the network by using devices on the network path or by
using information exported by an on-path device. Unencrypted
transport headers provide information that can support network
operations and management, and this section notes some ways in which
this has been done. Unencrypted transport header information also
contributes metadata that can be exploited for purposes unrelated to
network transport measurement, diagnostics, or troubleshooting (e.g.,
to block or to throttle traffic from a specific content provider),
and this section also notes some threats relating to unencrypted
transport headers.
Exposed transport information also provides a source of information
that contributes to linked data sets, which could be exploited to
deduce private information, e.g., user patterns, user location,
tracking behaviour, etc. This might reveal information the parties
did not intend to be revealed. [RFC6973] aims to make designers,
implementers, and users of Internet protocols aware of privacy-
related design choices in IETF protocols.
This section does not consider intentional modification of transport
headers by middleboxes, such as devices performing Network Address
Translation (NAT) or firewalls.
2.1. To Separate Flows in Network Devices
Some network-layer mechanisms separate network traffic by flow
without resorting to identifying the type of traffic: hash-based load
sharing across paths (e.g., Equal-Cost Multipath (ECMP)); sharing
across a group of links (e.g., using a Link Aggregation Group (LAG));
ensuring equal access to link capacity (e.g., Fair Queuing (FQ)); or
distributing traffic to servers (e.g., load balancing). To prevent
packet reordering, forwarding engines can consistently forward the
same transport flows along the same forwarding path, often achieved
by calculating a hash using an n-tuple gleaned from a combination of
link header information through to transport header information.
This n-tuple can use the Media Access Control (MAC) address and IP
addresses and can include observable transport header information.
When transport header information cannot be observed, there can be
less information to separate flows at equipment along the path. Flow
separation might not be possible when a transport forms traffic into
an encrypted aggregate. For IPv6, the Flow Label [RFC6437] can be
used even when all transport information is encrypted, enabling Flow
Label-based ECMP [RFC6438] and load sharing [RFC7098].
2.2. To Identify Transport Protocols and Flows
Information in exposed transport-layer headers can be used by the
network to identify transport protocols and flows [RFC8558]. The
ability to identify transport protocols, flows, and sessions is a
common function performed, for example, by measurement activities,
Quality of Service (QoS) classifiers, and firewalls. These functions
can be beneficial and performed with the consent of, and in support
of, the end user. Alternatively, the same mechanisms could be used
to support practises that might be adversarial to the end user,
including blocking, deprioritising, and monitoring traffic without
consent.
Observable transport header information, together with information in
the network header, has been used to identify flows and their
connection state, together with the set of protocol options being
used. Transport protocols, such as TCP [RFC7414] and the Stream
Control Transmission Protocol (SCTP) [RFC4960], specify a standard
base header that includes sequence number information and other data.
They also have the possibility to negotiate additional headers at
connection setup, identified by an option number in the transport
header.
In some uses, an assigned transport port (e.g., 0..49151) can
identify the upper-layer protocol or service [RFC7605]. However,
port information alone is not sufficient to guarantee identification.
Applications can use arbitrary ports and do not need to use assigned
port numbers. The use of an assigned port number is also not limited
to the protocol for which the port is intended. Multiple sessions
can also be multiplexed on a single port, and ports can be reused by
subsequent sessions.
Some flows can be identified by observing signalling data (e.g., see
[RFC3261] and [RFC8837]) or through the use of magic numbers placed
in the first byte(s) of a datagram payload [RFC7983].
When transport header information cannot be observed, this removes
information that could have been used to classify flows by passive
observers along the path. More ambitious ways could be used to
collect, estimate, or infer flow information, including heuristics
based on the analysis of traffic patterns, such as classification of
flows relying on timing, volumes of information, and correlation
between multiple flows. For example, an operator that cannot access
the Session Description Protocol (SDP) session descriptions [RFC8866]
to classify a flow as audio traffic might instead use (possibly less-
reliable) heuristics to infer that short UDP packets with regular
spacing carry audio traffic. Operational practises aimed at
inferring transport parameters are out of scope for this document,
and are only mentioned here to recognise that encryption does not
prevent operators from attempting to apply practises that were used
with unencrypted transport headers.
The IAB [RFC8546] has provided a summary of expected implications of
increased encryption on network functions that use the observable
headers and describe the expected benefits of designs that explicitly
declare protocol-invariant header information that can be used for
this purpose.
2.3. To Understand Transport Protocol Performance
This subsection describes use by the network of exposed transport-
layer headers to understand transport protocol performance and
behaviour.
2.3.1. Using Information Derived from Transport-Layer Headers
Observable transport headers enable explicit measurement and analysis
of protocol performance and detection of network anomalies at any
point along the Internet path. Some operators use passive monitoring
to manage their portion of the Internet by characterising the
performance of link/network segments. Inferences from transport
headers are used to derive performance metrics:
Traffic Rate and Volume:
Per-application traffic rate and volume measures can be used to
characterise the traffic that uses a network segment or the
pattern of network usage. Observing the protocol sequence number
and packet size offers one way to measure this (e.g., measurements
observing counters in periodic reports, such as RTCP [RFC3550]
[RFC3711] [RFC4585], or measurements observing protocol sequence
numbers in statistical samples of packet flows or specific control
packets, such as those observed at the start and end of a flow).
Measurements can be per endpoint or for an endpoint aggregate.
These could be used to assess usage or for subscriber billing.
Such measurements can be used to trigger traffic shaping and to
associate QoS support within the network and lower layers. This
can be done with consent and in support of an end user to improve
quality of service or could be used by the network to deprioritise
certain flows without user consent.
The traffic rate and volume can be determined, providing that the
packets belonging to individual flows can be identified, but there
might be no additional information about a flow when the transport
headers cannot be observed.
Loss Rate and Loss Pattern:
Flow loss rate can be derived (e.g., from transport sequence
numbers or inferred from observing transport protocol
interactions) and has been used as a metric for performance
assessment and to characterise transport behaviour. Network
operators have used the variation in patterns to detect changes in
the offered service. Understanding the location and root cause of
loss can help an operator determine whether this requires
corrective action.
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 preemption),
and policing (e.g., traffic management [RFC2475]). Understanding
flow loss rates requires maintaining the per-flow state (flow
identification often requires transport-layer information) and
either observing the increase in sequence numbers in the network
or transport headers or comparing a per-flow packet counter with
the number of packets that the flow actually sent. Per-hop loss
can also sometimes be monitored at the interface level by devices
on the network path or by using in-situ methods operating over a
network segment (see Section 3.3).
The pattern of loss can provide insight into the cause of loss.
Losses can often occur as bursts, randomly timed events, etc. It
can also be valuable to understand the conditions under which loss
occurs. This usually requires relating loss to the traffic
flowing at a network node or segment at the time of loss.
Transport header information can help identify cases where loss
could have been wrongly identified or where the transport did not
require retransmission of a lost packet.
Throughput and Goodput:
Throughput is the amount of payload data sent by a flow per time
interval. Goodput (the subset of throughput consisting of useful
traffic; see Section 2.5 of [RFC7928] and [RFC5166]) is a measure
of useful data exchanged. The throughput of a flow can be
determined in the absence of transport header information,
providing that the individual flow can be identified, and the
overhead known. Goodput requires the ability to differentiate
loss and retransmission of packets, for example, by observing
packet sequence numbers in the TCP or RTP headers [RFC3550].
Latency:
Latency is a key performance metric that impacts application and
user-perceived response times. It often indirectly impacts
throughput and flow completion time. This determines the reaction
time of the transport protocol itself, impacting flow setup,
congestion control, loss recovery, and other transport mechanisms.
The observed latency can have many components [Latency]. Of
these, unnecessary/unwanted queueing in buffers of the network
devices on the path 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) by
using packet sequence numbers and acknowledgements or by observing
header timestamp information. Such information allows an
observation point on the network path to determine not only the
path RTT but also allows measurement of the upstream and
downstream contribution to the RTT. This could be used to locate
a source of latency, e.g., by observing cases where the median RTT
is much greater than the minimum RTT for a part of a path.
The service offered by network operators can benefit from latency
information to understand the impact of configuration changes and
to tune deployed services. Latency metrics are key to evaluating
and deploying AQM [RFC7567], Diffserv [RFC2474], and Explicit
Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements
could identify excessively large buffers, indicating where to
deploy or configure AQM. An AQM method is often deployed in
combination with other techniques, such as scheduling [RFC7567]
[RFC8290], and although parameter-less methods are desired
[RFC7567], current methods often require tuning [RFC8290]
[RFC8289] [RFC8033] because they cannot scale across all possible
deployment scenarios.
Latency and round-trip time information can potentially expose
some information useful for approximate geolocation, as discussed
in [PAM-RTT].
Variation in Delay:
Some network applications are sensitive to (small) changes in
packet timing (jitter). Short- and long-term delay variation can
impact the latency of a flow and hence the perceived quality of
applications using a network path. For example, jitter metrics
are often cited when characterising paths supporting real-time
traffic. The expected performance of such applications can be
inferred from a measure of the variation in delay observed along a
portion of the path [RFC3393] [RFC5481]. The requirements
resemble those for the measurement of latency.
Flow Reordering:
Significant packet reordering within a flow can impact time-
critical applications and can be interpreted as loss by reliable
transports. Many transport protocol techniques are impacted by
reordering (e.g., triggering TCP retransmission or rebuffering of
real-time applications). Packet reordering can occur for many
reasons, e.g., from equipment design to misconfiguration of
forwarding rules. Flow identification is often required to avoid
significant packet misordering (e.g., when using ECMP, or LAG).
Network tools can detect and measure unwanted/excessive reordering
and the impact on transport performance.
There have been initiatives in the IETF transport area to reduce
the impact of reordering within a transport flow, possibly leading
to a reduction in the requirements for preserving ordering. These
have potential to simplify network equipment design as well as the
potential to improve robustness of the transport service.
Measurements of reordering can help understand the present level
of reordering and inform decisions about how to progress new
mechanisms.
Techniques for measuring reordering typically observe packet
sequence numbers. Metrics have been defined that evaluate whether
a network path has maintained packet order on a packet-by-packet
basis [RFC4737] [RFC5236]. 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. Metadata assists in understanding
the context under which the data was collected, including the
time, observation point [RFC7799], and way in which metrics were
accumulated. The RTCP protocol directly reports some of this
information in a form that can be directly visible by devices on
the network path.
In some cases, measurements could involve active injection of test
traffic to perform a measurement (see Section 3.4 of [RFC7799]).
However, most operators do not have access to user equipment;
therefore, the point of test is normally different from the transport
endpoint. Injection of test traffic can incur an additional cost in
running such tests (e.g., the implications of capacity tests in a
mobile network segment are obvious). Some active measurements
[RFC7799] (e.g., response under load or particular workloads) perturb
other traffic and could require dedicated access to the network
segment.
Passive measurements (see Section 3.6 of [RFC7799]) can have
advantages in terms of eliminating unproductive test traffic,
reducing the influence of test traffic on the overall traffic mix,
and having the ability to choose the point of observation (see
Section 2.4.1). Measurements can rely on observing packet headers,
which is not possible if those headers are encrypted, but could
utilise information about traffic volumes or patterns of interaction
to deduce metrics.
Passive packet sampling techniques are also often used to scale the
processing involved in observing packets on high-rate links. This
exports only the packet header information of (randomly) selected
packets. Interpretation of the exported information relies on
understanding of the header information. The utility of these
measurements depends on the type of network segment/link and number
of mechanisms used by the network devices. Simple routers are
relatively easy to manage, but a device with more complexity demands
understanding of the choice of many system parameters.
2.3.2. Using Information Derived from Network-Layer Header Fields
Information from the transport header can be used by a multi-field
(MF) classifier as a part of policy framework. Policies are commonly
used for management of the QoS or Quality of Experience (QoE) in
resource-constrained networks or by firewalls to implement access
rules (see also Section 2.2.2 of [RFC8404]). Policies can support
user applications/services or protect against unwanted or lower-
priority traffic (Section 2.4.4).
Transport-layer information can also be explicitly carried in
network-layer header fields that are not encrypted, serving as a
replacement/addition to the exposed transport header information
[RFC8558]. This information can enable a different forwarding
treatment by the devices forming the network path, even when a
transport employs encryption to protect other header information.
On the one hand, the user of a transport that multiplexes multiple
subflows might want to obscure the presence and characteristics of
these subflows. On the other hand, an encrypted transport could set
the network-layer information to indicate the presence of subflows
and to reflect the service requirements of individual subflows.
There are several ways this could be done:
IP Address:
Applications normally expose the endpoint addresses used in the
forwarding decisions in network devices. Address and other
protocol information can be used by an MF classifier to determine
how traffic is treated [RFC2475] and hence affects the quality of
experience for a flow. Common issues concerning IP address
sharing are described in [RFC6269].
Using the IPv6 Network-Layer Flow Label:
A number of Standards Track and Best Current Practice RFCs (e.g.,
[RFC8085], [RFC6437], and [RFC6438]) encourage endpoints to set
the IPv6 Flow Label field of the network-layer header. As per
[RFC6437], IPv6 source nodes "SHOULD assign each unrelated
transport connection and application data stream to a new flow."
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 queueing and forwarding, including use with
IPsec [RFC6294], Equal-Cost Multipath routing, and Link
Aggregation [RFC6438].
The choice of how to assign a flow label needs to avoid
introducing linkages between flows that a network device could not
otherwise 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 similarly).
Using the Network-Layer Differentiated Services Code Point:
Applications can expose their delivery expectations to network
devices 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 subflows (audio vs. video) based on the value of the
DSCP field [RFC8837]). This provides explicit information to
inform network-layer queueing and forwarding, rather than an
operator inferring traffic requirements from transport and
application headers via a multi-field classifier. Inappropriate
use by the transport can have privacy implications (e.g.,
assigning a different DSCP to a subflow could assist in a network
device discovering the traffic pattern used by an application).
The field is mutable, i.e., some network devices can be expected
to change this field. Since the DSCP value can impact the quality
of experience for a flow, observations of service performance have
to consider this field when a network path supports differentiated
service treatment.
Using Explicit Congestion Notification:
Explicit Congestion Notification (ECN) [RFC3168] is a transport
mechanism that uses the ECN field in the network-layer header.
Use of ECN explicitly informs the network layer that a transport
is ECN capable and requests ECN treatment of the flow. An ECN-
capable transport can offer benefits when used over a path with
equipment that implements an AQM method with Congestion
Experienced (CE) marking of IP packets [RFC8087], since it can
react to congestion without also having to recover from lost
packets.
ECN exposes the presence of congestion. The reception of CE-
marked packets can be used to estimate the level of incipient
congestion on the upstream portion of the path from the point of
observation (Section 2.5 of [RFC8087]). Interpreting the marking
behaviour (i.e., assessing congestion and diagnosing faults)
requires context from the transport layer, such as path RTT.
AQM and ECN offer a range of algorithms and configuration options.
Tools therefore have to be available to network operators and
researchers to understand the implication of configuration choices
and transport behaviour as the use of ECN increases and new
methods emerge [RFC7567].
Network-Layer Options:
Network protocols can carry optional headers (see Section 5.1).
These can explicitly expose transport header information to on-
path devices operating at the network layer (as discussed further
in Section 6).
IPv4 [RFC0791] has provisions for optional header fields. IP
routers can examine these headers and are required to ignore IPv4
options that they do not recognise. Many current paths include
network devices that forward packets that carry options on a
slower processing path. Some network devices (e.g., firewalls)
can be (and are) configured to drop these packets [RFC7126]. BCP
186 [RFC7126] provides guidance on how operators should treat IPv4
packets that specify options.
IPv6 can encode optional network-layer information in separate
headers that may be placed between the IPv6 header and the upper-
layer header [RFC8200] (e.g., the IPv6 Alternate Marking Method
[IPV6-ALT-MARK], which can be used to measure packet loss and
delay metrics). The Hop-by-Hop Options header, when present,
immediately follows the IPv6 header. IPv6 permits this header to
be examined by any node along the path if explicitly configured
[RFC8200].
Careful use of the network-layer features (e.g., extension headers
can; see Section 5) help provide similar information in the case
where the network is unable to inspect transport protocol headers.
2.4. To Support Network Operations
Some network operators make use of on-path observations of transport
headers to analyse the service offered to the users of a network
segment and inform operational practice and can help detect and
locate network problems. [RFC8517] gives an operator's perspective
about such use.
When observable transport header information is not available, those
seeking an understanding of transport behaviour and dynamics might
learn to work without that information. Alternatively, they might
use more limited measurements combined with pattern inference and
other heuristics to infer network behaviour (see Section 2.1.1 of
[RFC8404]). Operational practises aimed at inferring transport
parameters are out of scope for this document and are only mentioned
here to recognise that encryption does not necessarily stop operators
from attempting to apply practises that have been used with
unencrypted transport headers.
This section discusses topics concerning observation of transport
flows, with a focus on transport measurement.
2.4.1. Problem Location
Observations of transport header information can be used to locate
the source of problems or to assess the performance of a network
segment. Often issues can only be understood in the context of the
other flows that share a particular path, particular device
configuration, 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 implies knowledge of how
traffic is assigned to any subqueues used for flow scheduling but can
require information about how the traffic dynamics impact active
queue management, starvation prevention mechanisms, and circuit
breakers.
Sometimes correlating observations of headers at multiple points
along the path (e.g., at the ingress and egress of a network segment)
allows an observer to determine the contribution of a portion of the
path to an observed metric (e.g., to locate a source of delay,
jitter, loss, reordering, or congestion marking).
2.4.2. Network Planning and Provisioning
Traffic rate and volume measurements are used to help plan deployment
of new equipment and configuration in 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.
Trends in aggregate traffic can be observed and can be related to the
endpoint addresses being used, but when transport header information
is not observable, it might be impossible to correlate patterns in
measurements with changes in transport protocols. This increases the
dependency on other indirect sources of information to inform
planning and provisioning.
2.4.3. Compliance with Congestion Control
The traffic that can be observed by on-path network devices (the
"wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational
concern, although the ability to measure and monitor it is lower.
The desire to understand the traffic and protocol interactions
typically grows as the proportion of traffic increases. The
challenges increase when multiple instances of an evolving protocol
contribute to the traffic that share network capacity.
Operators can manage traffic load (e.g., when the network is severely
overloaded) by deploying rate limiters, traffic shaping, or network
transport circuit breakers [RFC8084]. The information provided by
observing transport headers is a source of data that can help to
inform such mechanisms.
Congestion Control Compliance of Traffic:
Congestion control is a key transport function [RFC2914]. Many
network operators implicitly accept that TCP traffic complies with
a behaviour that is acceptable for the shared Internet. TCP
algorithms have been continuously improved over decades and have
reached a level of efficiency and correctness that is difficult to
match in custom application-layer mechanisms [RFC8085].
A standards-compliant TCP stack provides congestion control that
is judged safe for use across the Internet. Applications
developed on top of well-designed transports can be expected to
appropriately control their network usage, reacting when the
network experiences congestion, by backing off and reducing the
load placed on the network. This is the normal expected behaviour
for IETF-specified transports (e.g., TCP and SCTP).
Congestion Control Compliance for UDP Traffic:
UDP provides a minimal message-passing datagram transport that has
no inherent congestion control mechanisms. Because congestion
control is critical to the stable operation of the Internet,
applications and other protocols that choose to use UDP as a
transport have to employ mechanisms to prevent collapse, avoid
unacceptable contributions to jitter/latency, and establish an
acceptable share of capacity with concurrent traffic [RFC8085].
UDP flows that expose a well-known header can be observed to gain
understanding of the dynamics of a flow and its congestion control
behaviour. For example, tools exist to monitor various aspects of
RTP header information and RTCP reports for real-time flows (see
Section 2.3). The Secure RTP and RTCP extensions [RFC3711] were
explicitly designed to expose some header information to enable
such observation while protecting the payload data.
A network operator can observe the headers of transport protocols
layered above UDP to understand if the datagram flows comply with
congestion control expectations. This can help inform a decision
on whether it might be appropriate to deploy methods, such as rate
limiters, to enforce acceptable usage. The available information
determines the level of precision with which flows can be
classified and the design space for conditioning mechanisms (e.g.,
rate-limiting, circuit breaker techniques [RFC8084], or blocking
uncharacterised traffic) [RFC5218].
When anomalies are detected, tools can interpret the transport header
information to help understand the impact of specific transport
protocols (or protocol mechanisms) on the other traffic that shares a
network. An observer on the network path can gain an understanding
of the dynamics of a flow and its congestion control behaviour.
Analysing observed flows can help to build confidence that an
application flow backs off its share of the network load under
persistent congestion and hence to understand whether the behaviour
is appropriate for sharing limited network capacity. For example, it
is common to visualise plots of TCP sequence numbers versus time for
a flow to understand how a flow shares available capacity, deduce its
dynamics in response to congestion, etc.
The ability to identify sources and flows that contribute to
persistent congestion is important to the safe operation of network
infrastructure and can inform configuration of network devices to
complement the endpoint congestion avoidance mechanisms [RFC7567]
[RFC8084] to avoid a portion of the network being driven into
congestion collapse [RFC2914].
2.4.4. To Characterise "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.
Encryption can increase the volume of "unknown" or "uncharacterised"
traffic seen by the network. If these traffic patterns form a small
part of the traffic aggregate passing through a network device or
segment of the network path, the dynamics of the uncharacterised
traffic might not have a significant collateral impact on the
performance of other traffic that shares this network segment. Once
the proportion of this traffic increases, monitoring the traffic can
determine if appropriate safety measures have to be put in place.
Tracking the impact of new mechanisms and protocols requires traffic
volume to be measured and new transport behaviours to be identified.
This is especially true of protocols operating over a UDP substrate.
The level and style of encryption needs to be considered in
determining how this activity is performed.
Traffic that cannot be classified typically receives a default
treatment. Some networks block or rate-limit traffic that cannot be
classified.
2.4.5. To Support Network Security Functions
On-path observation of the transport headers of packets can be used
for various security functions. For example, Denial of Service (DoS)
and Distributed DoS (DDoS) attacks against the infrastructure or
against an endpoint can be detected and mitigated by characterising
anomalous traffic (see Section 2.4.4) on a shorter timescale. Other
uses include support for security audits (e.g., verifying the
compliance with cipher suites), client and application fingerprinting
for inventory, and alerts provided for network intrusion detection
and other next generation firewall functions.
When using an encrypted transport, endpoints can directly provide
information to support these security functions. Another method, if
the endpoints do not provide this information, is to use an on-path
network device that relies on pattern inferences in the traffic and
heuristics or machine learning instead of processing observed header
information. An endpoint could also explicitly cooperate with an on-
path device (e.g., a QUIC endpoint could share information about
current uses of connection IDs).
2.4.6. Network Diagnostics and Troubleshooting
Operators monitor the health of a network segment to support a
variety of operational tasks [RFC8404], including procedures to
provide early warning and trigger action, e.g., to diagnose network
problems, to manage security threats (including DoS), to evaluate
equipment or protocol performance, or to respond to user performance
questions. Information about transport flows can assist in setting
buffer sizes and help identify whether link/network tuning is
effective. Information can also support debugging and diagnosis of
the root causes of faults that concern a particular user's traffic
and can support postmortem investigation after an anomaly. Sections
3.1.2 and 5 of [RFC8404] provide further examples.
Network segments vary in their complexity. The design trade-offs for
radio networks are often very different from those of wired networks
[RFC8462]. A radio-based network (e.g., cellular mobile, enterprise
Wireless LAN (WLAN), satellite access/backhaul, point-to-point radio)
adds a subsystem that performs radio resource management, with impact
on the available capacity and potentially loss/reordering of packets.
This impact can differ by traffic type and can be correlated with
link propagation and interference. These can impact the cost and
performance of a provided service and is expected to increase in
importance as operators bring together heterogeneous types of network
equipment and deploy opportunistic methods to access a shared radio
spectrum.
2.4.7. Tooling and Network Operations
A variety of open source and proprietary tools have been deployed
that use the transport header information observable with widely used
protocols, such as TCP or RTP/UDP/IP. Tools that dissect network
traffic flows can alert to potential problems that are hard to derive
from volume measurements, link statistics, or device measurements
alone.
Any introduction of a new transport protocol, protocol feature, or
application might require changes to such tools and could impact
operational practice and policies. Such changes have associated
costs that are incurred by the network operators that need to update
their tooling or develop alternative practises that work without
access to the changed/removed information.
The use of encryption has the desirable effect of preventing
unintended observation of the payload data, and these tools seldom
seek to observe the payload or other application details. A flow
that hides its transport header information could imply "don't touch"
to some operators. This might limit a trouble-shooting response to
"can't help, no trouble found".
An alternative that does not require access to an observable
transport headers is to access endpoint diagnostic tools or to
include user involvement in diagnosing and troubleshooting unusual
use cases or to troubleshoot nontrivial problems. Another approach
is to use traffic pattern analysis. Such tools can provide useful
information during network anomalies (e.g., detecting significant
reordering, high or intermittent loss); however, indirect
measurements need to be carefully designed to provide information for
diagnostics and troubleshooting.
If new protocols, or protocol extensions, are made to closely
resemble or match existing mechanisms, then the changes to tooling
and the associated costs can be small. Equally, more extensive
changes to the transport tend to require more extensive, and more
expensive, changes to tooling and operational practice. Protocol
designers can mitigate these costs by explicitly choosing to expose
selected information as invariants that are guaranteed not to change
for a particular protocol (e.g., the header invariants and the spin
bit in QUIC [RFC9000]). Specification of common log formats and
development of alternative approaches can also help mitigate the
costs of transport changes.
2.5. To Mitigate the Effects of Constrained Networks
Some link and network segments are constrained by the capacity they
can offer by the time it takes to access capacity (e.g., due to
underlying radio resource management methods) or by asymmetries in
the design (e.g., many link are designed so that the capacity
available is different in the forward and return directions; some
radio technologies have different access methods in the forward and
return directions resulting from differences in the power budget).
The impact of path constraints can be mitigated using a proxy
operating at or above the transport layer to use an alternate
transport protocol.
In many cases, one or both endpoints are unaware of the
characteristics of the constraining link or network segment, and
mitigations are applied below the transport layer. Packet
classification and QoS methods (described in various sections) can be
beneficial in differentially prioritising certain traffic when there
is a capacity constraint or additional delay in scheduling link
transmissions. Another common mitigation is to apply header
compression over the specific link or subnetwork (see Section 2.5.1).
2.5.1. To Provide Header Compression
Header compression saves link capacity by compressing network and
transport protocol headers on a per-hop basis. This has been widely
used with low bandwidth dial-up access links and still finds
application on wireless links that are subject to capacity
constraints. These methods are effective for bit-congestive links
sending small packets (e.g., reducing the cost for sending control
packets or small data packets over radio links).
Examples of header compression include use with TCP/IP and RTP/UDP/IP
flows [RFC2507] [RFC6846] [RFC2508] [RFC5795] [RFC8724]. Successful
compression depends on observing the transport headers and
understanding the way fields change between packets and is hence
incompatible with header encryption. Devices that compress transport
headers are dependent on a stable header format, implying
ossification of that format.
Introducing a new transport protocol, or changing the format of the
transport header information, will limit the effectiveness of header
compression until the network devices are updated. Encrypting the
transport protocol headers will tend to cause the header compression
to fall back to compressing only the network-layer headers, with a
significant reduction in efficiency. This can limit connectivity if
the resulting flow exceeds the link capacity or if the packets are
dropped because they exceed the link Maximum Transmission Unit (MTU).
The Secure RTP (SRTP) extensions [RFC3711] were explicitly designed
to leave the transport protocol headers unencrypted, but
authenticated, since support for header compression was considered
important.
2.6. To Verify SLA Compliance
Observable transport headers coupled with published transport
specifications allow operators and regulators to explore and verify
compliance with Service Level Agreements (SLAs). It can also be used
to understand whether a service is providing differential treatment
to certain flows.
When transport header information cannot be observed, other methods
have to be found to confirm that the traffic produced conforms to the
expectations of the operator or developer.
Independently verifiable performance metrics can be utilised to
demonstrate regulatory compliance in some jurisdictions and as a
basis for informing design decisions. This can bring assurance to
those operating networks, often avoiding deployment of complex
techniques that routinely monitor and manage Internet traffic flows
(e.g., avoiding the capital and operational costs of deploying flow
rate-limiting and network circuit breaker methods [RFC8084]).
3. Research, Development, and Deployment
Research and development of new protocols and mechanisms need to be
informed by measurement data (as described in the previous section).
Data can also help promote acceptance of proposed standards
specifications by the wider community (e.g., as a method to judge the
safety for Internet deployment).
Observed data is important to ensure the health of the research and
development communities and provides data needed to evaluate new
proposals for standardisation. Open standards motivate a desire to
include independent observation and evaluation of performance and
deployment data. Independent data helps compare different methods,
judge the level of deployment, and ensure the wider applicability of
the results. This is important when considering when a protocol or
mechanism should be standardised for use in the general Internet.
This, in turn, demands control/understanding about where and when
measurement samples are collected. This requires consideration of
the methods used to observe information and the appropriate balance
between encrypting all and no transport header information.
There can be performance and operational trade-offs in exposing
selected information to network tools. This section explores key
implications of tools and procedures that observe transport protocols
but does not endorse or condemn any specific practises.
3.1. Independent Measurement
Encrypting transport header information has implications on the way
network data is collected and analysed. Independent observations by
multiple actors is currently used by the transport community to
maintain an accurate understanding of the network within transport
area working groups, IRTF research groups, and the broader research
community. This is important to be able to provide accountability
and demonstrate that protocols behave as intended; although, when
providing or using such information, it is important to consider the
privacy of the user and their incentive for providing accurate and
detailed information.
Protocols that expose the state of the transport protocol in their
header (e.g., timestamps used to calculate the RTT, packet numbers
used to assess congestion, and requests for retransmission) provide
an incentive for a sending endpoint to provide consistent
information, because a protocol will not work otherwise. An on-path
observer can have confidence that well-known (and ossified) transport
header information represents the actual state of the endpoints when
this information is necessary for the protocol's correct operation.
Encryption of transport header information could reduce the range of
actors that can observe useful data. This would limit the
information sources available to the Internet community to understand
the operation of new transport protocols, reducing information to
inform design decisions and standardisation of the new protocols and
related operational practises. The cooperating dependence of
network, application, and host to provide communication performance
on the Internet is uncertain when only endpoints (i.e., at user
devices and within service platforms) can observe performance and
when performance cannot be independently verified by all parties.
3.2. Measurable Transport Protocols
Transport protocol evolution and the ability to measure and
understand the impact of protocol changes have to proceed hand-in-
hand. A transport protocol that provides observable headers can be
used to provide open and verifiable measurement data. Observation of
pathologies has a critical role in the design of transport protocol
mechanisms and development of new mechanisms and protocols and aides
in understanding the interactions between cooperating protocols and
network mechanisms, the implications of sharing capacity with other
traffic, and the impact of different patterns of usage. The ability
of other stakeholders to review transport header traces helps develop
insight into the performance and the traffic contribution of specific
variants of a protocol.
Development of new transport protocol mechanisms has to consider the
scale of deployment and the range of environments in which the
transport is used. Experience has shown that it is often difficult
to correctly implement new mechanisms [RFC8085] and that mechanisms
often evolve as a protocol matures or in response to changes in
network conditions, in network traffic, or to application usage.
Analysis is especially valuable when based on the behaviour
experienced across a range of topologies, vendor equipment, and
traffic patterns.
Encryption enables a transport protocol to choose which internal
state to reveal to devices on the network path, what information to
encrypt, and what fields to grease [RFC8701]. A new design can
provide summary information regarding its performance, congestion
control state, etc., or make explicit measurement information
available. For example, [RFC9000] specifies a way for a QUIC
endpoint to optionally set the spin bit to explicitly reveal the RTT
of an encrypted transport session to the on-path network devices.
There is a choice of what information to expose. For some
operational uses, the information has to contain sufficient detail to
understand, and possibly reconstruct, the network traffic pattern for
further testing. The interpretation of the information needs to
consider whether this information reflects the actual transport state
of the endpoints. This might require the trust of transport protocol
implementers to correctly reveal the desired information.
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. At
the time of writing, there has been 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, DCTCP, and methods proposed for Low Latency Low Loss
Scalable throughput (L4S)). The growth and diversity of applications
and protocols using the Internet also continues to expand. For each
new method or application, it is desirable to build a body of data
reflecting its behaviour under a wide range of deployment scenarios,
traffic load, and interactions with other deployed/candidate methods.
3.3. Other Sources of Information
Some measurements that traditionally rely on observable transport
information could be completed by utilising endpoint-based logging
(e.g., based on QUIC trace [Quic-Trace] and qlog [QLOG]). Such
information has a diversity of uses, including developers wishing to
debug/understand the transport/application protocols with which they
work, researchers seeking to spot trends and anomalies, and to
characterise variants of protocols. A standard format for endpoint
logging could allow these to be shared (after appropriate
anonymisation) to understand performance and pathologies.
When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network and
conditions in which the system was observed, is often necessary to
interpret this data to answer questions about network performance or
understand a pathology. Collecting and coordinating such metadata is
more difficult when the observation point is at a different location
to the bottleneck or device under evaluation [RFC7799].
Despite being applicable in some scenarios, endpoint logs do not
provide equivalent information to on-path measurements made by
devices in the network. In particular, endpoint logs contain only a
part of the information to understand the operation of network
devices and identify issues, such as link performance or capacity
sharing between multiple flows. An analysis can require coordination
between actors at different layers to successfully characterise flows
and correlate the performance or behaviour of a specific mechanism
with an equipment configuration and traffic using operational
equipment along a network path (e.g., combining transport and network
measurements to explore congestion control dynamics to understand the
implications of traffic on designs for active queue management or
circuit breakers).
Another source of information could arise from Operations,
Administration, and Maintenance (OAM) (see Section 6). Information
data records could be embedded into header information at different
layers to support functions, such as performance evaluation, path
tracing, path verification information, classification, and a
diversity of other uses.
In-situ OAM (IOAM) data fields [IOAM-DATA] can be encapsulated into a
variety of protocols to record operational and telemetry information
in an existing packet while that packet traverses a part of the path
between two points in a network (e.g., within a particular IOAM
management domain). IOAM-Data-Fields are independent from the
protocols into which IOAM-Data-Fields are encapsulated. For example,
IOAM can provide proof that a traffic flow takes a predefined path,
SLA verification for the live data traffic, and statistics relating
to traffic distribution.
4. Encryption and Authentication of Transport Headers
There are several motivations for transport header encryption.
One motive to encrypt transport headers is to prevent network
ossification from network devices that inspect well-known transport
headers. Once a network device observes a transport header and
becomes reliant upon using it, the overall use of that field can
become ossified, preventing new versions of the protocol and
mechanisms from being deployed. Examples include:
* During the development of TLS 1.3 [RFC8446], the design needed to
function in the presence of deployed middleboxes that relied on
the presence of certain header fields exposed in TLS 1.2
[RFC5426].
* The design of Multipath TCP (MPTCP) [RFC8684] had to account for
middleboxes (known as "TCP Normalizers") that monitor the
evolution of the window advertised in the TCP header and then
reset connections when the window did not grow as expected.
* TCP Fast Open [RFC7413] can experience problems due to middleboxes
that modify the transport header of packets by removing "unknown"
TCP options. Segments with unrecognised TCP options can be
dropped, segments that contain data and set the SYN bit can be
dropped, and some middleboxes that disrupt connections can send
data before completion of the three-way handshake.
* Other examples of TCP ossification have included middleboxes that
modify transport headers by rewriting TCP sequence and
acknowledgement numbers but are unaware of the (newer) TCP
selective acknowledgement (SACK) option and therefore fail to
correctly rewrite the SACK information to match the changes made
to the fixed TCP header, preventing correct SACK operation.
In all these cases, middleboxes with a hard-coded, but incomplete,
understanding of a specific transport behaviour (i.e., TCP)
interacted poorly with transport protocols after the transport
behaviour was changed. In some cases, the middleboxes modified or
replaced information in the transport protocol header.
Transport header encryption prevents an on-path device from observing
the transport headers and therefore stops ossified mechanisms being
used that directly rely on or infer semantics of the transport header
information. This encryption is normally combined with
authentication of the protected information. [RFC8546] summarises
this approach, stating that "[t]he wire image, not the protocol's
specification, determines how third parties on the network paths
among protocol participants will interact with that protocol"
(Section 1 of [RFC8546]), and it can be expected that header
information that is not encrypted will become ossified.
Encryption does not itself prevent ossification of the network
service. People seeking to understand or classify network traffic
could still come to rely on pattern inferences and other heuristics
or machine learning to derive measurement data and as the basis for
network forwarding decisions [RFC8546]. This can also create
dependencies on the transport protocol or the patterns of traffic it
can generate, also resulting in ossification of the service.
Another motivation for using transport header encryption is to
improve privacy and to decrease opportunities for surveillance.
Users value the ability to protect their identity and location and
defend against analysis of the traffic. Revelations about the use of
pervasive surveillance [RFC7624] have, to some extent, eroded trust
in the service offered by network operators and have led to an
increased use of encryption. Concerns have also been voiced about
the addition of metadata to packets by third parties to provide
analytics, customisation, advertising, cross-site tracking of users,
customer billing, or selectively allowing or blocking content.
Whatever the reasons, the IETF is designing protocols that include
transport header encryption (e.g., QUIC [RFC9000]) to supplement the
already widespread payload encryption and to further limit exposure
of transport metadata to the network.
If a transport protocol uses header encryption, the designers have to
decide whether to encrypt all or a part of the transport-layer
information. Section 4 of [RFC8558] states, "Anything exposed to the
path should be done with the intent that it be used by the network
elements on the path."
Certain transport header fields can be made observable to on-path
network devices or can define new fields designed to explicitly
expose observable transport-layer information to the network. Where
exposed fields are intended to be immutable (i.e., can be observed
but not modified by a network device), the endpoints are encouraged
to use authentication to provide a cryptographic integrity check that
can detect if these immutable fields have been modified by network
devices. Authentication can help to prevent attacks that rely on
sending packets that fake exposed control signals in transport
headers (e.g., TCP RST spoofing). Making a part of a transport
header observable or exposing new header fields can lead to
ossification of that part of a header as network devices come to rely
on observations of the exposed fields.
The use of transport header authentication and encryption therefore
exposes a tussle between middlebox vendors, operators, researchers,
applications developers, and end users:
* On the one hand, future Internet protocols that support transport
header encryption assist in the restoration of the end-to-end
nature of the Internet by returning complex processing to the
endpoints. Since middleboxes cannot modify what they cannot see,
the use of transport header encryption can improve application and
end-user privacy by reducing leakage of transport metadata to
operators that deploy middleboxes.
* On the other hand, encryption of transport-layer information has
implications for network operators and researchers seeking to
understand the dynamics of protocols and traffic patterns, since
it reduces the information that is available to them.
The following briefly reviews some security design options for
transport protocols. "A Survey of the Interaction between Security
Protocols and Transport Services" [RFC8922] provides more details
concerning commonly used encryption methods at the transport layer.
Security work typically employs a design technique that seeks to
expose only what is needed [RFC3552]. This approach provides
incentives to not reveal any information that is not necessary for
the end-to-end communication. The IETF has provided guidelines for
writing security considerations for IETF specifications [RFC3552].
Endpoint design choices impacting privacy also need to be considered
as a part of the design process [RFC6973]. The IAB has provided
guidance for analysing and documenting privacy considerations within
IETF specifications [RFC6973].
Authenticating the Transport Protocol Header:
Transport-layer header information can be authenticated. An
example transport authentication mechanism is TCP Authentication
Option (TCP-AO) [RFC5925]. This TCP option authenticates the IP
pseudo-header, TCP header, and TCP data. TCP-AO protects the
transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. Such
authentication might interact with middleboxes, depending on their
behaviour [RFC3234].
The IPsec Authentication Header (AH) [RFC4302] was designed to
work at the network layer and authenticate the IP payload. This
approach authenticates all transport headers and verifies their
integrity at the receiver, preventing modification by network
devices on the path. The IPsec Encapsulating Security Payload
(ESP) [RFC4303] can also provide authentication and integrity
without confidentiality using the NULL encryption algorithm
[RFC2410]. SRTP [RFC3711] is another example of a transport
protocol that allows header authentication.
Integrity Check:
Transport protocols usually employ integrity checks on the
transport header information. Security methods usually employ
stronger checks and can combine this with authentication. An
integrity check that protects the immutable transport header
fields, but can still expose the transport header information in
the clear, allows on-path network devices to observe these fields.
An integrity check is not able to prevent modification by network
devices on the path but can prevent a receiving endpoint from
accepting changes and avoid impact on the transport protocol
operation, including some types of attack.
Selectively Encrypting Transport Headers and Payload:
A transport protocol design that encrypts selected header fields
allows specific transport header fields to be made observable by
network devices on the path. This information is explicitly
exposed either in a transport header field or lower layer protocol
header. A design that only exposes immutable fields can also
perform end-to-end authentication of these fields across the path
to prevent undetected modification of the immutable transport
headers.
Mutable fields in the transport header provide opportunities where
on-path network devices can modify the transport behaviour (e.g.,
the extended headers described in [PLUS-ABSTRACT-MECH]). An
example of a method that encrypts some, but not all, transport
header information is GRE-in-UDP [RFC8086] when used with GRE
encryption.
Optional Encryption of Header Information:
There are implications to the use of optional header encryption in
the design of a transport protocol, where support of optional
mechanisms can increase the complexity of the protocol and its
implementation and in the management decisions that have to be
made to use variable format fields. Instead, fields of a specific
type ought to be sent with the same level of confidentiality or
integrity protection.
Greasing:
Protocols often provide extensibility features, reserving fields
or values for use by future versions of a specification. The
specification of receivers has traditionally ignored unspecified
values; however, on-path network devices have emerged that ossify
to require a certain value in a field or reuse 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 at random
[RFC8701]. This prevents a network device ossifying the use of a
specific observable field and can ease future deployment of new
uses of the value or code point. This is not a security
mechanism, although the use can be combined with an authentication
mechanism.
Different transports use encryption to protect their header
information to varying degrees. The trend is towards increased
protection.
5. Intentionally Exposing Transport Information to the Network
A transport protocol can choose to expose certain transport
information to on-path devices operating at the network layer by
sending observable fields. One approach is to make an explicit
choice not to encrypt certain transport header fields, making this
transport information observable by an on-path network device.
Another approach is to expose transport information in a network-
layer extension header (see Section 5.1). Both are examples of
explicit information intended to be used by network devices on the
path [RFC8558].
Whatever the mechanism used to expose the information, a decision to
expose only specific information places the transport endpoint in
control of what to expose outside of the encrypted transport header.
This decision can then be made independently of the transport
protocol functionality. This can be done by exposing part of the
transport header or as a network-layer option/extension.
5.1. Exposing Transport Information in Extension Headers
At the network layer, packets can carry optional headers that
explicitly expose transport header information to the on-path devices
operating at the network layer (Section 2.3.2). For example, an
endpoint that sends an IPv6 hop-by-hop option [RFC8200] can provide
explicit transport-layer information that can be observed and used by
network devices on the path. New hop-by-hop options are not
recommended in [RFC8200] "because nodes may be configured to ignore
the Hop-by-Hop Options header, drop packets containing a Hop-by-Hop
Options header, or assign packets containing a Hop-by-Hop Options
header to a slow processing path. Designers considering defining new
hop-by-hop options need to be aware of this likely behavior."
Network-layer optional headers explicitly indicate the information
that is exposed, whereas use of exposed transport header information
first requires an observer to identify the transport protocol and its
format. See Section 2.2.
An arbitrary path can include one or more network devices that drop
packets that include a specific header or option used for this
purpose (see [RFC7872]). This could impact the proper functioning of
the protocols using the path. Protocol methods can be designed to
probe to discover whether the specific option(s) can be used along
the current path, enabling use on arbitrary paths.
5.2. Common Exposed Transport Information
There are opportunities for multiple transport protocols to
consistently supply common observable information [RFC8558]. A
common approach can result in an open definition of the observable
fields. This has the potential that the same information can be
utilised across a range of operational and analysis tools.
5.3. Considerations for Exposing Transport Information
Considerations concerning what information, if any, it is appropriate
to expose include:
* On the one hand, explicitly exposing derived fields containing
relevant transport information (e.g., metrics for loss, latency,
etc.) can avoid network devices needing to derive this information
from other header fields. This could result in development and
evolution of transport-independent tools around a common
observable header and permit transport protocols to also evolve
independently of this ossified header [RFC8558].
* On the other hand, protocols and implementations might be designed
to avoid consistently exposing external information that
corresponds to the actual internal information used by the
protocol itself. An endpoint/protocol could choose to expose
transport header information to optimise the benefit it gets from
the network [RFC8558]. The value of this information for
analysing operation of the transport layer would be enhanced if
the exposed information could be verified to match the transport
protocol's observed behavior.
The motivation to include actual transport header information and the
implications of network devices using this information has to be
considered when proposing such a method. [RFC8558] summarises this
as:
| When signals from endpoints to the path are independent from the
| signals used by endpoints to manage the flow's state mechanics,
| they may be falsified by an endpoint without affecting the peer's
| understanding of the flow's state. For encrypted flows, this
| divergence is not detectable by on-path devices.
6. Addition of Transport OAM Information to Network-Layer Headers
Even when the transport headers are encrypted, on-path devices can
make measurements by utilising additional protocol headers carrying
OAM information in an additional packet header. OAM information can
be included with packets to perform functions, such as identification
of transport protocols and flows, to aide understanding of network or
transport performance or to support network operations or mitigate
the effects of specific network segments.
Using network-layer approaches to reveal information has the
potential that the same method (and hence same observation and
analysis tools) can be consistently used by multiple transport
protocols. This approach also could be applied to methods beyond OAM
(see Section 5). There can also be less desirable implications from
separating the operation of the transport protocol from the
measurement framework.
6.1. Use of OAM within a Maintenance Domain
OAM information can be restricted to a maintenance domain, typically
owned and operated by a single entity. OAM information can be added
at the ingress to the 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 [IOAM-DATA] or as a part of the
encapsulation protocol). This additional header information is not
delivered to the endpoints and is typically removed at the egress of
the maintenance domain.
Although some types of measurements are supported, this approach does
not cover the entire range of measurements described in this
document. In some cases, it can be difficult to position measurement
tools at the appropriate segments/nodes, and there can be challenges
in correlating the downstream/upstream information when in-band OAM
data is inserted by an on-path device.
6.2. Use of OAM across Multiple Maintenance Domains
OAM information can also be added at the network layer by the sender
as an IPv6 extension header or an IPv4 option or in an encapsulation/
tunnel header that also includes an extension header or 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 carries header fields that can be
used to observe timestamps and packet sequence numbers. This
information could be authenticated by a receiving transport endpoint
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 needs to be explicitly enabled at the sender.
7. Conclusions
Header authentication and encryption and strong integrity checks are
being incorporated into new transport protocols and have important
benefits. The pace of the development of transports using the WebRTC
data channel and the rapid deployment of the QUIC transport protocol
can both be attributed to using the combination of UDP as a substrate
while providing confidentiality and authentication of the
encapsulated transport headers and payload.
This document has described some current practises, and the
implications for some stakeholders, when transport-layer header
encryption is used. It does not judge whether these practises are
necessary or endorse the use of any specific practise. Rather, the
intent is to highlight operational tools and practises to consider
when designing and modifying transport protocols, so protocol
designers can make informed choices about what transport header
fields to encrypt and whether it might be beneficial to make an
explicit choice to expose certain fields to devices on the network
path. In making such a decision, it is important to balance:
User Privacy:
The less transport header information that is exposed to the
network, the lower the risk of leaking metadata that might have
user privacy implications. Transports that chose to expose some
header fields need to make a privacy assessment to understand the
privacy cost versus benefit trade-off in making that information
available. The design of the QUIC spin bit to the network is an
example of such considered analysis.
Transport Ossification:
Unencrypted transport header fields are likely to ossify rapidly,
as network devices come to rely on their presence, making it
difficult to change the transport in future. This argues that the
choice to expose information to the network is made deliberately
and with care, since it is essentially defining a stable interface
between the transport and the network. Some protocols will want
to make that interface as limited as possible; other protocols
might find value in exposing certain information to signal to the
network or in allowing the network to change certain header fields
as signals to the transport. The visible wire image of a protocol
should be explicitly designed.
Network Ossification:
While encryption can reduce ossification of the transport
protocol, it does not itself prevent ossification of the network
service. People seeking to understand network traffic could still
come to rely on pattern inferences and other heuristics or machine
learning to derive measurement data and as the basis for network
forwarding decisions [RFC8546]. This creates dependencies on the
transport protocol or the patterns of traffic it can generate,
resulting in ossification of the service.
Impact on Operational Practice:
The network operations community has long relied on being able to
understand Internet traffic patterns, both in aggregate and at the
flow level, to support network management, traffic engineering,
and troubleshooting. Operational practice has developed based on
the information available from unencrypted transport headers. The
IETF has supported this practice by developing operations and
management specifications, interface specifications, and
associated Best Current Practices. Widespread deployment of
transport protocols that encrypt their information will impact
network operations unless operators can develop alternative
practises that work without access to the transport header.
Pace of Evolution:
Removing obstacles to change can enable an increased pace of
evolution. If a protocol changes its transport header format
(wire image) or its transport behaviour, this can result in the
currently deployed tools and methods becoming no longer relevant.
Where this needs to be accompanied by development of appropriate
operational support functions and procedures, it can incur a cost
in new tooling to catch up with each change. Protocols that
consistently expose observable data do not require such
development but can suffer from ossification and need to consider
if the exposed protocol metadata has privacy implications. There
is no single deployment context; therefore, designers need to
consider the diversity of operational networks (ISPs, enterprises,
DDoS mitigation and firewall maintainers, etc.).
Supporting Common Specifications:
Common, open, transport specifications can stimulate engagement by
developers, users, researchers, and the broader community.
Increased protocol diversity can be beneficial in meeting new
requirements, but the ability to innovate without public scrutiny
risks point solutions that optimise for specific cases and that
can accidentally disrupt operations of/in different parts of the
network. The social contract that maintains the stability of the
Internet relies on accepting common transport specifications and
on it being possible to detect violations. The existence of
independent measurements, transparency, and public scrutiny of
transport protocol behaviour helps the community to enforce the
social norm that protocol implementations behave fairly and
conform (at least mostly) to the specifications. It is important
to find new ways of maintaining that community trust as increased
use of transport header encryption limits visibility into
transport behaviour (see also Section 5.3).
Impact on Benchmarking and Understanding Feature Interactions:
An appropriate vantage point for observation, coupled with timing
information about traffic flows, provides a valuable tool for
benchmarking network devices, endpoint stacks, and/or
configurations. This can help understand complex feature
interactions. An inability to observe transport header
information can make it harder to diagnose and explore
interactions between features at different protocol layers, a side
effect of not allowing a choice of vantage point from which this
information is observed. New approaches might have to be
developed.
Impact on Research and Development:
Hiding transport header information can impede independent
research into new mechanisms, measurements of behaviour, and
development initiatives. Experience shows that transport
protocols are complicated to design and complex to deploy and that
individual mechanisms have to be evaluated while considering other
mechanisms across a broad range of network topologies and with
attention to the impact on traffic sharing the capacity. If
increased use of transport header encryption results in reduced
availability of open data, it could eliminate the independent
checks to the standardisation process that have previously been in
place from research and academic contributors (e.g., the role of
the IRTF Internet Congestion Control Research Group (ICCRG) and
research publications in reviewing new transport mechanisms and
assessing the impact of their deployment).
Observable transport header information might be useful to various
stakeholders. Other sets of stakeholders have incentives to limit
what can be observed. This document does not make recommendations
about what information ought to be exposed, to whom it ought to be
observable, or how this will be achieved. There are also design
choices about where observable fields are placed. For example, one
location could be a part of the transport header outside of the
encryption envelope; another alternative is to carry the information
in a network-layer option or extension header. New transport
protocol designs ought to explicitly identify any fields that are
intended to be observed, consider if there are alternative ways of
providing the information, and reflect on the implications of
observable fields being used by on-path network devices and how this
might impact user privacy and protocol evolution when these fields
become ossified.
As [RFC7258] notes, "Making networks unmanageable to mitigate PM is
not an acceptable outcome, but ignoring PM would go against the
consensus documented here." Providing explicit information can help
avoid traffic being inappropriately classified, impacting application
performance. An appropriate balance will emerge over time as real
instances of this tension are analysed [RFC7258]. This balance
between information exposed and information hidden ought to be
carefully considered when specifying new transport protocols.
8. Security Considerations
This document is about design and deployment considerations for
transport protocols. Issues relating to security are discussed
throughout this document.
Authentication, confidentiality protection, and integrity protection
are identified as transport features by [RFC8095]. As currently
deployed in the Internet, these features are generally provided by a
protocol or layer on top of the transport protocol [RFC8922].
Confidentiality and strong integrity checks have properties that can
also be incorporated into the design of a transport protocol or to
modify an existing transport. Integrity checks can protect an
endpoint from undetected modification of protocol fields by on-path
network devices, whereas encryption and obfuscation or greasing can
further prevent these headers being utilised by network devices
[RFC8701]. Preventing observation of headers provides an opportunity
for greater freedom to update the protocols and can ease
experimentation with new techniques and their final deployment in
endpoints. A protocol specification needs to weigh the costs of
ossifying common headers versus the potential benefits of exposing
specific information that could be observed along the network path to
provide tools to manage new variants of protocols.
Header encryption can provide confidentiality of some or all of the
transport header information. This prevents an on-path device from
gaining knowledge of the header field. It therefore prevents
mechanisms being built that directly rely on the information or seeks
to infer semantics of an exposed header field. Reduced visibility
into transport metadata can limit the ability to measure and
characterise traffic and conversely can provide privacy benefits.
Extending the transport payload security context to also include the
transport protocol header protects both types of information with the
same key. A privacy concern would arise if this key was shared with
a third party, e.g., providing access to transport header information
to debug a performance issue would also result in exposing the
transport payload data to the same third party. Such risks would be
mitigated using a layered security design that provides one domain of
protection and associated keys for the transport payload and
encrypted transport headers and a separate domain of protection and
associated keys for any observable transport header fields.
Exposed transport headers are sometimes utilised as a part of the
information to detect anomalies in network traffic. As stated in
[RFC7258], "While PM is an attack, other forms of monitoring that
might fit the definition of PM can be beneficial and not part of any
attack, e.g., network management functions monitor packets or flows
and anti-spam mechanisms need to see mail message content." 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, for malware detection, or to perform
packet "scrubbing" (the normalisation of packets so that there are no
ambiguities in interpretation by the ultimate destination of the
packet). These techniques are currently used by some operators to
also defend from distributed DoS attacks.
Exposed transport header fields can also form a part of the
information used by the receiver of a transport protocol to protect
the transport layer from data injection by an attacker. In
evaluating this use of exposed header information, it is important to
consider whether it introduces a significant DoS threat. For
example, an attacker could construct a DoS attack by sending packets
with a sequence number that falls within the currently accepted range
of sequence numbers at the receiving endpoint. This would then
introduce additional work at the receiving endpoint, even though the
data in the attacking packet might not finally be delivered by the
transport layer. This is sometimes known as a "shadowing attack".
An attack can, for example, disrupt receiver processing, trigger loss
and retransmission, or make a receiving endpoint perform unproductive
decryption of packets that cannot be successfully decrypted (forcing
a receiver to commit decryption resources, or to update and then
restore protocol state).
One mitigation to off-path attacks is to deny knowledge of what
header information is accepted by a receiver or obfuscate the
accepted header information, e.g., setting a nonpredictable initial
value for a sequence number during a protocol handshake, as in
[RFC3550] and [RFC6056], or a port value that cannot be predicted
(see Section 5.1 of [RFC8085]). A receiver could also require
additional information to be used as a part of a validation check
before accepting packets at the transport layer, e.g., utilising a
part of the sequence number space that is encrypted or by verifying
an encrypted token not visible to an attacker. This would also
mitigate against on-path attacks. An additional processing cost can
be incurred when decryption is attempted before a receiver discards
an injected packet.
The existence of open transport protocol standards and a research and
operations community with a history of independent observation and
evaluation of performance data encourage fairness and conformance to
those standards. This suggests careful consideration will be made
over where, and when, measurement samples are collected. An
appropriate balance between encrypting some or all of the transport
header information needs to be considered. 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 "A Framework for Large-
Scale Measurement of Broadband Performance (LMAP)" [RFC7594] contain
considerations for Active and Passive measurement techniques and
supporting material on measurement context.
Addition of observable transport information to the path increases
the information available to an observer and may, when this
information can be linked to a node or user, reduce the privacy of
the user. See the security considerations of [RFC8558].
9. IANA Considerations
This document has no IANA actions.
10. Informative References
[bufferbloat]
Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
the Internet", Communications of the ACM, Vol. 55, no. 1,
pp. 57-65, DOI 10.1145/2063176.2063196, January 2012,
<https://doi.org/10.1145/2063176.2063196>.
[DTLS] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
dtls13-43>.
[IOAM-DATA]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", Work in Progress, Internet-Draft, draft-
ietf-ippm-ioam-data-12, 21 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-ippm-
ioam-data-12>.
[IPV6-ALT-MARK]
Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
Pang, "IPv6 Application of the Alternate Marking Method",
Work in Progress, Internet-Draft, draft-ietf-6man-ipv6-
alt-mark-06, 31 May 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
ipv6-alt-mark-06>.
[Latency] Briscoe, B., Brunstrom, A., Petlund, A., Hayes, D., Ros,
D., Tsang, I., Gjessing, S., Fairhurst, G., Griwodz, C.,
and M. Welzl, "Reducing Internet Latency: A Survey of
Techniques and Their Merits", IEEE Communications Surveys
& Tutorials, vol. 18, no. 3, pp. 2149-2196, thirdquarter
2016, DOI 10.1109/COMST.2014.2375213, November 2014,
<https://doi.org/10.1109/COMST.2014.2375213>.
[Measurement]
Fairhurst, G., Kuehlewind, M., and D. Lopez, "Measurement-
based Protocol Design", European Conference on Networks
and Communications, Oulu, Finland., June 2017.
[PAM-RTT] Trammell, B. and M. Kuehlewind, "Revisiting the Privacy
Implications of Two-Way Internet Latency Data", Passive
and Active Measurement, March 2018.
[PLUS-ABSTRACT-MECH]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", Work in Progress, Internet-
Draft, draft-trammell-plus-abstract-mech-00, 28 September
2016, <https://datatracker.ietf.org/doc/html/draft-
trammell-plus-abstract-mech-00>.
[QLOG] Marx, R., Niccolini, L., and M. Seemann, "Main logging
schema for qlog", Work in Progress, Internet-Draft, draft-
ietf-quic-qlog-main-schema-00, 10 June 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
qlog-main-schema-00>.
[Quic-Trace]
"QUIC trace utilities", Commit 413c3a4,
<https://github.com/google/quic-trace>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, DOI 10.17487/RFC2410,
November 1998, <https://www.rfc-editor.org/info/rfc2410>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2507, DOI 10.17487/RFC2507, February
1999, <https://www.rfc-editor.org/info/rfc2507>.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508,
DOI 10.17487/RFC2508, February 1999,
<https://www.rfc-editor.org/info/rfc2508>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[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>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[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>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion
Control Mechanisms", RFC 5166, DOI 10.17487/RFC5166, March
2008, <https://www.rfc-editor.org/info/rfc5166>.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A., and R.
Whitner, "Improved Packet Reordering Metrics", RFC 5236,
DOI 10.17487/RFC5236, June 2008,
<https://www.rfc-editor.org/info/rfc5236>.
[RFC5426] Okmianski, A., "Transmission of Syslog Messages over UDP",
RFC 5426, DOI 10.17487/RFC5426, March 2009,
<https://www.rfc-editor.org/info/rfc5426>.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
March 2009, <https://www.rfc-editor.org/info/rfc5481>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.
[RFC6294] Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
2011, <https://www.rfc-editor.org/info/rfc6294>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC6846] Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West,
"RObust Header Compression (ROHC): A Profile for TCP/IP
(ROHC-TCP)", RFC 6846, DOI 10.17487/RFC6846, January 2013,
<https://www.rfc-editor.org/info/rfc6846>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7098] Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
Flow Label for Load Balancing in Server Farms", RFC 7098,
DOI 10.17487/RFC7098, January 2014,
<https://www.rfc-editor.org/info/rfc7098>.
[RFC7126] Gont, F., Atkinson, R., and C. Pignataro, "Recommendations
on Filtering of IPv4 Packets Containing IPv4 Options",
BCP 186, RFC 7126, DOI 10.17487/RFC7126, February 2014,
<https://www.rfc-editor.org/info/rfc7126>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC7594] Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
Aitken, P., and A. Akhter, "A Framework for Large-Scale
Measurement of Broadband Performance (LMAP)", RFC 7594,
DOI 10.17487/RFC7594, September 2015,
<https://www.rfc-editor.org/info/rfc7594>.
[RFC7605] Touch, J., "Recommendations on Using Assigned Transport
Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
August 2015, <https://www.rfc-editor.org/info/rfc7605>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/info/rfc7872>.
[RFC7928] Kuhn, N., Ed., Natarajan, P., Ed., Khademi, N., Ed., and
D. Ros, "Characterization Guidelines for Active Queue
Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July
2016, <https://www.rfc-editor.org/info/rfc7928>.
[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
<https://www.rfc-editor.org/info/rfc7983>.
[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
March 2017, <https://www.rfc-editor.org/info/rfc8086>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8250] Elkins, N., Hamilton, R., and M. Ackermann, "IPv6
Performance and Diagnostic Metrics (PDM) Destination
Option", RFC 8250, DOI 10.17487/RFC8250, September 2017,
<https://www.rfc-editor.org/info/rfc8250>.
[RFC8289] Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
Iyengar, Ed., "Controlled Delay Active Queue Management",
RFC 8289, DOI 10.17487/RFC8289, January 2018,
<https://www.rfc-editor.org/info/rfc8289>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.
[RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of
Pervasive Encryption on Operators", RFC 8404,
DOI 10.17487/RFC8404, July 2018,
<https://www.rfc-editor.org/info/rfc8404>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8462] Rooney, N. and S. Dawkins, Ed., "Report from the IAB
Workshop on Managing Radio Networks in an Encrypted World
(MaRNEW)", RFC 8462, DOI 10.17487/RFC8462, October 2018,
<https://www.rfc-editor.org/info/rfc8462>.
[RFC8517] Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
Jacquenet, "An Inventory of Transport-Centric Functions
Provided by Middleboxes: An Operator Perspective",
RFC 8517, DOI 10.17487/RFC8517, February 2019,
<https://www.rfc-editor.org/info/rfc8517>.
[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[RFC8548] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic Protection of TCP Streams
(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
<https://www.rfc-editor.org/info/rfc8548>.
[RFC8558] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zúñiga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
[RFC8837] Jones, P., Dhesikan, S., Jennings, C., and D. Druta,
"Differentiated Services Code Point (DSCP) Packet Markings
for WebRTC QoS", RFC 8837, DOI 10.17487/RFC8837, January
2021, <https://www.rfc-editor.org/info/rfc8837>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
[RFC8922] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Wood, "A Survey of the Interaction between Security
Protocols and Transport Services", RFC 8922,
DOI 10.17487/RFC8922, October 2020,
<https://www.rfc-editor.org/info/rfc8922>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
Acknowledgements
The authors would like to thank Mohamed Boucadair, Spencer Dawkins,
Tom Herbert, Jana Iyengar, Mirja Kühlewind, Kyle Rose, Kathleen
Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris
Wood, Thomas Fossati, Mohamed Boucadair, Martin Thomson, David Black,
Martin Duke, Joel Halpern, and members of TSVWG for their comments
and feedback.
This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No 688421 and
the EU Stand ICT Call 4. The opinions expressed and arguments
employed reflect only the authors' views. The European Commission is
not responsible for any use that might be made of that information.
This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
Authors' Addresses
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen, Scotland
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow, Scotland
G12 8QQ
United Kingdom
Email: csp@csperkins.org
URI: https://csperkins.org/
ERRATA
