TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational C. Perkins
Expires: March 31, 2019 University of Glasgow
September 27, 2018

The Impact of Transport Header Confidentiality on Network Operation and Evolution of the Internet


This document describes implications of applying end-to-end encryption at the transport layer. It identifies in-network uses of transport layer header information. It then reviews the implications of developing end-to-end transport protocols that use authentication to protect the integrity of transport information or encryption to provide confidentiality of the transport protocol header and expected implications of transport protocol design and network operation. Since transport measurement and analysis of the impact of network characteristics have been important to the design of current transport protocols, it also considers the impact on transport and application evolution.

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This Internet-Draft will expire on March 31, 2019.

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Table of Contents

1. Introduction

This document describes implications of applying end-to-end encryption at the transport layer. It reviews the implications of developing end-to-end transport protocols that use encryption to provide confidentiality of the transport protocol header and expected implications of transport protocol design and network operation. It also considers anticipated implications on transport and application evolution.

2. Context and Rationale

The transport layer provides end-to-end interactions between endpoints (processes) using an Internet path. Transport protocols layer directly over the network-layer service and are sent in the payload of network-layer packets. They support end-to-end communication between applications, supported by higher-layer protocols, running on the end systems (or transport endpoints). This simple architectural view hides one of the core functions of the transport, however, to discover and adapt to the properties of the Internet path that is currently being used. The design of Internet transport protocols is as much about trying to avoid the unwanted side effects of congestion on a flow and other capacity-sharing flows, avoiding congestion collapse, adapting to changes in the path characteristics, etc., as it is about end-to-end feature negotiation, flow control and optimising for performance of a specific application.

To achieve stable Internet operations the IETF transport community has to date relied heavily on measurement and insights of the network operations community to understand the trade-offs, and to inform selection of appropriate mechanisms, to ensure a safe, reliable, and robust Internet (e.g., [RFC1273]). In turn, the network operations community relies on being able to understand the pattern and requirements of traffic passing over the Internet, both in aggregate and at the flow level.

There are many motivations for deploying encrypted transports [RFC7624] (i.e., transport protocols that use encryption to provide confidentiality of some or all of the transport-layer header information), and encryption of transport payloads (i.e. confidentiality of the payload data). The increasing public concerns about the interference with Internet traffic have led to a rapidly expanding deployment of encryption to protect end-user privacy, in protocols like QUIC [I-D.ietf-quic-transport], but also expected to form a basis of future protocol designs.

Some network operators and access providers, have come to rely on the in-network measurement of transport properties and the functionality provided by middleboxes to both support network operations and enhance performance. There can therefore be implications when working with encrypted transport protocols that hide transport header information from the network. These present architectural challenges and considerations in the way transport protocols are designed, and ability to characterise and compare different transport solutions [Measure], Section 3.2. Implementations of network devices are encouraged to avoid side-effects when protocols are updated. Introducing cryptographic integrity checks to header fields can also prevent undetected manipulation of the field by network devices, or undetected addition of information to a packet. However, this does not prevent inspection of the information by a device on path, and it is possible that such devices could develop mechanisms that rely on the presence of such a field, or a known value in the field.

Reliance on the presence and semantics of specific header information leads to ossification: An endpoint could be required to supply a specific header to receive the network service that it desires. In some cases, this could be benign or advantageous to the protocol (e.g., recognising the start of a connection, or explicitly exposing protocol information can be expected to provide more consistent decisions by on-path devices than the use of diverse methods to infer semantics from other flow properties). In some cases, this is not beneficial (e.g., a mechanism implemented in a network device, such as a firewall, that required a header field to have only a specific known set of values could prevent the device from forwarding packets using a different version of a protocol that introduces a new feature that changes the value present in this field, preventing evolution of the protocol).

Examples of the impact of ossification on transport protocol design and ease of deployment can be seen in the case of Multipath TCP (MPTCP) and the TCP Fast Open option. The design of MPTCP had to be revised to account for middleboxes, so called "TCP Normalizers", that monitor the evolution of the window advertised in the TCP headers and that reset connections if the window does not grow as expected. Similarly, TCP Fast Open has had issues with middleboxes that remove unknown TCP options, that drop segments with unknown TCP options, that drop segments that contain data and have the SYN bit set, that drop packets with SYN/ACK that acknowledge data, or that disrupt connections that send data before the three-way handshake completes. In both cases, the issue was caused by middleboxes that had a hard-coded understanding of transport behaviour, and that interacted poorly with transports that tried to change that behaviour. Other examples have included middleboxes that rewrite TCP sequence and acknowledgement numbers but are unaware of the (newer) SACK option and don't correctly rewrite selective acknowledgements to match the changes made to the fixed TCP header; or devices that inspect, and change, TCP MSS options that can interfere with path MTU discovery.

A protocol design that uses header encryption can provide confidentiality of some or all of the protocol header information. This prevents an on-path device from knowledge of the header field. It therefore prevents mechanisms being built that directly rely on the information or seeks to imply semantics of an exposed header field. Using encryption to provide confidentiality of the transport layer brings some well-known privacy and security benefits and can therefore help reduce ossification of the transport layer. In particular, it is important that protocols either do not expose information where the usage may change in future protocols, or that methods that utilise the information are robust to potential changes as protocols evolve over time. To avoid unwanted inspection, a protocol could also intentionally vary the format and value of header fields (sometimes known as Greasing [I-D.thomson-quic-grease]). However, while encryption hides the protocol header information, it does not prevent ossification of the network service: People seeking understanding of network traffic could come to rely on pattern inferences and other heuristics as the basis for network decision and to derive measurement data, creating new dependencies on the transport protocol.

A level of ossification of the transport header can offer trade-offs around authentication, and confidentiality of transport protocol headers and has the potential to explicitly support for other uses of this header information. For example, a design that provides confidentiality of protocol header information can impact the following activities that rely on measurement and analysis of traffic flows:

Network Operations and Research:
Observable transport headers enable both operators and the research community to measure and analyse protocol performance, network anomalies, and failure pathologies.
This information can help inform capacity planning, and assist in determining the need for equipment and/or configuration changes by network operators.
The data can also inform Internet engineering research, and help in the development of new protocols, methodologies, and procedures. Concealing the transport protocol header information makes the stream performance unavailable to passive observers along the path, and likely leads to the development of alternative methods to collect or infer that data.
Providing confidentiality of the transport payload, but leaving some, or all, of the transport headers unencrypted, possibly with authentication, can provide the majority of the privacy and security benefits while allowing some measurement.
Protection from Denial of Service:
Observable transport headers currently provide useful input to classify traffic and detect anomalous events (e.g., changes in application behaviour, distributed denial of service attacks). To be effective, this protection needs to be able to uniquely disambiguate unwanted traffic. An inability to separate this traffic using packet header information may result in less-efficient identification of unwanted traffic or development of different methods (e.g. rate-limiting of uncharacterised traffic).
Network Troubleshooting and Diagnostics:
Encrypting transport header information eliminates the incentive for operators to troubleshoot what they cannot interpret. A flow experiencing packet loss or jitter looks like an unaffected flow when only observing network layer headers (if transport sequence numbers and flow identifiers are obscured). This limits understanding of the impact of packet loss or latency on the flows, or even localizing the network segment causing the packet loss or latency. Encrypted traffic may imply "don't touch" to some, and could limit a trouble-shooting response to "can't help, no trouble found". The additional mechanisms that will need to be introduced to help reconstruct transport-level metrics add complexity and operational costs (e.g., in deploying additional functions in equipment or adding traffic overhead).
Network Traffic Analysis:
Hiding transport protocol header information can make it harder to determine which transport protocols and features are being used across a network segment and to measure trends in the pattern of usage. This could impact the ability for an operator to anticipate the need for network upgrades and roll-out. It can also impact the on-going traffic engineering activities performed by operators (such as determining which parts of the path contribute delay, jitter or loss). While the impact may, in many cases, be small there are scenarios where operators directly support particular services (e.g., to troubleshoot issues relating to Quality of Service, QoS; the ability to perform fast re-routing of critical traffic, or support to mitigate the characteristics of specific radio links). The more complex the underlying infrastructure the more important this impact.
Open and Verifiable Network Data:
Hiding transport protocol header information can reduce the range of actors that can capture useful measurement data. For example, one approach could be to employ an existing transport protocol that reveals little information (e.g., UDP), and perform traditional transport functions at higher layers protecting the confidentiality of transport information. Such a design, limits the information sources available to the Internet community to understand the operation of new transport protocols, so preventing access to the information necessary to inform design decisions and standardisation of the new protocols and related operational practices.
The cooperating dependence of network, application, and host to provide communication performance on the Internet is uncertain when only endpoints (i.e., at user devices and within service platforms) can observe performance, and performance cannot be independently verified by all parties. The ability of other stakeholders to review code can help develop deeper insight. In the heterogeneous Internet, this helps extend the range of topologies, vendor equipment, and traffic patterns that are evaluated.
Independently captured data is important to help ensure the health of the research and development communities. It can provide input and test scenarios to support development of new transport protocol mechanisms, especially when this analysis can be based on the behaviour experienced in a diversity of deployed networks.
Independently verifiable performance metrics might also be important to demonstrate regulatory compliance in some jurisdictions, and provides an important basis for informing design decisions.

The last point leads us to consider the impact of hiding transport headers in the specification and development of protocols and standards. This has potential impact on:

In summary, there are trade offs. On the one hand, protocol designers have often ignored the implications of whether the information in transport header fields can or will be used by in-network devices, and the implications this places on protocol evolution. This motivates a design that provides confidentiality of the header information. On the other hand, it can be expected that a lack of visibility of transport header information can impact the ways that protocols are deployed, standardised, and their operational support. The choice of whether future transport protocols encrypt their protocol headers therefore needs to be taken based not solely on security and privacy considerations, but also taking into account the impact on operations, standards, and research. Any new Internet transport need to provide appropriate transport mechanisms and operational support to assure the resulting traffic can not result in persistent congestion collapse [RFC2914]. This document suggests that the balance between information exposed and concealed should be carefully considered when specifying new protocols.

3. Current uses of Transport Headers within the Network

Despite transport headers having end-to-end meaning, some of these transport headers have come to be used in various ways within the Internet. In response to pervasive monitoring [RFC7624] revelations and the IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258], efforts are underway to increase encryption of Internet traffic,. Applying confidentiality to transport header fields would affect how protocol information is used [RFC8404]. To understand these implications, it is first necessary to understand how transport layer headers are currently observed and/or modified by middleboxes within the network.

Transport protocols can be designed to encrypt or authenticate transport header fields. Authentication at the transport layer can be used to detect any changes to an immutable header field that were made by a network device along a path. The intentional modification of transport headers by middleboxes (such as Network Address Translation, NAT, or Firewalls) is not considered. Common issues concerning IP address sharing are described in [RFC6269].

3.1. Observing Transport Information in the Network

If in-network observation of transport protocol headers is needed, this requires knowledge of the format of the transport header:

The following subsections describe various ways that observable transport information has been utilised.

3.1.1. Flow Identification

Transport protocol header information (together with information in the network header), has been used to identify a flow and the connection state of the flow, together with the protocol options being used. In some usages, a low-numbered (well-known) transport port number has been used to identify a protocol (although port information alone is not sufficient to guarantee identification of a protocol, since applications can use arbitrary ports, multiple sessions can be multiplexed on a single port, and ports can be re-used by subsequent sessions).

Transport protocols, such as TCP and Stream Control Transport Protocol (SCTP) specify a standard base header that includes sequence number information and other data, with the possibility to negotiate additional headers at connection setup, identified by an option number in the transport header. UDP-based protocols can use, but sometimes do not use, well-known port numbers. Some flows can instead be identified by signalling protocols or through the use of magic numbers placed in the first byte(s) of the datagram payload.

Flow identification is a common function. For example, performed by measurement activities, QoS classification, firewalls, Denial of Service, DOS, prevention. It becomes more complex and less easily achieved when multiplexing is used at or above the transport layer.

3.1.2. Metrics derived from Transport Layer Headers

Some actors manage their portion of the Internet by characterizing the performance of link/network segments. Passive monitoring uses observed traffic to makes inferences from transport headers to derive these measurements. A variety of open source and commercial tools have been deployed that utilise this information. The following metrics can be derived from transport header information:

Traffic Rate and Volume:
Header information e.g., (sequence number, length) allows derivation of volume measures per-application, to characterise the traffic that uses a network segment or the pattern of network usage. This may be measured per endpoint or for an aggregate of endpoints (e.g., by an operator to assess subscriber usage). It can also be used to trigger measurement-based traffic shaping and to implement QoS support within the network and lower layers. Volume measures can be valuable for capacity planning (providing detail of trends rather than the volume per subscriber).
Loss Rate and Loss Pattern:
Flow loss rate may be derived (e.g., from sequence number) and has been used as a metric for performance assessment and to characterise transport behaviour. Understanding the root cause of loss can help an operator determine whether this requires corrective action. Network operators have used the variation in patterns of loss as a key performance metric, utilising this to detect changes in the offered service.
There are various causes of loss, including: corruption of link frames (e.g., interference on a radio link), buffer overflow (e.g., due to congestion), policing (traffic management), buffer management (e.g., Active Queue Management, AQM [RFC7567]), inadequate provision of traffic preemption. Understanding flow loss rate requires either maintaining per flow packet counters or by observing sequence numbers in transport headers. Loss can be monitored at the interface level by devices in the network. It is often important to understand the conditions under which packet loss occurs. This usually requires relating loss to the traffic flowing on the network node/segment at the time of loss.
Observation of transport feedback information (observing loss reports, e.g., RTP Control Protocol (RTCP) [RFC3550], TCP SACK) can increase understanding of the impact of loss and help identify cases where loss may have been wrongly identified, or the transport did not require the lost packet. It is sometimes more important to understand the pattern of loss, than the loss rate, because losses can often occur as bursts, rather than randomly-timed events.
Throughput and Goodput:
The throughput achieved by a flow can be determined even when a flow is encrypted, providing the individual flow can be identified. Goodput [RFC7928] is a measure of useful data exchanged (the ratio of useful/total volume of traffic sent by a flow). This requires ability to differentiate loss and retransmission of packets (e.g., by observing packet sequence numbers in the TCP or the Real Time Protocol, RTP, headers [RFC3550]).
Latency is a key performance metric that impacts application response time and user-perceived response time. It often indirectly impacts throughput and flow completion time. Latency determines the reaction time of the transport protocol itself, impacting flow setup, congestion control, loss recovery, and other transport mechanisms. The observed latency can have many components [Latency]. Of these, unnecessary/unwanted queuing in network buffers has often been observed as a significant factor. Once the cause of unwanted latency has been identified, this can often be eliminated.
To measure latency across a part of a path, an observation point can measure the experienced round trip time (RTT) using packet sequence numbers, and acknowledgements, or by observing header timestamp information. Such information allows an observation point in the network to determine not only the path RTT, but also to measure the upstream and downstream contribution to the RTT. This has been used to locate a source of latency, e.g., by observing cases where the ratio of median to minimum RTT is large for a part of a path.
The service offered by operators can benefit from latency information to understand the impact of deployment and tune deployed services. Latency metrics are key to evaluating and deploying AQM [RFC7567], DiffServ [RFC2474], and Explicit Congestion Notification (ECN) [RFC3168] [RFC8087]. Measurements could identify excessively large buffers, indicating where to deploy or configure AQM. An AQM method is often deployed in combination with other techniques, such as scheduling [RFC7567] [RFC8290] and although parameter-less methods are desired [RFC7567], current methods [RFC8290] [RFC8289] [RFC8033] often cannot scale across all possible deployment scenarios.
Variation in delay:
Some network applications are sensitive to small changes in packet timing. To assess the performance of such applications, it can be necessary to measure the variation in delay observed along a portion of the path [RFC3393] [RFC5481]. The requirements resemble those for the measurement of latency.
Flow Reordering:
Significant flow reordering can impact time-critical applications and can be interpreted as loss by reliable transports. Many transport protocol techniques are impacted by reordering (e.g., triggering TCP retransmission, or re-buffering of real-time applications). Packet reordering can occur for many reasons (from equipment design to misconfiguration of forwarding rules). Since this impacts transport performance, network tools are needed to detect and measure unwanted/excessive reordering.
There have been initiatives in the IETF transport area to reduce the impact of reordering within a transport flow, possibly leading to a reduction in the requirements for preserving ordering. These have promise to simplify network equipment design as well as the potential to improve robustness of the transport service. Measurements of reordering can help understand the present level of reordering within deployed infrastructure, and inform decisions about how to progress such mechanisms.

Operational tools to detect mis-ordered packet flows and quantify the degree or reordering. Key performance indicators are retransmission rate, packet drop rate, sector utilisation level, a measure of reordering, peak rate, the ECN congestion experienced (CE) marking rate, etc.

Metrics have been defined that evaluate whether a network has maintained packet order on a packet-by-packet basis [RFC4737] and [RFC5236].

Techniques for measuring reordering typically observe packet sequence numbers. Some protocols provide in-built monitoring and reporting functions. Transport fields in the RTP header [RFC3550] [RFC4585] can be observed to derive traffic volume measurements and provide information on the progress and quality of a session using RTP. As with other measurement, metadata is often important to understand the context under which the data was collected, including the time, observation point, and way in which metrics were accumulated. The RTCP protocol directly reports some of this information in a form that can be directly visible in the network. A user of summary measurement data needs to trust the source of this data and the method used to generate the summary information.

3.1.3. Metrics derived from Network Layer Headers

Some transport information is made visible in the network-layer protocol header. These header fields are not encrypted and have been utilised to make flow observations.

Use of IPv6 Network-Layer Flow Label:
Endpoints are encouraged expose flow information in the IPv6 Flow Label field of the network-layer header (e.g., [RFC8085]). This can be used to inform network-layer queuing, forwarding (e.g., for Equal Cost Multi-Path, ECMP, routing, and Link Aggregation, LAG). This can provide useful information to assign packets to flows in the data collected by measurement campaigns. Although important to characterising a path, it does not directly provide performance data.
Use Network-Layer Differentiated Services Code Point Point:
Applications can expose their delivery expectations to the network by setting the Differentiated Services Code Point (DSCP) field of IPv4 and IPv6 packets. This can be used to inform network-layer queuing and forwarding, and can also provide information on the relative importance of packet information collected by measurement campaigns, but does not directly provide performance data.
This field provides explicit information that can be used in place of inferring traffic requirements (e.g., by inferring QoS requirements from port information via a multi-field classifier). The DSCP value can therefore impact the quality of experience for a flow. Observations of service performance need to consider this field when a network path has support for differentiated service treatment.
Use of Explicit Congestion Marking:
ECN [RFC3168] is an optional transport mechanism that uses a code point in the network-layer header. Use of ECN can offer gains in terms of increased throughput, reduced delay, and other benefits when used over a path that includes equipment that supports an AQM method that performs Congestion Experienced (CE) marking of IP packets [RFC8087].
ECN exposes the presence of congestion on a network path to the transport and network layer. The reception of CE-marked packets can therefore be used to monitor the presence and estimate the level of incipient congestion on the upstream portion of the path from the point of observation (Section 2.5 of [RFC8087]). Because ECN marks are carried in the IP protocol header, it is much easier to measure ECN than to measure packet loss. However, interpreting the marking behaviour (i.e., assessing congestion and diagnosing faults) requires context from the transport layer (path RTT, visibility of loss - that could be due to queue overflow, congestion response, etc) [RFC7567].
Some ECN-capable network devices can provide richer (more frequent and fine-grained) indication of their congestion state. Setting congestion marks proportional to the level of congestion (e.g., Data Center TCP, DCTP [RFC8257], and Low Latency Low Loss Scalable throughput, L4S, [I-D.ietf-tsvwg-l4s-arch].
Use of ECN requires a transport to feed back reception information on the path towards the data sender. Exposure of this Transport ECN feedback provides an additional powerful tool to understand ECN-enabled AQM-based networks [RFC8087].
AQM and ECN offer a range of algorithms and configuration options, it is therefore important for tools to be available to network operators and researchers to understand the implication of configuration choices and transport behaviour as use of ECN increases and new methods emerge [RFC7567] [RFC8087]. ECN-monitoring is expected to become important as AQM is deployed that supports ECN [RFC8087].

3.2. Transport Measurement

The common language between network operators and application/content providers/users is packet transfer performance at a layer that all can view and analyse. For most packets, this has been transport layer, until the emergence of QUIC, with the obvious exception of Virtual Private Networks (VPNs) and IPsec.

When encryption conceals more layers in each packet, people seeking understanding of the network operation rely more on pattern inferences and other heuristics reliance on pattern inferences and accuracy suffers. For example, the traffic patterns between server and browser are dependent on browser supplier and version, even when the sessions use the same server application (e.g., web e-mail access). It remains to be seen whether more complex inferences can be mastered to produce the same monitoring accuracy (see section 2.1.1 of [RFC8404]).

When measurement datasets are made available by servers or client endpoints, additional metadata, such as the state of the network, is often required to interpret this data. Collecting and coordinating such metadata is more difficult when the observation point is at a different location to the bottleneck/device under evaluation.

Packet sampling techniques can be used to scale the processing involved in observing packets on high rate links. This exports only the packet header information of (randomly) selected packets. The utility of these measurements depends on the type of bearer and number of mechanisms used by network devices. Simple routers are relatively easy to manage, a device with more complexity demands understanding of the choice of many system parameters. This level of complexity exists when several network methods are combined.

This section discusses topics concerning observation of transport flows, with a focus on transport measurement.

3.2.1. Point of Measurement

Often measurements can only be understood in the context of the other flows that share a bottleneck. A simple example is monitoring of AQM. For example, FQ-CODEL [RFC8290], combines sub queues (statistically assigned per flow), management of the queue length (CODEL), flow-scheduling, and a starvation prevention mechanism. Usually such algorithms are designed to be self-tuning, but current methods typically employ heuristics that can result in more loss under certain path conditions (e.g., large RTT, effects of multiple bottlenecks [RFC7567]).

In-network measurements can distinguish between upstream and downstream metrics with respect to a measurement point. These are particularly useful for locating the source of problems or to assess the performance of a network segment or a particular device configuration. By correlating observations of headers at multiple points along the path (e.g., at the ingress and egress of a network segment), an observer can determine the contribution of a portion of the path to an observed metric (to locate a source of delay, jitter, loss, reordering, congestion marking, etc.).

3.2.2. Use by Operators to Plan and Provision Networks

Traffic measurements (e.g., traffic volume, loss, latency) is used by operators to help plan deployment of new equipment and configurations in their networks. Data is also important to equipment vendors who need to understand traffic trends and patterns of usage as inputs to decisions about planning products and provisioning for new deployments. This measurement information can also be correlated with billing information when this is also collected by an operator.

A network operator supporting traffic that uses transport header encryption may not have access to per-flow measurement data. Trends in aggregate traffic can be observed and can be related to the endpoint addresses being used, but it may not be possible to correlate patterns in measurements with changes in transport protocols (e.g., the impact of changes in introducing a new transport protocol mechanism). This increases the dependency on other indirect sources of information to inform planning and provisioning.

3.2.3. Service Performance Measurement

Traffic measurements (e.g., traffic volume, loss, latency) can be used by various actors to help analyse the performance offered to the users of a network segment, and inform operational practice.

While active measurements may be used in-network, passive measurements can have advantages in terms of eliminating unproductive test traffic, reducing the influence of test traffic on the overall traffic mix, and the ability to choose the point of measurement Section 3.2.1. However, passive measurements may rely on observing transport headers.

3.2.4. Measuring Transport to Support Network Operations

Information provided by tools observing transport headers can help determine whether mechanisms are needed in the network to prevent flows from acquiring excessive network capacity. Operators can implement operational practices to manage traffic flows (e.g., to prevent flows from acquiring excessive network capacity under severe congestion) by deploying rate-limiters, traffic shaping or network transport circuit breakers [RFC8084].

Congestion Control Compliance of Traffic:
Congestion control is a key transport function [RFC2914]. Many network operators implicitly accept that TCP traffic to comply with a behaviour that is acceptable for use in the shared Internet. TCP algorithms have been continuously improved over decades, and they have reached a level of efficiency and correctness that custom application-layer mechanisms will struggle to easily duplicate [RFC8085].
A standards-compliant TCP stack provides congestion control may therefore be judged safe for use across the Internet. Applications developed on top of well-designed transports can be expected to appropriately control their network usage, reacting when the network experiences congestion, by back-off and reduce the load placed on the network. This is the normal expected behaviour for IETF-specified transport (e.g., TCP and SCTP).
However, when anomalies are detected, tools can interpret the transport protocol header information to help understand the impact of specific transport protocols (or protocol mechanisms) on the other traffic that shares a network. An observation in the network can gain understanding of the dynamics of a flow and its congestion control behaviour. Analysing observed packet sequence numbers can be used to help build confidence that an application flow backs-off its share of the network load in the face of persistent congestion, and hence to understand whether the behaviour is appropriate for sharing limited network capacity. For example, it is common to visualise plots of TCP sequence numbers versus time for a flow to understand how a flow shares available capacity, deduce its dynamics in response to congestion, etc.
Congestion Control Compliance for UDP traffic
UDP provides a minimal message-passing datagram transport that has no inherent congestion control mechanisms. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as a transport are required to employ mechanisms to prevent congestion collapse, avoid unacceptable contributions to jitter/latency, and to establish an acceptable share of capacity with concurrent traffic [RFC8085].
A network operator needs tools to understand if datagram flows comply with congestion control expectations and therefore whether there is a need to deploy methods such as rate-limiters, transport circuit breakers or other methods to enforce acceptable usage for the offered service.
UDP flows that expose a well-known header by specifying the format of header fields can allow information to be observed to gain understanding of the dynamics of a flow and its congestion control behaviour. For example, tools exist to monitor various aspects of the RTP and RTCP header information of real-time flows (see Section 3.1.2.

3.3. Use for Network Diagnostics and Troubleshooting

Transport header information can be useful for a variety of operational tasks [RFC8404]: to diagnose network problems, assess network provider performance, evaluate equipment/protocol performance, capacity planning, management of security threats (including denial of service), and responding to user performance questions. Sections 3.1.2 and 5 of [RFC8404] provide further examples. These tasks seldom involve the need to determine the contents of the transport payload, or other application details.

A network operator supporting traffic that uses transport header encryption can see only encrypted transport headers. This prevents deployment of performance measurement tools that rely on transport protocol information. Choosing to encrypt all the information reduces the operator’s ability to observe transport performance, and may limit the ability of network operators to trace problems, make appropriate QoS decisions, or response to other queries about the network service. For some this will be blessing, for others it may be a curse. For example, operational performance data about encrypted flows needs to be determined by traffic pattern analysis, rather than relying on traditional tools. This can impact the ability of the operator to respond to faults, it could require reliance on endpoint diagnostic tools or user involvement in diagnosing and troubleshooting unusual use cases or non-trivial problems. A key need here is for tools to provide useful information during network anomalies (e.g., significant reordering, high or intermittent loss). Although many network operators utilise transport information as a part of their operational practice, the network will not break because transport headers are encrypted, and this may require alternative tools may need to be developed and deployed.

3.3.1. Examples of measurements

Measurements can be used to monitor the health of a portion of the Internet, to provide early warning of the need to take action. They can assist in debugging and diagnosing the root causes of faults that concern a particular user's traffic. They can also be used to support post-mortem investigation after an anomaly to determine the root cause of a problem.

In some case, measurements may involve active injection of test traffic to complete a measurement. However, most operators do not have access to user equipment, and injection of test traffic may be associated with costs in running such tests (e.g., the implications of bandwidth tests in a mobile network are obvious). Some active measurements (e.g., response under load or particular workloads) perturb other traffic, and could require dedicated access to the network segment. An alternative approach is to use in-network techniques that observe transport packet headers in operational networks to make the measurements.

In other cases, measurement involves dissecting network traffic flows. The observed transport layer information can help identify whether the link/network tuning is effective and alert to potential problems that can be hard to derive from link or device measurements alone. The design trade-offs for radio networks are often very different to those of wired networks. A radio-based network (e.g., cellular mobile, enterprise WiFi, satellite access/back-haul, point-to-point radio) has the complexity of a subsystem that performs radio resource management,s with direct impact on the available capacity, and potentially loss/reordering of packets. The impact of the pattern of loss and congestion, differs for different traffic types, correlation with propagation and interference can all have significant impact on the cost and performance of a provided service. The need for this type of information is expected to increase as operators bring together heterogeneous types of network equipment and seek to deploy opportunistic methods to access radio spectrum.

3.4. Observing Headers to Implement Network Policy

Information from the transport protocol can be used by a multi-field classifier as a part of policy framework. Policies are commonly used for management of the QoS or Quality of Experience (QoE) in resource-constrained networks and by firewalls that use the information to implement access rules (see also section 2.2.2 of [RFC8404]). Traffic that cannot be classified, will typically receive a default treatment.

4. Encryption and Authentication of Transport Headers

End-to-end encryption can be applied at various protocol layers. It can be applied above the transport to encrypt the transport payload. Encryption methods can hide information from an eavesdropper in the network. Encryption can also help protect the privacy of a user, by hiding data relating to user/device identity or location. Neither an integrity check nor encryption methods prevent traffic analysis, and usage needs to reflect that profiling of users, identification of location and fingerprinting of behaviour can take place even on encrypted traffic flows.

There are several motivations:

[RFC6437], the DSCP and ECN.

Authentication methods (that provide integrity checks of protocols fields) have also been specified at the network layer, and this also protects transport header fields. The network layer itself carries protocol header fields that are increasingly used to help forwarding decisions reflect the need of transport protocols, such as the IPv6 Flow Label

The use of transport layer authentication and encryption exposes a tussle between middlebox vendors, operators, applications developers and users.

Whatever the motives, a decision to use pervasive of transport header encryption will have implications on the way in which design and evaluation is performed, and which can in turn impact the direction of evolution of the TCP/IP stack. While the IETF can specify protocols, the success in actual deployment is often determined by many factors [RFC5218] that are not always clear at the time when protocols are being defined.

The next subsections briefly review some security design options for transport protocols. A Survey of Transport Security Protocols [I-D.ietf-taps-transport-security] provides more details concerning commonly used encryption methods at the transport layer.

4.1. Authenticating the Transport Protocol Header

Transport layer header information can be authenticated. An integrity check that protects the immutable transport header fields, but can still expose the transport protocol header information in the clear, allowing in-network devices to observes these fields. An integrity check can not prevent in-network modification, but can avoid a receiving accepting changes and avoid impact on the transport protocol operation.

An example transport authentication mechanism is TCP-Authentication (TCP-AO) [RFC5925]. This TCP option authenticates the IP pseudo header, TCP header, and TCP data. TCP-AO protects the transport layer, preventing attacks from disabling the TCP connection itself and provides replay protection. TCP-AO may interact with middleboxes, depending on their behaviour [RFC3234].

The IPsec Authentication Header (AH) [RFC4302] was designed to work at the network layer and authenticate the IP payload. This approach authenticates all transport headers, and verifies their integrity at the receiver, preventing in-network modification.

4.2. Encrypting the Transport Payload

The transport layer payload can be encrypted to protect the content of transport segments. This leaves transport protocol header information in the clear. The integrity of immutable transport header fields could be protected by combining this with an integrity check (Section 4.1).

Examples of encrypting the payload include Transport Layer Security (TLS) over TCP [RFC8446] [RFC7525], Datagram TLS (DTLS) over UDP [RFC6347] [RFC7525], and TCPcrypt [I-D.ietf-tcpinc-tcpcrypt], which permits opportunistic encryption of the TCP transport payload.

4.3. Encrypting the Transport Header

The network layer payload could be encrypted (including the entire transport header and the payload). This method provides confidentiality of the entire transport packet. It therefore does not expose any transport information to devices in the network, which also prevents modification along a network path.

One example of encryption at the network layer is use of IPsec Encapsulating Security Payload (ESP) [RFC4303] in tunnel mode. This encrypts and authenticates all transport headers, preventing visibility of the transport headers by in-network devices. Some Virtual Private Network (VPN) methods also encrypt these headers.

4.4. Authenticating Transport Information and Selectively Encrypting the Transport Header

A transport protocol design can encrypt selected header fields, while also choosing to authenticate fields in the transport header. This allows specific transport header fields to be made observable by network devices. End-to end integrity checks can prevent an endpoint from undetected modification of the immutable transport headers.

Mutable fields in the transport header provide opportunities for middleboxes to modify the transport behaviour (e.g., the extended headers described in [I-D.trammell-plus-abstract-mech]). This considers only immutable fields in the transport headers, that is, fields that may be authenticated End-to-End across a path.

An example of a method that encrypts some, but not all, transport information is GRE-in-UDP [RFC8086] when used with GRE encryption.

4.5. Optional Encryption of Header Information

There are implications to the use of optional header encryption in the design of a transport protocol, where support of optional mechanisms can increase the complexity of the protocol and its implementation and in the management decisions that are required to use variable format fields. Instead, fields of a specific type ought to always be sent with the same level of confidentiality or integrity protection.

5. Addition of Transport Information to Network-Layer Protocol Headers

Transport protocol information can be made visible in a network-layer header. This has the advantage that this information can then be observed by in-network devices. This has the advantage that a single header can support all transport protocols, but there may also be less desirable implications of separating the operation of the transport protocol from the measurement framework.

Some measurements may be made by adding additional protocol headers carrying operations, administration and management (OAM) information to packets at the ingress to a maintenance domain (e.g., an Ethernet protocol header with timestamps and sequence number information using a method such as 802.11ag or in-situ OAM [I-D.ietf-ippm-ioam-data]) and removing the additional header at the egress of the maintenance domain. This approach enables some types of measurements, but does not cover the entire range of measurements described in this document. In some cases, it can be difficult to position measurement tools at the required segments/nodes and there can be challenges in correlating the downsream/upstream information when in-band OAM data is inserted by an on-path device.

Another example of a network-layer approach is the IPv6 Performance and Diagnostic Metrics (PDM) Destination Option [RFC8250]. This allows a sender to optionally include a destination option that caries header fields that can be used to observe timestamps and packet sequence numbers. This information could be authenticated by receiving transport endpoints when the information is added at the sender and visible at the receiving endpoint, although methods to do this have not currently been proposed. This method needs to be explicitly enabled at the sender.

It can be undesirable to rely on methods requiring the presence of network options or extension headers. IPv4 network options are often not supported (or are carried on a slower processing path) and some IPv6 networks are also known to drop packets that set an IPv6 header extension (e.g., [RFC7872]). Another disadvantage is that protocols that separately expose header information do not necessarily have an advantage to expose the information that is utilised by the protocol itself, and could manipulate this header information to gain an advantage from the network.

6. Implications of Protecting the Transport Headers

The choice of which fields to expose and which to encrypt is a design choice for the transport protocol. Any selective encryption method requires trading two conflicting goals for a transport protocol designer to decide which header fields to encrypt. Security work typically employs a design technique that seeks to expose only what is needed. However, there can be performance and operational benefits in exposing selected information to network tools.

This section explores key implications of working with encrypted transport protocols.

6.1. Independent Measurement

Independent observation by multiple actors is important for scientific analysis. Encrypting transport header encryption changes the ability for other actors to collect and independently analyse data. Internet transport protocols employ a set of mechanisms. Some of these need to work in cooperation with the network layer - loss detection and recovery, congestion detection and congestion control, some of these need to work only End-to-End (e.g., parameter negotiation, flow-control).

When encryption conceals information in the transport header, it could be possible for an applications to provide summary data on performance and usage of the network. This data could be made available to other actors. However, this data needs to contain sufficient detail to understand (and possibly reconstruct the network traffic pattern for further testing) and to be correlated with the configuration of the network paths being measured.

Sharing information between actors needs also to consider the privacy of the user and the incentives for providing accurate and detailed information. Protocols that expose the state information used by the transport protocol in their header information (e.g., timestamps used to calculate the RTT, packet numbers used to asses congestion and requests for retransmission) provide an incentive for the sending endpoint to provide correct information, increasing confidence that the observer understands the transport interaction with the network. This becomes important when considering changes to transport protocols, changes in network infrastructure, or the emergence of new traffic patterns.

6.2. Characterising "Unknown" Network Traffic

The patterns and types of traffic that share Internet capacity changes with time as networked applications, usage patterns and protocols continue to evolve.

If "unknown" or "uncharacterised" traffic patterns form a small part of the traffic aggregate passing through a network device or segment of the network the path, the dynamics of the uncharacterised traffic may not have a significant collateral impact on the performance of other traffic that shares this network segment. Once the proportion of this traffic increases, the need to monitor the traffic and determine if appropriate safety measures need to be put in place.

Tracking the impact of new mechanisms and protocols requires traffic volume to be measured and new transport behaviours to be identified. This is especially true of protocols operating over a UDP substrate. The level and style of encryption needs to be considered in determining how this activity is performed. On a shorter timescale, information may also need to be collected to manage denial of service attacks against the infrastructure.

6.3. Accountability and Internet Transport Protocols

Information provided by tools observing transport headers can be used to classify traffic, and to limit the network capacity used by certain flows. Operators can potentially use this information to prioritise or de-prioritise certain flows or classes of flow, with potential implications for network neutrality, or to rate limit malicious or otherwise undesirable flows (e.g., for Distributed Denial of Service, DDOS, protection, or to ensure compliance with a traffic profile Section 3.2.4). Equally, operators could use analysis of transport headers and transport flow state to demonstrate that they are not providing differential treatment to certain flows. Obfuscating or hiding this information using encryption is expected to lead operators and maintainers of middleboxes (firewalls, etc.) to seek other methods to classify, and potentially other mechanisms to condition, network traffic.

A lack of data reduces the level of precision with which flows can be classified and conditioning mechanisms are applied (e.g., rate limiting, circuit breaker techniques [RFC8084], or blocking of uncharacterised traffic), and this needs to be considered when evaluating the impact of designs for transport encryption [RFC5218].

6.4. Impact on Research, Development and Deployment

The majority of present Internet applications use two well-known transport protocols: e.g., TCP and UDP. Although TCP represents the majority of current traffic, some important real-time applications use UDP, and much of this traffic utilises RTP format headers in the payload of the UDP datagram. Since these protocol headers have been fixed for decades, a range of tools and analysis methods have became common and well-understood. Over this period, the transport protocol headers have mostly changed slowly, and so also the need to develop tools track new versions of the protocol.

Looking ahead, there will be a need to update these protocols and to develop and deploy new transport mechanisms and protocols. There are both opportunities and also challenges to the design, evaluation and deployment of new transport protocol mechanisms.

Integrity checks can protect an endpoint from undetected modification of protocol fields by network devices, whereas encryption and obfuscation can further prevent these headers being utilised by network devices. Hiding headers can therefore provide the opportunity for greater freedom to update the protocols and can ease experimentation with new techniques and their final deployment in endpoints.

Hiding headers can limit the ability to measure and characterise traffic. Measurement data is increasingly being used to inform design decisions in networking research, during development of new mechanisms and protocols and in standardisation. Measurement has a critical role in the design of transport protocol mechanisms and their acceptance by the wider community (e.g., as a method to judge the safety for Internet deployment). Observation of pathologies are also important in understanding the interactions between cooperating protocols and network mechanism, the implications of sharing capacity with other traffic and the impact of different patterns of usage.

Evolution and the ability to understand (measure) the impact need to proceed hand-in-hand. Attention needs to be paid to the expected scale of deployment of new protocols and protocol mechanisms. Whatever the mechanism, experience has shown that it is often difficult to correctly implement combination of mechanisms [RFC8085]. These mechanisms therefore typically evolve as a protocol matures, or in response to changes in network conditions, changes in network traffic or changes to application usage.

New transport protocol formats are expected to facilitate an increased pace of transport evolution, and with it the possibility to experiment with and deploy a wide range of protocol mechanisms. There has been recent interest in a wide range of new transport methods, e.g., Larger Initial Window, Proportional Rate Reduction (PRR), congestion control methods based on measuring bottleneck bandwidth and round-trip propagation time, the introduction of AQM techniques and new forms of ECN response (e.g., Data Centre TCP, DCTP, and methods proposed for L4S).The growth and diversity of applications and protocols using the Internet also continues to expand. For each new method or application it is desirable to build a body of data reflecting its behaviour under a wide range of deployment scenarios, traffic load, and interactions with other deployed/candidate methods.

Open standards motivate a desire for this evaluation to include independent observation and evaluation of performance data, which in turn suggests control over where and when measurement samples are collected. This requires consideration of the appropriate balance between encrypting all and no transport information.

7. Conclusions

The majority of present Internet applications use two well-known transport protocols: e.g., TCP and UDP. Although TCP represents the majority of current traffic, some important real-time applications have used UDP, and much of this traffic utilises RTP format headers in the payload of the UDP datagram. Since these protocol headers have been fixed for decades, a range of tools and analysis methods have became common and well-understood. Over this period, the transport protocol headers have mostly changed slowly, and so also the need to develop tools track new versions of the protocol.

Confidentiality and strong integrity checks have properties that are being incorporated into new protocols and which have important benefits. The pace of development of transports using the WebRTC data channel and the rapid deployment of QUIC prototype transports can both be attributed to using a combination of UDP transport and confidentiality of the UDP payload.

The traffic that can be observed by on-path network devices is a function of transport protocol design/options, network use, applications and user characteristics. In general, when only a small proportion of the traffic has a specific (different) characteristic. Such traffic seldom leads to an operational issue although the ability to measure and monitor it is less. The desire to understand the traffic and protocol interactions typically grows as the proportion of traffic increases in volume. The challenges increase when multiple instances of an evolving protocol contribute to the traffic that share network capacity.

An increased pace of evolution therefore needs to be accompanied by methods that can be successfully deployed and used across operational networks. This leads to a need for network operators (at various level (ISPs, enterprises, firewall maintainer, etc) to identify appropriate operational support functions and procedures.

Protocols that change their transport header format (wire format) or their behaviour (e.g., algorithms that are needed to classify and characterise the protocol), will require new tooling needs to be developed to catch-up with the changes. If the currently deployed tools and methods are no longer relevant and performance may not be correctly measured. This can increase the response-time after faults, and can impact the ability to manage the network resulting in traffic causing traffic to be treated inappropriately (e.g., rate limiting because of being incorrectly classified/monitored). There are benefits in exposing consistent information to the network that avoids traffic being mis-classified and then receiving a default treatment by the network.

As a part of its design a new protocol specification therefore needs to weigh the benefits of ossifying common headers, versus the potential demerits of exposing specific information that could be observed along the network path to provide tools to manage new variants of protocols. Several scenarios to illustrate different ways this could evolve are provided below:

The outcome could have significant implications on the way the Internet architecture develops. It exposes a risk that significant actors (e.g., developers and transport designers) achieve more control of the way in which the Internet architecture develops.In particular, there is a possibility that designs could evolve to significantly benefit of customers for a specific vendor, and that communities with very different network, applications or platforms could then suffer at the expense of benefits to their vendors own customer base. In such a scenario, there could be no incentive to support other applications/products or to work in other networks leading to reduced access for new approaches.

8. Security Considerations

This document is about design and deployment considerations for transport protocols. Issues relating to security are discussed in the various sections of the document.

Authentication, confidentiality protection, and integrity protection are identified as Transport Features by [RFC8095]. As currently deployed in the Internet, these features are generally provided by a protocol or layer on top of the transport protocol [I-D.ietf-taps-transport-security].

Confidentiality and strong integrity checks have properties that can also be incorporated into the deisgn of a transport protocol. Integrity checks can protect an endpoint from undetected modification of protocol fields by network devices, whereas encryption and obfuscation can further prevent these headers being utilised by network devices. Hiding headers can therefore provide the opportunity for greater freedom to update the protocols and can ease experimentation with new techniques and their final deployment in endpoints. A protocol specification needs to weigh the benefits of ossifying common headers, versus the potential demerits of exposing specific information that could be observed along the network path to provide tools to manage new variants of protocols.

A protocol design that uses header encryption can provide confidentiality of some or all of the protocol header information. This prevents an on-path device from knowledge of the header field. It therefore prevents mechanisms being built that directly rely on the information or seeks to imply semantics of an exposed header field. Hiding headers can limit the ability to measure and characterise traffic.

Exposed transport headers are sometimes utilised as a part of the information to detect anomalies in network traffic. This can be used as the first line of defence yo identify potential threats from DOS or malware and redirect suspect traffic to dedicated nodes responsible for DOS analysis, malware detection, or to perform packet scrubbing "Scrubbing" (the normalization of packets so that there are no ambiguities in interpretation by the ultimate destination of the packet). These techniques are currently used by some operators to also defend from distributed DOS attacks.

Exposed transport headers are sometimes also utilised as a part of the information used by the receiver of a transport protocol to protect the transport layer from data injection by an attacker. In evaluating this use of exposed header information, it is important to consider whether it introduces a significant DOS threat. For example, an attacker could construct a DOS attack by sending packets with a sequence number that falls within the currently accepted range of sequence numbers at the receiving endpoint, this would then introduce additional work at the receiving endpoint, even though the data in the attacking packet may not finally be delivered by the transport layer. This is sometimes known as a “shadowing attack”. An attack can, for example, disrupt receiver processing, trigger loss and retransmission, or make a receiving endpoint perform unproductive decryption of packets that cannot be successfully decrypted (forcing a receiver to commit decryption resources, or to update and then restore protocol state).

One mitigation to off-path attack is to deny knowledge of what header information is accepted by a receiver or obfusticate the accepted header information, e.g., setting a non-predictable initial value for a sequence number during a protocol handshake, as in [RFC3550] and [RFC6056], or a port value that can not be predicted (see section 5.1 of [RFC8085]). A receiver could also require additional information to be used as a part of 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 on-path attacks. An additional processing cost can be incurred when decryption needs to be attempted before a receiver is able to discard injected packets.

Open standards motivate a desire for this evaluation to include independent observation and evaluation of performance data, which in turn suggests control over where and when measurement samples are collected. This requires consideration of the appropriate balance between encrypting all and no transport information. Open data, and accessibility to tools that can help understand trends in application deployment, network traffic and usage patterns can all contribute to understanding security challenges.

9. IANA Considerations


This memo includes no request to IANA.

10. Acknowledgements

The authors would like to thank Mohamed Boucadair, Spencer Dawkins, Jana Iyengar, Mirja Kuehlewind, Kathleen Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, and other members of the TSVWG for their comments and feedback.

This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688421.The opinions expressed and arguments employed reflect only the authors' view. The European Commission is not responsible for any use that may be made of that information.

This work has received funding from the UK Engineering and Physical Sciences Research Council under grant EP/R04144X/1.

11. Informative References

[I-D.ietf-ippm-ioam-data] Brockners, F., Bhandari, S., Pignataro, C., Gredler, H., Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov, P., Chang, R., daniel.bernier@bell.ca, d. and J. Lemon, "Data Fields for In-situ OAM", Internet-Draft draft-ietf-ippm-ioam-data-03, June 2018.
[I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-14, August 2018.
[I-D.ietf-taps-transport-security] Pauly, T., Perkins, C., Rose, K. and C. Wood, "A Survey of Transport Security Protocols", Internet-Draft draft-ietf-taps-transport-security-02, June 2018.
[I-D.ietf-tcpinc-tcpcrypt] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, Q. and E. Smith, "Cryptographic protection of TCP Streams (tcpcrypt)", Internet-Draft draft-ietf-tcpinc-tcpcrypt-12, June 2018.
[I-D.ietf-tsvwg-l4s-arch] Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture", Internet-Draft draft-ietf-tsvwg-l4s-arch-02, March 2018.
[I-D.thomson-quic-grease] Thomson, M., "More Apparent Randomization for QUIC", Internet-Draft draft-thomson-quic-grease-00, December 2017.
[I-D.trammell-plus-abstract-mech] Trammell, B., "Abstract Mechanisms for a Cooperative Path Layer under Endpoint Control", Internet-Draft draft-trammell-plus-abstract-mech-00, September 2016.
[Latency] Briscoe, B., "Reducing Internet Latency: A Survey of Techniques and Their Merits", November 2014.
[Measure] Fairhurst, G., Kuehlewind, M. and D. Lopez, "Measurement-based Protocol Design", June 2017.
[RFC1273] Schwartz, M., "Measurement Study of Changes in Service-Level Reachability in the Global TCP/IP Internet: Goals, Experimental Design, Implementation, and Policy Considerations", RFC 1273, DOI 10.17487/RFC1273, November 1991.
[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.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, DOI 10.17487/RFC3135, June 2001.
[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.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, DOI 10.17487/RFC3393, November 2002.
[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.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, DOI 10.17487/RFC4302, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[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.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, S. and J. Perser, "Packet Reordering Metrics", RFC 4737, DOI 10.17487/RFC4737, November 2006.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A. and R. Whitner, "Improved Packet Reordering Metrics", RFC 5236, DOI 10.17487/RFC5236, June 2008.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation Applicability Statement", RFC 5481, DOI 10.17487/RFC5481, March 2009.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011.
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P. and P. Roberts, "Issues with IP Address Sharing", RFC 6269, DOI 10.17487/RFC6269, June 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/RFC6437, November 2011.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.
[RFC7567] Baker, F. and G. Fairhurst, "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015.
[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.
[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.
[RFC7928] Kuhn, N., Natarajan, P., Khademi, N. and D. Ros, "Characterization Guidelines for Active Queue Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July 2016.
[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.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017.
[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017.
[RFC8086] Yong, L., Crabbe, E., Xu, X. and T. Herbert, "GRE-in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, March 2017.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using Explicit Congestion Notification (ECN)", RFC 8087, DOI 10.17487/RFC8087, March 2017.
[RFC8095] Fairhurst, G., Trammell, B. and M. Kuehlewind, "Services Provided by IETF Transport Protocols and Congestion Control Mechanisms", RFC 8095, DOI 10.17487/RFC8095, March 2017.
[RFC8250] Elkins, N., Hamilton, R. and M. Ackermann, "IPv6 Performance and Diagnostic Metrics (PDM) Destination Option", RFC 8250, DOI 10.17487/RFC8250, September 2017.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L. and G. Judd, "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257, October 2017.
[RFC8289] Nichols, K., Jacobson, V., McGregor, A. and J. Iyengar, "Controlled Delay Active Queue Management", RFC 8289, DOI 10.17487/RFC8289, January 2018.
[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.
[RFC8404] Moriarty, K. and A. Morton, "Effects of Pervasive Encryption on Operators", RFC 8404, DOI 10.17487/RFC8404, July 2018.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018.

Appendix A. Revision information

-00 This is an individual draft for the IETF community.

-01 This draft was a result of walking away from the text for a few days and then reorganising the content.

-02 This draft fixes textual errors.

-03 This draft follows feedback from people reading this draft.

-04 This adds an additional contributor and includes significant reworking to ready this for review by the wider IETF community Colin Perkins joined the author list.

Comments from the community are welcome on the text and recommendations.

-05 Corrections received and helpful inputs from Mohamed Boucadair.

-06 Updated following comments from Stephen Farrell, and feedback via email. Added a draft conclusion section to sketch some strawman scenarios that could emerge.

-07 Updated following comments from Al Morton, Chris Seal, and other feedback via email.

-08 Updated to address comments sent to the TSVWG mailing list by Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on 11/05/2018, and Spencer Dawkins.

-09 Updated security considerations.

-10 Updated references, split the Introduction, and added a paragraph giving some examples of why ossification has been an issue.

-00 This is the first revision submitted as a working group document.

Authors' Addresses

Godred Fairhurst University of Aberdeen Department of Engineering Fraser Noble Building Aberdeen, AB24 3UE Scotland EMail: gorry@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk/
Colin Perkins University of Glasgow School of Computing Science Glasgow, G12 8QQ Scotland EMail: csp@csperkins.org URI: https://csperkins.org//