]> University of Aberdeen

Department of Engineering Fraser Noble Building Aberdeen AB24 3UE Scotland gorry@erg.abdn.ac.uk http://www.erg.abdn.ac.uk/

University of Glasgow

School of Computing Science Glasgow G12 8QQ Scotland csp@csperkins.org https://csperkins.org//

Transport TSVWG transport design, operations and management 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.

There is increased interest in, and deployment of, new protocols that employ end-to-end encryption at the transport layer, including the transport layer headers. An example of such a transport is the QUIC transport protocol , currently being standardised in the IETF. Encryption of transport layer headers and payload data has many benefits in terms of protecting user privacy. These benefits have been widely discussed, and we strongly support them. There are also, however, some costs, in that the wide use of transport encryption requires changes to network operations, and complicates network measurement for research, operational, and standardisation purposes. This document discusses some consequences of applying end-to-end encryption at the transport layer. It reviews the implications of developing end-to-end transport protocols that use encryption to provide confidentiality of the transport protocol header, and consider the effect of such changes on transport protocol design and network operation. It also considers anticipated implications on transport and application evolution.

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., ). 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 (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 interference with Internet traffic have led to a rapidly expanding deployment of encryption to protect end-user privacy, e.g., QUIC . Encryption is also expected to form a basis of future transport 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 . 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 other 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. 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 seek to infer 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 could 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/or value of header fields (sometimes known as Greasing ). 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. Specification of non-encrypted transport header fields explicitly allows protocol designers to make specific header information observable in the network. This supports other uses of this information by on-path devices, and at the same time this can be expected to lead to ossification of the transport header, because network forwarding could evolve to depend on the presence and/or value of these fields. The decision about which transport headers fields are made observable offers trade-offs around authentication, and confidentiality. 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: 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 supporting operations and research. 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 could result in less-efficient identification of unwanted traffic or development of different methods (e.g. rate-limiting of uncharacterised traffic). 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 could 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). 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 could, 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. 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 utilised to demonstrate regulatory compliance in some jurisdictions, and provides a 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: Understanding Feature Interactions: An appropriate vantage point, coupled with timing information about traffic flows, provides a valuable tool for benchmarking equipment, functions, and/or configurations, and to understand complex feature interactions. An inability to observe transport protocol information can limit the ability to diagnose and explore interactions between features at different protocol layers, a side-effect of not allowing a choice of vantage point from which this information is observed. Supporting Common Specifications: Transmission Control Protocol (TCP) is currently the predominant transport protocol used over Internet paths. Its many variants have broadly consistent approaches to avoiding congestion collapse, and to ensuring the stability of the Internet. Increased use of transport layer encryption can overcome ossification, allowing deployment of new transports and different types of congestion control. This flexibility can be beneficial, but it can come at the cost of fragmenting the ecosystem. There is little doubt that developers will try to produce high quality transports for their intended target uses, but it is not clear there are sufficient incentives to ensure good practice that benefits the wide diversity of requirements for the Internet community as a whole. Increased diversity, and the ability to innovate without public scrutiny, risks point solutions that optimise for specific needs, but accidentally disrupt operations of/in different parts of the network. The social contract that maintains the stability of the Internet relies on accepting common specifications. Operational Practice: The network operations community relies on being able to understand the pattern and requirements of traffic passing over the Internet, both in aggregate and at the flow level. These operational practices have developed based on the information available from unencrypted transport headers. If this information is only carried in encrypted transport headers, operators will not be able to use this information directly, but if operators still wish to use these practices, they may turn to more ambitious ways of discovering this information. For example, if an operator wants to know that traffic is audio traffic, and no longer has access to Session Description Protocol (SDP) session descriptions that would explicitly say a flow "is audio", the operator might use heuristics to guess that short UDP packets with regular spacing are carrying audio traffic. Operational practices aimed at guessing transport parameters are out of scope for this document, and are only mentioned here to recognize that encryption may not prevent operators from attempting to apply the same practices they used with unencrypted transport headers. Compliance: Published transport specifications allow operators and regulators to check compliance. This can bring assurance to those operating networks, often avoiding the need to deploy complex techniques that routinely monitor and manage TCP/IP traffic flows (e.g. Avoiding the capital and operational costs of deploying flow rate-limiting and network circuit-breaker methods ). When it is not possible to observe transport header information, methods are still needed to confirm that the traffic produced conforms to the expectations of the operator or developer. Restricting research and development: Hiding transport information can impede independent research into new mechanisms, measurement of behaviour, and development initiatives. Experience shows that transport protocols are complicated to design and complex to deploy, and that individual mechanisms need to be evaluated while considering other mechanisms, across a broad range of network topologies and with attention to the impact on traffic sharing the capacity. If this results in reduced availability of open data, it could eliminate the independent self-checks to the standardisation process that have previously been in place from research and academic contributors (e.g., the role of the IRTF ICCRG, and research publications in reviewing new transport mechanisms and assessing the impact of their experimental deployment) 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. 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., ,). 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 will need to provide appropriate transport mechanisms and operational support to assure the resulting traffic can not result in persistent congestion collapse . This document suggests that the balance between information exposed and concealed should be carefully considered when specifying new protocols.

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 revelations and the IETF consensus that "Pervasive Monitoring is an Attack" , efforts are underway to increase encryption of Internet traffic,. Applying confidentiality to transport header fields would affect how protocol information is used . 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 .

If in-network observation of transport protocol headers is needed, this requires knowledge of the format of the transport header: Flows need to be identified at the level required to perform the observation; The protocol and version of the header need to be visible, e.g., by defining the wire image . As protocols evolve over time and there could be a need to introduce new transport headers. This could require interpretation of protocol version information or connection setup information; The location and syntax of any observed transport headers needs to be known. IETF transport protocols can specify this information. The following subsections describe various ways that observable transport information has been utilised.

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.

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: 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 can 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). Flow loss rate can 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 ), 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 valuable 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) , TCP SACK) can increase understanding of the impact of loss and help identify cases where loss could have been wrongly identified, or the transport did not require the lost packet. It is sometimes more helpful to understand the pattern of loss, than the loss rate, because losses can often occur as bursts, rather than randomly-timed events. The throughput achieved by a flow can be determined even when a flow is encrypted, providing the individual flow can be identified. Goodput 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 Transport Protocol, RTP, headers ). 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 . 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 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 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 , DiffServ , and Explicit Congestion Notification (ECN) . 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 and although parameter-less methods are desired , current methods often cannot scale across all possible deployment scenarios. 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 . The requirements resemble those for the measurement of latency. 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 and . Techniques for measuring reordering typically observe packet sequence numbers. Some protocols provide in-built monitoring and reporting functions. Transport fields in the RTP header can be observed to derive traffic volume measurements and provide information on the progress and quality of a session using RTP. As with other measurement, metadata is often needed to understand the context under which the data was collected, including the time, observation point, 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.

Network-layer classification methods that rely on a multi-field classifier (e.g. infering QoS from the 5-tuple or choice of application protocol) are incompatible with transport protocols that encrypt the transport information. In contrast, network-layer header fields are not encrypted and can explicitly provide information from the transport layer to enable a different forwarding treatment by the network. This information can be provided by a transport that employs encryption. When a transport multiplexes multiple subflows, the user of the transport could wish to hide the presence and characteristics of these subflows. other uses of an encrypted transport could set the network-layer information to indicate the presence of subflows and to reflect the network needs of individual subflows (e.g., a WebRTC can identify different forwarding treatments for individual packets based on the value of the Differentiated Services Code Point (DSCP) field ). Endpoints are encouraged to set the IPv6 Flow Label field of the network-layer header (e.g., ). The label can provide information that can help inform network-layer queuing, forwarding (e.g., for Equal Cost Multi-Path, ECMP, routing, and Link Aggregation, LAG) . A multiplexing transport could choose to use multiple flow labels to allow the network to independently forward subflows. Applications can expose their delivery expectations to the network by setting the DSCP field of IPv4 and IPv6 packets .This provides explicit information to inform network-layer queuing and forwarding, rather than an operator inferring traffic requirements from transport and application headers via a multi-field classifier. Since the DSCP value can impact the quality of experience for a flow., observations of service performance need to consider this field when a network path has support for differentiated service treatment. ECN is a transport mechanism that utilises the ECN field in the network-layer header. Use of ECN explicitly informs the network-layer that a transport is ECN-capable, and requests ECN treatment of the flows packets. An ECN-capable transport can offer benefits when used over a path with equipment taht implements an AQM method with Congestion Experienced (CE) marking of IP 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 ). Interpreting the marking behaviour (i.e., assessing congestion and diagnosing faults) requires context from the transport layer (such as path RTT). AQM and ECN offer a range of algorithms and configuration options. Tools therefore need to be available to network operators and researchers to understand the implication of configuration choices and transport behaviour as use of ECN increases and new methods emerge .

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 ). 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.

On-path measurements are particularly useful for locating the source of problems or to assess the performance of a network segment or a particular device configuration. Often issues can only be understood in the context of the other flows that share a particular path, common network device, interface port, etc. A simple example is monitoring of a network device that uses a scheduler or active queue management technique , where it could be desirable to understand whether algorithms are correctly controlling latency, or overload protection. This understanding implies knowledge of how traffic is assigned to any sub-queues used for flow scheduling, but can also require information about how the traffic dynamics impact active queue management, starvation prevention mechanisms and circuit-breakers. Sometimes multiple on-path observation points are needed. By correlating observations of headers at multiple points along the path (e.g., at the ingress and egress of a network segment), an observer can determine the contribution of a portion of the path to an observed metric (to locate a source of delay, jitter, loss, reordering, congestion marking, etc.).

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 valuable to equipment vendors who want to understand traffic trends and patterns of usage as inputs to decisions about planning products and provisioning for new deployments. This measurement information can also be correlated with billing information when this is also collected by an operator. A network operator supporting traffic that uses transport header encryption 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 be impossible to correlate patterns in measurements with changes in transport protocols (e.g., the impact of changes in introducing a new transport protocol mechanism). This increases the dependency on other indirect sources of information to inform planning and provisioning.

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 observation . However, passive measurements can rely on observing transport headers.

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 . Congestion control is a key transport function . Many network operators implicitly accept that TCP traffic complies with a behaviour that is acceptable for use in the shared Internet. TCP algorithms have been continuously improved over decades, and they have reached a level of efficiency and correctness that custom application-layer mechanisms will struggle to easily duplicate . 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 flows can help to build confidence that an application flow backs-off its share of the network load in the face of persistent congestion, and hence to understand whether the behaviour is appropriate for sharing limited network capacity. For example, it is common to visualise plots of TCP sequence numbers versus time for a flow to understand how a flow shares available capacity, deduce its dynamics in response to congestion, etc. The ability to identify sources that contribute excessive congestion is important to safe operation of network infrastructure, and mechanisms can inform configuration of network devices to complement the endpoint congestion avoidance mechanisms to avoid a portion of the network being driven into congestion collapse . 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 . 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 .

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

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.

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 ). Network-layer classification methods that rely on a multi-field classifier (e.g. Inferring QoS from the 5-tuple or choice of application protocol) are incompatible with transport protocols that encrypt the transport information. Traffic that cannot be classified, will typically receive a default treatment. Transport information can also be explicitly set in network-layer header fields that are not encrypted. This can provide information to enable a different forwarding treatment by the network, even when a transport employs encryption to protect other header information. When a transport multiplexes multiple subflows, a transport could choose to hide the presence and characteristics of these subflows from the network. However, a transport is permitted to set the network-layer information to indicate the presence of subflows and to reflect the needs of individual subflows (e.g., a WebRTC can identify different forwarding treatments for individual packets based on the value of the DS field ). Endpoints are encouraged to set the IPv6 Flow Label field of the network-layer header (e.g., ). The label can provide information that can help inform network-layer queuing, forwarding (e.g., for Equal Cost Multi-Path, ECMP, routing, and Link Aggregation, LAG) . A multiplexing transport could choose to use multiple flow labels to allow the network to independently forward subflows. Applications can expose their delivery expectations to the network by setting the DSCP field of IPv4 and IPv6 packets . This provides explicit information to inform network-layer queuing and forwarding, rather than an operator inferring traffic requirements from transport and application headers via a multi-field classifier. Since the DSCP value can impact the quality of experience for a flow, observations of service performance need to consider this field when a network path has support for differentiated service treatment. ECN is a transport mechanism that utilises the ECN field in the network-layer header to explicitly inform the network-layer that a transport is ECN-capable and request ECN treatment of the flows packets. This can offer benefits when used over a path with equipment that implements an AQM method with Congestion Experienced (CE) marking of IP packets . 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 ). 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, 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 .

End-to-end encryption can be applied at various protocol layers. It can be applied above the transport to encrypt the transport payload. Encryption methods can hide information from an eavesdropper in the network. Encryption can also help protect the privacy of a user, by hiding data relating to user/device identity or location. Neither an integrity check nor encryption methods prevent traffic analysis, and usage needs to reflect that profiling of users, identification of location and fingerprinting of behaviour can take place even on encrypted traffic flows. Any header information that has a clear definition in the protocol's message format(s), or is implied by that definition, and is not cryptographically confidentiality-protected can be unambiguously interpreted by on-path observers . There are several motivations: One motive to use encryption is a response to perceptions that the network has become ossified by over-reliance on middleboxes that prevent new protocols and mechanisms from being deployed. This has lead to a perception that there is too much “manipulation” of protocol headers within the network, and that designing to deploy in such networks is preventing transport evolution. In the light of this, a method that authenticates transport headers may help improve the pace of transport development, by eliminating the need to always consider deployed middleboxes , or potentially to only explicitly enable middlebox use for particular paths with particular middleboxes that are deliberately deployed to realise a useful function for the network and/or users. Another motivation stems from increased concerns about privacy and surveillance. Some Internet users have valued the ability to protect identity, user location, and defend against traffic analysis, and have used methods such as IPsec Encapsulated Security Payload (ESP), Virtual Private Networks (VPNs) and other encrypted tunnel technologies. Revelations about the use of pervasive surveillance have, to some extent, eroded trust in the service offered by network operators, and following the Snowden revelation in the USA in 2013 has led to an increased desire for people to employ encryption to avoid unwanted "eavesdropping" on their communications. Concerns have also been voiced about the addition of information to packets by third parties to provide analytics, customization, advertising, cross-site tracking of users, to bill the customer, or to selectively allow or block content. Whatever the reasons, there are now activities in the IETF to design new protocols that could include some form of transport header encryption (e.g., QUIC ). 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 DSCP and ECN. The use of transport layer authentication and encryption exposes a tussle between middlebox vendors, operators, applications developers and users. On the one hand, future Internet protocols that enable large-scale encryption assist in the restoration of the end-to-end nature of the Internet by returning complex processing to the endpoints, since middleboxes cannot modify what they cannot see. On the other hand, encryption of transport layer header information has implications for people who are responsible for operating networks and researchers and analysts seeking to understand the dynamics of protocols and traffic patterns. Whatever the motives, a decision to use pervasive 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 that are not always clear at the time when protocols are being defined. The following briefly reviews some security design options for transport protocols. A Survey of Transport Security Protocols provides more details concerning commonly used encryption methods at the transport layer. 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) . 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 . The IPsec Authentication Header (AH) 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. Transport layer header information that is observable can be observed in the network. Protocols often provide extensibility features, reserving fields or values for use by future versions of a specification. The specification of receivers has traditionally ignored unspecified values, however in-network devices have emerged that ossify to require a certain value in a field, or re-use a field for another purpose. When the speicfication is later updated, it is impossible to deploy the new use of the field, and forwarding of the protocol could even become conditional on a specific header field value. A protocol can intentionally vary the value, format, and/or presence of observable transport header fields. This behaviour, known as GREASE (Generate Random Extensions And Sustain Extensibility), is designed to avoid a network device ossifying the use of a specific observable field. Greasing seeks to ease deployment of new methods. It can be designed to avoid in-network devices utilising the information in a transport header, or can make an observation robust to a set of changing values, rather than a specific set of values. The transport layer payload can be encrypted to protect the content of transport segments. This leaves transport protocol header information in the clear. The integrity of immutable transport header fields could be protected by combining this with an integrity check. Examples of encrypting the payload include Transport Layer Security (TLS) over TCP , Datagram TLS (DTLS) over UDP , and TCPcrypt , which permits opportunistic encryption of the TCP transport payload. The network layer payload could be encrypted (including the entire transport header and the payload). This method provides confidentiality of the entire transport packet. It therefore does not expose any 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) 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. A transport protocol design can encrypt selected header fields, while also choosing to authenticate the entire transport header. This allows specific transport header fields to be made observable by network devices. End-to end integrity checks can prevent an endpoint from undetected modification of the immutable transport headers. Mutable fields in the transport header provide opportunities for middleboxes to modify the transport behaviour (e.g., the extended headers described in ). This considers only immutable fields in the transport headers, that is, fields that can be authenticated End-to-End across a path. An example of a method that encrypts some, but not all, transport information is GRE-in-UDP when used with GRE encryption. 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.

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.

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 ). Network-layer classification methods that rely on a multi-field classifier (e.g. Inferring QoS from the 5-tuple or choice of application protocol) are incompatible with transport protocols that encrypt the transport information. Traffic that cannot be classified, will typically receive a default treatment. Transport information can also be explicitly set in network-layer header fields that are not encrypted. This can provide information to enable a different forwarding treatment by the network, even when a transport employs encryption to protect other header information. When a transport multiplexes multiple subflows, a transport could choose to hide the presence and characteristics of these subflows from the network. However, a transport is permitted to set the network-layer information to indicate the presence of subflows and to reflect the needs of individual subflows (e.g., a WebRTC can identify different forwarding treatments for individual packets based on the value of the Differentiated Services Code Point (DSCP) field ). Endpoints are encouraged to set the IPv6 Flow Label field of the network-layer header (e.g., ). The label can provide information that can help inform network-layer queuing, forwarding (e.g., for Equal Cost Multi-Path, ECMP, routing, and Link Aggregation, LAG) . A multiplexing transport could choose to use multiple flow labels to allow the network to independently forward subflows. Applications can expose their delivery expectations to the network by setting the DSCP field of IPv4 and IPv6 packets .This provides explicit information to inform network-layer queuing and forwarding, rather than an operator inferring traffic requirements from transport and application headers via a multi-field classifier. Since the DSCP value can impact the quality of experience for a flow., observations of service performance need to consider this field when a network path has support for differentiated service treatment. ECN is a transport mechanism that utilises the ECN field in the network-layer header. Use of ECN explicitly informs the network-layer that a transport is ECN-capable, and requests ECN treatment of the flows packets. 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 . 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 ). 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, 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 .

Some measurements can 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 ) 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. This has the advantage that a single header can support all transport protocols, but there could also be less desirable implications of separating the operation of the transport protocol from the measurement framework. Another example of a network-layer approach is the IPv6 Performance and Diagnostic Metrics (PDM) Destination Option . 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.XXX Current measurements suggest 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., ). 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.

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.

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 can support decisions when considering changes to transport protocols, changes in network infrastructure, or the emergence of new traffic patterns.

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.

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 ). 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 , or blocking of uncharacterised traffic), and this needs to be considered when evaluating the impact of designs for transport encryption .

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 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 . 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.

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 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 that have important benefits. The pace of development of transports using the WebRTC data channel and the rapid deployment of 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. The traffic that can be observed by on-path network devices is a function of transport protocol design/options, network use, applications, and user characteristics. In general, when only a small proportion of the traffic has a specific (different) characteristic, such traffic seldom leads to operational concern, although the ability to measure and monitor it is less. The desire to understand the traffic and protocol interactions typically grows as the proportion of traffic increases in volume. The challenges increase when multiple instances of an evolving protocol contribute to the traffic that share network capacity. An increased pace of evolution therefore needs to be accompanied by methods that can be successfully deployed and used across operational networks. This leads to a need for network operators (at various 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 to be developed to catch-up with the changes. If the currently deployed tools and methods are no longer relevant then it may no longer be posisble to correctly measure performance. This can increase the response-time after faults, and can impact the ability to manage the network resulting in traffic causing traffic to be treated inappropriately (e.g., rate limiting because of being incorrectly classified/monitored). There are benefits in exposing consistent information to the network that avoids traffic being mis-classified and then receiving a default treatment by the network. The flow label and DSCP fields provide examples of how transport information can be made available for network-layer decisions. Extension headers could also be used to carry transport information that can inform network-layer decisions. 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. This can be done for the entire transport header, or by dividing header fields between those that are observable and mutable; those that are observable, but imutable; and those that are hidden/obfusticated. Several scenarios to illustrate different ways this could evolve are provided below: One scenario is when transport protocols provide consistent information to the network by intentionally exposing a part of the transport header. The design fixes the format of this information between versions of the protocol. This ossification of the transport header allows an operator to establish tooling and procedures that enable it to provide consistent traffic management as the protocol evolves. In contrast to TCP (where all protocol information is exposed), evolution of the transport is facilitated by providing cryptographic integrity checks of the transport header fields (preventing undetected middlebox changes) and encryption of other protocol information (preventing observation within the network, or incentivising the use of the exposed information, rather than inferring information from other characteristics of the flow traffic). The exposed transport information can be used by operators to provide troubleshooting, measurement and any necessary functions appropriate to the class of traffic (priority, retransmission, reordering, circuit breakers, etc). An alternative scenario adopts different design goals, with a different outcome. A protocol that encrypts all header information forces network operators to act independently from apps/transport developments to extract the information they need to manage their network. A range of approaches could proliferate, as in current networks. Some operators can add a shim header to each packet as a flow as it crosses the network; other operators/managers could develop heuristics and pattern recognition to derive information that classifies flows and estimates quality metrics for the service being used; some could decide to rate-limit or block traffic until new tooling is in place. In many cases, the derived information can be used by operators to provide necessary functions appropriate to the class of traffic (priority, retransmission, reordering, circuit breakers, etc). Troubleshooting, and measurement becomes more difficult, and more diverse. This could require additional information beyond that visible in the packet header and when this information is used to inform decisions by on-path devices it can lead to dependency on other characteristics of the flow. In some cases, operators might need access to keying information to interpret encrypted data that they observe. Some use cases could demand use of transports that do not use encryption. The direction in which this evolves 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.

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 . As currently deployed in the Internet, these features are generally provided by a protocol or layer on top of the transport protocol . 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 or greasing can further prevent these headers being utilised by network devices. Hiding headers can therefore provide the opportunity for greater freedom to update the protocols and can ease experimentation with new techniques and their final deployment in endpoints. A protocol specification needs to weigh the benefits of ossifying common headers, versus the potential demerits of exposing specific information that could be observed along the network path to provide tools to manage new variants of protocols. A protocol design that uses header encryption can provide confidentiality of some or all of the protocol header information. This prevents an on-path device from knowledge of the header field. It therefore prevents mechanisms being built that directly rely on the information or seeks to infer semantics of an exposed header field. Hiding headers can limit the ability to measure and characterise traffic. Exposed transport headers are sometimes utilised as a part of the information to detect anomalies in network traffic. This can be used as the first line of defence 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" (the normalization of packets so that there are no ambiguities in interpretation by the ultimate destination of the packet). These techniques are currently used by some operators to also defend from distributed DOS attacks. Exposed transport header fields are sometimes also utilised as a part of the information used by the receiver of a transport protocol to protect the transport layer from data injection by an attacker. In evaluating this use of exposed header information, it is important to consider whether it introduces a significant DOS threat. For example, an attacker could construct a DOS attack by sending packets with a sequence number that falls within the currently accepted range of sequence numbers at the receiving endpoint, this would then introduce additional work at the receiving endpoint, even though the data in the attacking packet may not finally be delivered by the transport layer. This is sometimes known as a “shadowing attack”. 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 and , or a port value that can not be predicted (see section 5.1 of ). 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.

XX RFC ED - PLEASE REMOVE THIS SECTION XXX This memo includes no request to IANA.

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.

&RFC1273; &RFC2474; &RFC2914; &RFC3135; &RFC3168; &RFC3234; &RFC3393; &RFC3550; &RFC4302; &RFC4303; &RFC4585; &RFC4737; &RFC5218; &RFC5236; &RFC8446; &RFC5481; &RFC5925; &RFC6056; &RFC6294; &RFC6269; &RFC6347; &RFC6437; &RFC7258; &RFC7525; &RFC7567; &RFC7624; &RFC7872; &RFC7928; &RFC8033; &RFC8084; &RFC8085; &RFC8086; &RFC8087; &RFC8095; &RFC8250; &RFC8289; &RFC8290; &RFC8404; &I-D.thomson-quic-grease; &I-D.ietf-quic-transport; &I-D.trammell-plus-abstract-mech; &I-D.ietf-tcpinc-tcpcrypt; &I-D.ietf-tsvwg-l4s-arch; &I-D.ietf-ippm-ioam-data; &I-D.ietf-taps-transport-security; &I-D.trammell-wire-image; &I-D.ietf-tsvwg-rtcweb-qos;

-00 This is an individual draft for the IETF community. -01 This draft was a result of walking away from the text for a few days and then reorganising the content. -02 This draft fixes textual errors. -03 This draft follows feedback from people reading this draft. -04 This adds an additional contributor and includes significant reworking to ready this for review by the wider IETF community Colin Perkins joined the author list. Comments from the community are welcome on the text and recommendations. -05 Corrections received and helpful inputs from Mohamed Boucadair. -06 Updated following comments from Stephen Farrell, and feedback via email. Added a draft conclusion section to sketch some strawman scenarios that could emerge. -07 Updated following comments from Al Morton, Chris Seal, and other feedback via email. -08 Updated to address comments sent to the TSVWG mailing list by Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on 11/05/2018, and Spencer Dawkins. -09 Updated security considerations. -10 Updated references, split the Introduction, and added a paragraph giving some examples of why ossification has been an issue. -01 This resolved some reference issues. Updated section on observation by devices on the path. -02 Comments received from Kyle Rose, Spencer Dawkins and Tom Herbert. The network-layer information has also been re-organised after comments at IETF-103.