]> 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 To protect user data and privacy, Internet transport protocols have supported payload encryption and authentication for some time. Such encryption and authentication is now also starting to be applied to the transport protocol headers. This helps avoid transport protocol ossification by middleboxes, mitigate attacks against the transport protocol, and protect metadata about the communication. Current operational practice in some networks inspect transport header information within the network, but this is no longer possible when those transport headers are encrypted. This document discusses the possible impact when network traffic uses a protocol with an encrypted transport header. It suggests issues to consider when designing new transport protocols or features.

The transport layer supports the end-to-end flow of data across a network path, providing features such as connection establishment, reliability, framing, ordering, congestion control, flow control, etc., as needed to support applications. One of the core functions of an Internet transport: to discover and adapt to the characteristics of the network path that is currently being used. For some years, it has been common for the transport layer payload to be protected by encryption and authentication, but for the transport layer headers to be sent unprotected. Examples of protocols that behave in this manner include Transport Layer Security (TLS) over TCP, Datagram TLS , the Secure Real-time Transport Protocol, and tcpcrypt . The use of unencrypted transport headers has led some network operators, researchers, and others to develop tools and processes that rely on observations of transport headers both in aggregate and at the flow level to infer details of the network's behaviour and inform operational practice. Transport protocols are now being developed that encrypt some or all of the transport headers, in addition to the transport payload data. The QUIC transport protocol is an example of such a protocol. Such transport header encryption makes it difficult to observe transport protocol behaviour within the network. This document discusses some implications of transport header encryption for network operators, researchers, and others that have previously observed transport headers, and highlights some issues to consider for transport protocol designers. As discussed in , the IETF has concluded that Pervasive Monitoring (PM) is a technical attack that needs to be mitigated in the design of IETF protocols. This document supports that conclusion. It also recognises that RFC7258 states "Making networks unmanageable to mitigate PM is not an acceptable outcome, but ignoring PM would go against the consensus documented here. An appropriate balance will emerge over time as real instances of this tension are considered". This document is written to provide input to the discussion around what is an appropriate balance, by highlighting some implications of transport header encryption. This document explains current uses of transport header information in the network, which can be beneficial or malicious. It is written to provide input to the discussion around what is an appropriate balance, by highlighting some implications of transport header encryption.

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 can improve privacy, and can help to mitigate certain attacks, but can also affect network operations . When considering what parts of the transport headers should be encrypted to provide confidentiality, and what parts should be visible to the network (including non-encrypted but authenticated headers), it is necessary to consider both the impact on network operations and management, and the implications for ossification and user privacy . Different parties will view the relative importance of these concerns differently. For some, the benefits of encrypting all the transport headers outweigh the impact of doing so; others might analyse the security, privacy, and ossification impacts and arrive at a different trade-off. This section reviews examples of the observation of transport layer headers within the network. Unencrypted transport headers provide information can support network operations and management, and this section notes some ways in which this has been done. Unencrypted transport header information also contributes metadata that can be exploited for purposes unrelated to network transport measurement, diagnostics or troubleshooting (e.g., to block or to throttle traffic from a specific content provider), and this section also notes some threats relating to unencrypted transport headers. Exposed transport information also provides a source of information that contributes to linked data sets, which could be exploited to deduce private information, e.g., user patterns, user location, tracking behaviour, etc. This might reveal information the parties did not intend to be revealed. aims to make designers, implementers, and users of Internet protocols aware of privacy-related design choices in IETF protocols. This section does not consider intentional modification of transport headers by middleboxes, such as in Network Address Translation (NAT) or Firewalls. Common issues concerning IP address sharing are described in .

Information in exposed transport layer headers can be used by the network to identify transport protocols and flows . The ability to identify transport protocols, flows, and sessions is a common function performed, for example, by measurement activities, QoS classifiers, and firewalls. These functions can be beneficial, and performed with the consent of, and in support of, the end user. Alternatively, a network operator could use the same mechanisms to support practises that are adversarial to the end user, including blocking, de-prioritising, and monitoring traffic without consent. Observable transport header information, together with information in the network header, has been used to identify flows and their connection state, together with the set of protocol options being used. Transport protocols, such as TCP and the Stream Control Transport Protocol (SCTP), specify a standard base header that includes sequence number information and other data. They also have the possibility to negotiate additional headers at connection setup, identified by an option number in the transport header. In some uses, an assigned transport port (e.g., 0..49151) can identify the upper-layer protocol or service . However, port information alone is not sufficient to guarantee identification. Applications can use arbitrary ports and do not need to use assigned port numbers. The use of an assigned port number is also not limited to the protocol for which the port is intended. Multiple sessions can also be multiplexed on a single port, and ports can be re-used by subsequent sessions. Some flows can be identified by observing signalling protocol data (e.g., , ) or through the use of magic numbers placed in the first byte(s) of the datagram payload . When transport header information cannot be observed, this removes information that could have been used to classify flows by passive observers along the path. More ambitious ways could be used to collect, estimate, or infer flow information, including heuristics based on the analysis of traffic patterns. For example, an operator that cannot access the Session Description Protocol (SDP) session descriptions to classify a flow as audio traffic, might instead use (possibly less-reliable) heuristics to infer that short UDP packets with regular spacing carry audio traffic. Operational practises aimed at inferring transport parameters are out of scope for this document, and are only mentioned here to recognise that encryption does not prevent operators from attempting to apply practises that were used with unencrypted transport headers. The IAB have provided a summary of expected implications of increased encryption on network functions that use the observable headers and describe the expected benefits of designs that explicitly declare protocol invariant header information that can be used for this purpose.

Information in exposed transport layer headers can be used by the network to understand transport protocol performance and behaviour.

Observable transport headers enable explicit measurement and analysis of protocol performance, network anomalies, and failure pathologies at any point along the Internet path. Some operators use passive monitoring to manage their portion of the Internet by characterising the performance of link/network segments. Inferences from transport headers are used to derive performance metrics. A variety of open source and commercial tools have been deployed that utilise transport header information in this way to derive the following metrics: Volume measures per-application can be used to characterise the traffic that uses a network segment or the pattern of network usage. Observing the protocol sequence number and packet size offers one way to measure this (e.g., measurements observing counters in periodic reports such as RTCP; or measurements observing protocol sequence numbers in statistical samples of packet flows, or specific control packets, such as those observed at the start and end of a flow). Measurements can be per endpoint, or for an endpoint aggregate. This can be used, for example, to assess subscriber usage or for billing purposes. Measurements can also be used to trigger traffic shaping, and to associate QoS support within the network and lower layers. This can be done with consent and in support of an end user, to improve quality of service; or can be used by the network to de-prioritise certain flows without user consent. Volume measures can also be valuable for capacity planning and providing detail of trends in usage. The traffic rate and volume can be determined providing that the packets belonging to individual flows can be identified, but there might be no additional information about a flow when the transport headers cannot be observed. Flow loss rate can be derived (e.g., from transport sequence numbers or inferred from observing transport protocol interactions) and has been used as a metric for performance assessment and to characterise transport behaviour. Understanding the location and root cause of loss can help an operator determine whether this requires corrective action. Network operators have used the variation in patterns of loss as a key performance metric, utilising this to detect changes in the offered service. There are various causes of loss, including: corruption of link frames (e.g., due to interference on a radio link), buffering loss (e.g., overflow due to congestion, Active Queue Management (AQM) , or inadequate provision following traffic pre-emption), and policing (traffic management) . Understanding flow loss rates requires either observing sequence numbers in network or transport headers, or maintaining per-flow packet counters (flow identification often requires transport layer information). Per-hop loss can also sometimes be monitored at the interface level by devices in the network. Losses can often occur as bursts, randomly-timed events, etc. The pattern of loss can provide insight into the cause of loss. It can also be valuable to understand the conditions under which loss occurs, which usually requires relating loss to the traffic flowing at a network node or segment at the time of loss. This can also help identify cases where loss could have been wrongly identified, or where the transport did not require transmission of a lost packet. Throughput is the amount of payload data sent by a flow per time interval. Goodput (see Section 2.5 of ) is a measure of useful data exchanged (the ratio of useful data to total volume of traffic sent by a flow). The throughput of a flow can be determined in the absence of transport header information, providing that the individual flow can be identified, and the overhead known. Goodput requires ability to differentiate loss and retransmission of packets, for example by observing packet sequence numbers in the TCP or RTP headers . Latency is a key performance metric that impacts application and user-perceived response times. It often indirectly impacts throughput and flow completion time. This determines the reaction time of the transport protocol itself, impacting flow setup, congestion control, loss recovery, and other transport mechanisms. The observed latency can have many components . Of these, unnecessary/unwanted queueing 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 allows measurement of the upstream and downstream contribution to the RTT. This could be used to locate a source of latency, e.g., by observing cases where the median RTT is much greater than the minimum RTT for a part of a path. The service offered by network operators can benefit from latency information to understand the impact of configuration changes and to tune deployed services. Latency metrics are key to evaluating and deploying AQM , 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 require tuning because they cannot scale across all possible deployment scenarios. Latency and round-trip time information can potentially expose some information useful for approximate geolocation, as discussed in . Encrypting transport headers can reduce the latency information that is available. Some network applications are sensitive to (small) changes in packet timing (jitter). Short and long-term delay variation can impact on the latency of a flow and hence the perceived quality of applications using the network. For example, jitter metrics are often cited when characterising paths supporting real-time traffic. The expected performance of such applications, can be inferred from a measure the variation in delay observed along a portion of the path . The requirements resemble those for the measurement of latency. Significant packet reordering within a flow can impact time-critical applications and can be interpreted as loss by reliable transports. Many transport protocol techniques are impacted by reordering (e.g., triggering TCP retransmission or re-buffering of real-time applications). Packet reordering can occur for many reasons, from equipment design to misconfiguration of forwarding rules. Network tools can detect and measure unwanted/excessive reordering, and the impact on transport performance. There have been initiatives in the IETF transport area to reduce the impact of reordering within a transport flow, possibly leading to a reduction in the requirements for preserving ordering. These have potential to simplify network equipment design as well as the potential to improve robustness of the transport service. Measurements of reordering can help understand the present level of reordering within deployed infrastructure, and inform decisions about how to progress such mechanisms. Key performance indicators are retransmission rate, packet drop rate, sector utilisation level, a measure of reordering, peak rate, the ECN congestion experienced (CE) marking rate, etc. Metrics have been defined that evaluate whether a network has maintained packet order on a packet-by-packet basis . 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 assist in understanding 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 has to trust the source of this data and the method used to generate the summary information. These metrics can support network operations, inform capacity planning, and assist in determining the demand for equipment and/or configuration changes by network operators. They can also inform Internet engineering activities by informing the development of new protocols, methodologies, and procedures. In some cases, measurements could involve active injection of test traffic to perform a measurement (see Section 3.4 of ). However, most operators do not have access to user equipment, therefore the point of test is normally different from the transport endpoint. Injection of test traffic can incur an additional cost in running such tests (e.g., the implications of capacity tests in a mobile network are obvious). Some active measurements (e.g., response under load or particular workloads) perturb other traffic, and could require dedicated access to the network segment. Passive measurements (see Section 3.6 of ) can have advantages in terms of eliminating unproductive test traffic, reducing the influence of test traffic on the overall traffic mix, and the ability to choose the point of observation (see ). Measurements can rely on observing packet headers, which is not possible if those headers are encrypted, but could utilise information about traffic volumes or patterns of interaction to deduce metrics. One alternative approach is to use in-network techniques, which does not require the cooperation of an endpoint (see ).

Information from the transport header 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, or by firewalls to implement access rules (see also Section 2.2.2 of ). Operators can use such policies to support user applications and to protect against unwanted traffic. Such policies can also be used without user consent, to de-prioritise certain applications or services, for example. 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 header information. Traffic that cannot be classified typically receives a default treatment. Some networks block or rate traffic that cannot be classified. Transport layer information can also be explicitly carried in network-layer header fields that are not encrypted, serving as a replacement/addition to the exposed transport header information . This information can enable a different forwarding treatment by the network, even when a transport employs encryption to protect other header information. The user of a transport that multiplexes multiple sub-flows might want to obscure the presence and characteristics of these sub-flows. On the other hand, an encrypted transport could set the network-layer information to indicate the presence of sub-flows, and to reflect the service requirements of individual sub-flows. There are several ways this could be done: Applications normally expose the addresses used by endpoints, and this is used in the forwarding decisions in network devices. Address and other protocol information can be used by a Multi-Field (MF) classifier to determine how traffic is treated , and hence affect the quality of experience for a flow. A number of Standards Track and Best Current Practice RFCs (e.g., , , ) encourage endpoints to set the IPv6 flow label field of the network-layer header. IPv6 “source nodes SHOULD assign each unrelated transport connection and application data stream to a new flow” . A multiplexing transport could choose to use multiple flow labels to allow the network to independently forward sub-flows. RFC6437 provides further guidance on choosing a flow label value, stating these “should be chosen such that their bits exhibit a high degree of variability”, and chosen so that “third parties should be unlikely to be able to guess the next value that a source of flow labels will choose”. Once set, a flow label can provide information that can help inform network-layer queueing and forwarding , for example with Equal Cost Multi-Path routing and Link Aggregation . Considerations when using IPsec are further described in . The choice of how to assign a flow label needs to avoid introducing linkability that a network device could observe. Inappropriate use by the transport can have privacy implications (e.g., assigning the same label to two independent flows that ought not to be classified the same). Applications can expose their delivery expectations to the network by setting the Differentiated Services Code Point (DSCP) field of IPv4 and IPv6 packets . For example, WebRTC applications identify different forwarding treatments for individual sub-flows (audio vs. video) based on the value of the DSCP field ). This provides explicit information to inform network-layer queueing and forwarding, rather than an operator inferring traffic requirements from transport and application headers via a multi-field classifier. Inappropriate use by the transport can have privacy implications (e.g., assigning a different DSCP to a subflow could assist in a network device discovering the traffic pattern used by an application, assigning the same label to two independent flows that ought not to be classified the same). The field is mutable, i.e., some network devices can be expected to change this field (use of each DSCP value is defined by an RFC). Since the DSCP value can impact the quality of experience for a flow, observations of service performance has to consider this field when a network path supports differentiated service treatment. ECN is a transport mechanism that uses the ECN field in the network-layer header. Use of ECN explicitly informs the network-layer that a transport is ECN-capable, and requests ECN treatment of the flow. An ECN-capable transport can offer benefits when used over a path with equipment that implements an AQM method with CE marking of IP packets , since it can react to congestion without also having to recover from lost packets. ECN exposes the presence of congestion. The reception of CE-marked packets can be used to estimate the level of incipient congestion on the upstream portion of the path from the point of observation (Section 2.5 of ). Interpreting the marking behaviour (i.e., assessing congestion and diagnosing faults) requires context from the transport layer, such as path RTT. AQM and ECN offer a range of algorithms and configuration options. Tools therefore have to be available to network operators and researchers to understand the implication of configuration choices and transport behaviour as the use of ECN increases and new methods emerge . Network protocols can carry optional headers. These can be used to explicitly expose transport header information to on-path devices operating at the network layer (as discussed further in ). IPv4 has provision for optional header fields identified by an option type field. IP routers can examine these headers and are required to ignore IPv4 options that they does not recognise. Many current paths include network devices that forward packets that carry options on a slower processing path. Some network devices (e.g., firewalls) can be (and are) configured to drop these packets . BCP 186 provides Best Current Practice guidance on how operators should treat IPv4 packets that specify options. IPv6 can encode optional network-layer information in separate headers that may be placed between the IPv6 header and the upper-layer header . The Hop-by-Hop options header, when present, immediately follows the IPv6 header. IPv6 permits this header to be examined by any node along the path. While required all nodes to examine and process the Hop-by-Hop options header, it is now expected that nodes along a path only examine and process the Hop-by-Hop options header if explicitly configured to do so. Careful use of the network layer features can help provide similar information in the case where the network is unable to inspect transport protocol headers. describes use of network extension headers.

Some network operators make use of on-path observations of transport headers to monitor the performance of their networks, and to support network operations. Transport protocols with observable headers allow such operators to explicitly measurement and analyse transport protocol performance, and in some cases this can help detect and locate network problems. gives an operator's perspective about such use. When encryption hides the transport headers, making it difficult to directly observe transport behaviour and dynamics, those seeking an understanding of network operations might learn to work without that information. Alternatively, they might use more limited measurements combined with pattern inference and other heuristics to infer network behaviour (see Section 2.1.1 of ). Operational practises aimed at inferring transport parameters are out of scope for this document, and are only mentioned here to recognise that encryption does not necessarily stop operators from attempting to apply practises that have been used with unencrypted transport headers. When measurement datasets are made available by servers or client endpoints, additional metadata, such as the state of the network, is often necessary to interpret this data to answer questions about network performance or understand a pathology. Collecting and coordinating such metadata is more difficult when the observation point is at a different location to the bottleneck or device under evaluation . Packet sampling techniques are used to scale the processing involved in observing packets on high rate links. This exports only the packet header information of (randomly) selected packets. The utility of these measurements depends on the type of bearer and number of mechanisms used by network devices. Simple routers are relatively easy to manage, but 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.

In network measurements of transport header information can be used to locate 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 the algorithms are correctly controlling latency, or if overload protection is working. This understanding implies knowledge of how traffic is assigned to any sub-queues used for flow scheduling, but can also require information about how the traffic dynamics impact active queue management, starvation prevention mechanisms, and circuit-breakers. Sometimes multiple in network observation points have to be used. 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 rate and volume measurements are used by operators to help plan deployment of new equipment and configuration in their networks. Data is also valuable to equipment vendors who want to understand traffic trends and patterns of usage as inputs to decisions about planning products and provisioning for new deployments. This measurement information can also be correlated with billing information when this is also collected by an operator. Trends in aggregate traffic can be observed and can be related to the endpoint addresses being used, but when transport header information is not observable, it might be impossible to correlate patterns in measurements with changes in transport protocols. This increases the dependency on other indirect sources of information to inform planning and provisioning.

Performance measurements (e.g., throughput, loss, latency) can be used by various actors to analyse the service offered to the users of a network segment, and to inform operational practice.

The traffic that can be observed by on-path network devices (the "wire image") is a function of transport protocol design/options, network use, applications, and user characteristics. In general, when only a small proportion of the traffic has a specific (different) characteristic, such traffic seldom leads to operational concern, although the ability to measure and monitor it is lower. The desire to understand the traffic and protocol interactions typically grows as the proportion of traffic increases in volume. The challenges increase when multiple instances of an evolving protocol contribute to the traffic that share network capacity. Operators can manage traffic load (e.g., when the network is severely overloaded) by deploying rate-limiters, traffic shaping, or network transport circuit breakers . The information provided by observing transport headers is a source of data that can help to inform such mechanisms. Congestion control is a key transport function . Many network operators implicitly accept that TCP traffic complies with a behaviour that is acceptable for the shared Internet. TCP algorithms have been continuously improved over decades, and have reached a level of efficiency and correctness that is difficult to match in custom application-layer mechanisms . A standards-compliant TCP stack provides congestion control that is judged safe for use across the Internet. Applications developed on top of well-designed transports can be expected to appropriately control their network usage, reacting when the network experiences congestion, by back-off and reduce the load placed on the network. This is the normal expected behaviour for IETF-specified transports (e.g., TCP and SCTP). However, when anomalies are detected, tools can interpret the transport protocol header information to help understand the impact of specific transport protocols (or protocol mechanisms) on the other traffic that shares a network. An observation in the network can gain an understanding of the dynamics of a flow and its congestion control behaviour. Analysing observed flows can help to build confidence that an application flow backs-off its share of the network load under persistent congestion, and hence to understand whether the behaviour is appropriate for sharing limited network capacity. For example, it is common to visualise plots of TCP sequence numbers versus time for a flow to understand how a flow shares available capacity, deduce its dynamics in response to congestion, etc. The ability to identify sources and flows that contribute to persistent congestion is important to the safe operation of network infrastructure, and can inform configuration of network devices to complement the endpoint congestion avoidance mechanisms 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 have to employ mechanisms to prevent collapse, avoid unacceptable contributions to jitter/latency, and to establish an acceptable share of capacity with concurrent traffic . A network operator can observe the headers of transport protocols layered above UDP to understand if the datagram flows comply with congestion control expectations. This can help inform a decision on whether it might be appropriate to deploy methods such as rate-limiters to enforce acceptable usage. UDP flows that expose a well-known header can be observed to gain understanding of the dynamics of a flow and its congestion control behaviour. For example, tools exist to monitor various aspects of RTP header information and RTCP reports for real-time flows (see ). The Secure RTP and RTCP extensions were explicitly designed to expose some header information to enable such observation, while protecting the payload data.

Transport header information can be utilised for a variety of operational tasks : to diagnose network problems, assess network provider performance, evaluate equipment or protocol performance, capacity planning, management of security threats (including DoS), and responding to user performance questions. Section 3.1.2 and Section 5 of provide further examples. Operators can monitor the health of a portion of the Internet, to provide early warning and trigger action. Traffic and performance measurements can assist in setting buffer sizes, debugging and diagnosing the root causes of faults that concern a particular user's traffic. They can also be used to support post-mortem investigation after an anomaly to determine the root cause of a problem. In other cases, measurement involves dissecting network traffic flows. Observed transport header information can help identify whether link/network tuning is effective and alert to potential problems that can be hard to derive from link or device measurements alone. An alternative could rely on access to endpoint diagnostic tools or user involvement in diagnosing and troubleshooting unusual use cases or to troubleshoot non-trivial problems. Another approach is to use traffic pattern analysis. Such tools can provide useful information during network anomalies (e.g., detecting significant reordering, high or intermittent loss), however indirect measurements would need to be carefully designed to provide information for diagnostics and troubleshooting. The design trade-offs for radio networks are often very different from those of wired networks. A radio-based network (e.g., cellular mobile, enterprise Wireless LAN (WLAN), satellite access/back-haul, point-to-point radio) has the complexity of a subsystem that performs radio resource management, with direct impact on the available capacity, and potentially loss/reordering of packets. The impact of the pattern of loss and congestion and differences between traffic types, and their correlation with link propagation and interference can all have significant impact on the cost and performance of a provided service. For radio links, the use 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. Lack of tools and resulting information can reduce 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 respond to other queries about the network service. A network operator supporting traffic that uses transport header encryption is unable to use tools that rely on transport protocol information. However, the use of encryption has the desirable effect of preventing unintended observation of the payload data and these tools seldom seek to observe the payload, or other application details. A flow that hides its transport header information could imply "don't touch" to some operators. This might limit a trouble-shooting response to "can't help, no trouble found".

Header compression saves link capacity by compressing network and transport protocol headers on a per-hop basis. It was widely used with low bandwidth dial-up access links, and still finds application on wireless links that are subject to capacity constraints. Examples of header compression include use with TCP/IP and RTP/UDP/IP flows , , , . Successful compression depends on observing the transport headers and understanding of the way header fields change packet-by-packet, and is hence incompatible with header encryption. Devices that compress transport headers are dependent on a stable header format, implying ossification of that format. Introducing a new transport protocol, or changing the format of the transport header information, will limit the effectiveness of header compression until the network devices are updated. Encrypting the transport protocol headers will tend to cause the header compression to a fall back to compressing only the network layer headers, with a significant reduction in efficiency. This can limit connectivity if the resulting flow exceeds the link capacity, or if the packets are dropped because they exceed the link MTU. The Secure RTP (SRTP) extensions were explicitly designed to leave the transport protocol headers unencrypted, but authenticated, since support for header compression was considered important.

Observable transport headers coupled with published transport specifications allow operators and regulators to explore and verify compliance with Service Level Agreements (SLAs). When transport header information cannot be observed, other methods have to be found to confirm that the traffic produced conforms to the expectations of the operator or developer. Independently verifiable performance metrics can be utilised to demonstrate regulatory compliance in some jurisdictions, and as a basis for informing design decisions. This can bring assurance to those operating networks, often avoiding deployment of complex techniques that routinely monitor and manage Internet traffic flows (e.g., avoiding the capital and operational costs of deploying flow rate-limiting and network circuit-breaker methods ).

The choice of which transport header fields to expose and which to encrypt is a design decision for the transport protocol. Selective encryption requires trading conflicting goals of observability and network support, privacy, and risk of ossification, to decide what header fields to protect and which to make visible. Security work typically employs a design technique that seeks to expose only what is needed . This approach provides incentives to not reveal any information that is not necessary for the end-to-end communication. The IAB has provided guidelines for writing Security Considerations for IETF specifications . Endpoint design choices impacting privacy also need to be considered as a part of the design process . The IAB has provided guidance for analyzing and documenting privacy considerations within IETF specifications . There can also be performance and operational trade-offs in exposing selected information to network tools. This section explores key implications of working with encrypted transport protocols, but does not endorse or condemn these practices.

The patterns and types of traffic that share Internet capacity change over time as networked applications, usage patterns and protocols continue to evolve. If "unknown" or "uncharacterised" traffic patterns form a small part of the traffic aggregate passing through a network device or segment of the network the path, the dynamics of the uncharacterised traffic might not have a significant collateral impact on the performance of other traffic that shares this network segment. Once the proportion of this traffic increases, monitoring the traffic can determine if appropriate safety measures have to be put in place. Tracking the impact of new mechanisms and protocols requires traffic volume to be measured and new transport behaviours to be identified. This is especially true of protocols operating over a UDP substrate. The level and style of encryption has to be considered in determining how this activity is performed. On a shorter timescale, information could also have to be collected to manage DoS 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, as discussed in ). Equally, operators could use analysis of transport headers and transport flow state to demonstrate that they are not providing differential treatment to certain flows. Obfuscating or hiding this information using encryption could lead operators and maintainers of middleboxes (firewalls, etc.) to seek other methods to classify, and potentially other mechanisms to condition network traffic. A lack of data that reduces the level of precision with which flows can be classified also reduces the design space for conditioning mechanisms (e.g., rate limiting, circuit breaker techniques , or blocking of uncharacterised traffic) .

A variety and open source and proprietary tools have been deployed to can make use of the transport header information that's observable in widely used protocols such as TCP or RTP/UDP/IP. Changes to the transport, whether to protect the transport headers, introduce a new transport protocol, protocol feature, or application might require changes to such tools, and so could impact operational practice and policies. Such changes have associated costs that are incurred by the network operators that need to update their tooling or develop alternative practises that work without access to the changed/removed information. If new protocols, or protocol extensions, are made to closely resemble or match existing mechanisms, then these changes and the associated costs can be small. Equally, more extensive changes to the transport tend to require more extensive, and more expensive, changes to tooling and operational practice. Protocol designers can mitigate these costs by explicitly choosing to expose selected information as invariants that are guaranteed not to change for a particular protocol (e.g., the header invariants and the spin-bit in QUIC ). Specification of common log formats and development of alternative approaches can also help mitigate the costs of transport changes.

Independent observation by multiple actors is currently used by the transport community to maintain an accurate understanding of the network. Encrypting transport header encryption changes the ability to collect and independently analyse data. 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 assess congestion and requests for retransmission) provide an incentive for the sending endpoint to provide correct information, since the protocol will not work otherwise. This increases confidence that the observer understands the transport interaction with the network. For example, when TCP is used over an unencrypted network path (i.e., one that does not use IPsec or other encryption below the transport), it implicitly exposes transport header information that can be used for measurement at any point along the path. This information is necessary for the protocol's correct operation, therefore there is no incentive for a TCP or RTP implementation to put incorrect information in this transport header. A network device can have confidence that the well-known (and ossified) transport header information represents the actual state of the endpoints. When encryption is used to hide some or all of the transport headers, the transport protocol chooses which information to reveal to the network about its internal state, what information to leave encrypted, and what fields to grease to protect against future ossification . Such a transport could provide summary data regarding its performance, congestion control state, etc., or to make available explicit measurement information. For example, a QUIC endpoint can optionally set the spin bit to reflect to explicitly reveal the RTT of an encrypted transport session to the on-path network devices ). When providing or using such information, it is important to consider the privacy of the user and their incentive for providing accurate and detailed information. Protocols that selectively reveal some transport state or measurable information are choosing to establish a trust relationship with the network operators. There is no protocol mechanism that can guarantee that the information provided represents the actual transport state of the endpoints, since those endpoints can always send additional information in the encrypted part of the header, to update or replace whatever they reveal. This reduces the ability to independently measure and verify that a protocol is behaving as expected. For some operational uses, the information has to contain sufficient detail to understand, and possibly reconstruct, the network traffic pattern for further testing. In this case, operators have to gain the trust of transport protocol implementers if the transport headers are to correctly reveal such information. OAM data records could be embedded into a variety of encapsulation methods at different layers to support the goals of a specific operational domain. OAM-related metadata can support functions such as performance evaluation, path-tracing, path verification information, classification and a diversity of other uses. When encryption is used to hide some or all of the transport headers, analysis requires coordination between actors at different layers to successfully characterise flows and correlate the performance or behaviour of a specific mechanism with the configuration and traffic using operational equipment (e.g., combining transport and network measurements to explore congestion control dynamics, the implications of designs for active queue management or circuit breakers). Some measurements could be completed by utilising endpoint-based logging (e.g., based on Quic-Trace). Such information has a diversity of uses, including developers wishing to debug/understand the transport/application protocols with which they work, researchers seeking to spot trends and anomalies, and to characterise variants of protocols. A standard format for endpoint logging could allow these to be shared (after appropriate anonymisation) to understand performance and pathologies. Measurements based on logging have to establish the validity and provenance of the logged information to establish how and when traces were captured. Despite being applicable in some scenarios, endpoint logs do not provide equivalent information to in-network measurements. In particular, endpoint logs contain only a part of the information to understand the operation of network devices and identify issues such as link performance or capacity sharing between multiple flows. Additional information has to be combined to determine which equipment/links are used and the configuration of equipment along the network paths being measured.

Transport protocol evolution, and the ability to measure and understand the impact of protocol changes, have to proceed hand-in-hand. A transport protocol that provides observable headers can be used to provide open and verifiable measurement data. Observation of pathologies has a critical role in the design of transport protocol mechanisms and development of new mechanisms and protocols. This helps understanding of the interactions between cooperating protocols and network mechanisms, the implications of sharing capacity with other traffic and the impact of different patterns of usage. The ability of other stakeholders to review transport header traces helps develop insight into performance and traffic contribution of specific variants of a protocol. Development of new transport protocol mechanisms has to consider the scale of deployment and the range of environments in which the transport is used. Experience has shown that it is often difficult to correctly implement new mechanisms , and that mechanisms often evolve as a protocol matures, or in response to changes in network conditions, changes in network traffic, or changes to application usage. Analysis is especially valuable when based on the behaviour experienced across a range of topologies, vendor equipment, and traffic patterns. New transport protocol formats are expected to facilitate an increased pace of transport evolution, and with it the possibility to experiment with and deploy a wide range of protocol mechanisms. There has been recent interest in a wide range of new transport methods, e.g., Larger Initial Window, Proportional Rate Reduction (PRR), congestion control methods based on measuring bottleneck bandwidth and round-trip propagation time, the introduction of AQM techniques and new forms of ECN response (e.g., Data Centre TCP, DCTP, and methods proposed for L4S). The growth and diversity of applications and protocols using the Internet also continues to 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. Encryption of transport header information could reduce the range of actors that can observe useful data. This would limit the information sources available to the Internet community to understand the operation of new transport protocols, reducing information to inform design decisions and standardisation of the new protocols and related operational practises. The cooperating dependence of network, application, and host to provide communication performance on the Internet is uncertain when only endpoints (i.e., at user devices and within service platforms) can observe performance, and when performance cannot be independently verified by all parties. Independently observed data is also important to ensure the health of the research and development communities and can help promote acceptance of proposed specifications by the wider community (e.g., as a method to judge the safety for Internet deployment) and provides valuable input during standardisation. Open standards motivate a desire to include independent observation and evaluation of performance data, which in turn demands control/understanding about where and when measurement samples are collected. This requires consideration of the methods used to observe data and the appropriate balance between encrypting all and no transport header information.

End-to-end encryption can be applied at various protocol layers. It can be applied above the transport to encrypt the transport payload (e.g., using TLS). This can hide information from an eavesdropper in the network. It can also help protect the privacy of a user, by hiding data relating to user/device identity or location. Encryption and authentication is also increasingly used to protect the transport headers.

There are several motivations for transport header encryption. One motive to encrypt transport headers is to prevent network ossification from network devices that inspect transport headers. Once a network device observes a transport header and becomes reliant upon using it, the overall use of that field can become ossified, preventing new protocols and mechanisms from being deployed. One of the benefits of encrypting transport headers is that it can help improve the pace of transport development by eliminating interference by deployed middleboxes. Examples of this include: During the development of TLS 1.3 , the design needed to be modified to function in the presence of deployed middleboxes that relied on the presence of certain header fields exposed in TLS 1.2 . The design of Multipath TCP (MPTCP) also had to be revised to account for middleboxes (known as "TCP Normalizers") that monitor the evolution of the window advertised in the TCP header and then reset connections when the window did not grow as expected. TCP Fast Open can experience problems due to middleboxes that modify the transport header of packets by removing "unknown" TCP options, segments with unrecognised TCP options can be dropped, segments that contain data and set the SYN bit can be dropped, or middleboxes that disrupt connections that send data before completion of the three-way handshake. Other examples of ossification have included middleboxes that modify transport headers by rewriting TCP sequence and acknowledgement numbers, but are unaware of the (newer) TCP selective acknowledgement (SACK) option and therefore fail to correctly rewrite the SACK information to match the changes that were made to the fixed TCP header, preventing SACK from operating correctly. In all these cases, middleboxes with a hard-coded, but incomplete, understanding of transport behaviour, interacted poorly with transport protocols after the transport behaviour was changed. In some case, the middleboxes modified or replaced information in the transport protocol header. Transport header encryption prevents an on-path device from observing the transport headers, and therefore stops mechanisms being built that directly rely on or infer semantics of the transport header information. Encryption is normally combined with authentication of the protected information. RFC 8546 summarises this approach, stating that it is "The wire image, not the protocol's specification, determines how third parties on the network paths among protocol participants will interact with that protocol" (Section 1 of ), and it can be expected that header information that is not encrypted will become ossified. Encryption can reduce ossification of the transport protocol, but does not itself prevent ossification of the network service. People seeking to understand network traffic could still come to rely on pattern inferences and other heuristics or machine learning to derive measurement data and as the basis for network forwarding decisions . This can also create dependencies on the transport protocol, or the patterns of traffic it can generate, also in time resulting in ossification of the service. Another motivation stems from increased concerns about privacy and surveillance. Users value the ability to protect their identity and location, and defend against analysis of the traffic. Revelations about the use of pervasive surveillance have, to some extent, eroded trust in the service offered by network operators and have led to an increased use of encryption to avoid unwanted eavesdropping on communications. Concerns have also been voiced about the addition of information to packets by third parties to provide analytics, customisation, advertising, cross-site tracking of users, to bill the customer, or to selectively allow or block content. Whatever the reasons, the IETF is designing protocols that include transport header encryption (e.g., QUIC ) to supplement the already widespread payload encryption, and to further limit exposure of transport metadata to the network. The use of transport header authentication and encryption exposes a tussle between middlebox vendors, operators, applications developers and users: On the one hand, future Internet protocols that support transport header encryption assist in the restoration of the end-to-end nature of the Internet by returning complex processing to the endpoints, since middleboxes cannot modify what they cannot see, and can improve privacy by reducing leakage of transport metadata. On the other hand, encryption of transport layer 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. A decision to use transport header encryption can improve user privacy, and can reduce protocol ossification and help the evolution of the transport protocol stack, but is also has implications for network operations and management.

The designers of a transport protocol have to decide whether to encrypt all, or a part of, the transport layer information. Section 4 of states: "Anything exposed to the path should be done with the intent that it be used by the network elements on the path". Protocol designers can decide not to encrypt certain transport header fields, making those fields observable in the network, or can define new fields designed to explicitly expose observable transport layer information to the network. Where exposed fields are intended to be immutable (i.e., can be observed, but not modified by a network device), the endpoints are encouraged to use authentication to provide a cryptographic integrity check that can detect if these immutable fields have been modified by network devices. Authentication can also help to prevent attacks that rely on sending packets that fake exposed control signals in transport headers (e.g., TCP RST spoofing). Making a part of a transport header observable or exposing new header fields can lead to ossification of that part of a header as network devices come to rely on observations of the exposed fields. The following briefly reviews some security design options for transport protocols. A Survey of the Interaction between Security Protocols and Transport Services 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, allows in-network devices to observe these fields. An integrity check is not able to prevent in-network modification, but can prevent a receiving endpoint from accepting changes and avoid impact on the transport protocol operation, including some types of attack. 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. Such authentication might 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. The IPsec Encapsulating Security Payload (ESP) can also provide authentication and integrity without confidentiality using the NULL encryption algorithm . SRTP is another example of a transport protocol that allows header authentication. 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 (explicitly exposed either in a transport header field or lower layer protocol header). A design that only exposes immutable fields can also perform end-to-end authentication of these fields across the path to prevent undetected modification of the immutable transport headers. Mutable fields in the transport header provide opportunities where network devices can modify the transport behaviour (e.g., the extended headers described in ). An example of a method that encrypts some, but not all, transport header 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 have to be made 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. Protocols often provide extensibility features, reserving fields or values for use by future versions of a specification. The specification of receivers has traditionally ignored unspecified values, however in-network devices have emerged that ossify to require a certain value in a field, or re-use a field for another purpose. When the specification is later updated, it is impossible to deploy the new use of the field, and forwarding of the protocol could even become conditional on a specific header field value. A protocol can intentionally vary the value, format, and/or presence of observable transport header fields. This behaviour, known as GREASE (Generate Random Extensions And Sustain Extensibility) is designed to avoid a network device ossifying the use of a specific observable field. Greasing seeks to ease deployment of new methods. This seeks to prevent in-network devices utilising the information in a transport header, or can make an observation robust to a set of changing values, rather than a specific set of values. It is not a security mechanism, although use can be combined with an authentication mechanism. As seen, different transports use encryption to protect their header information to varying degrees. The trend is towards increased protection.

A transport protocol can choose to expose certain transport information to on-path devices operating at the network layer by sending observable fields. One approach is to make an explicit choice not to encrypt certain transport header fields, making this transport information observable by the network. Another approach is to choose to expose transport information through the use of network-layer extension headers (see ). Both are examples of explicit information intended to be used by network devices on the path . Whatever the mechanism used to expose the information, a decision to only expose specific transport information, places the transport endpoint in control of what to expose or not to expose outside of the encrypted transport header. This decision can then be made independently of the transport protocol functionality. This can be done by exposing part of the transport header or as a network layer option/extension.

At the network-layer, packets can carry optional headers (similar to ) that may be used to explicitly expose transport header information to the on-path devices operating at the network layer (). For example, an endpoint that sends an IPv6 Hop-by-Hop option can provide explicit transport layer information that can be observed and used by network devices on the path. Network-layer optional headers explicitly indicate the information that is exposed, whereas use of exposed transport header information first requires an observer to identify the transport protocol and its format. See for further discussion of transport protocol identification. An arbitrary path can include one or more network devices that drop packets that include a specific header or option used for this purpose (see ). This could impact the proper functioning of the protocols using the path. Protocol methods can be designed to probe to discover whether the specific option(s) can be used along the current path, enabling use on arbitrary paths.

There are opportunities for multiple transport protocols to consistently supply common observable information . A common approach can result in an open definition of the observable fields. This has the potential that the same information can be utilised across a range of operational and analysis tools.

Considerations concerning what information, if any, it is appropriate to expose include: On the one hand, explicitly exposing derived fields containing relevant transport information (e.g., metrics for loss, latency, etc) can avoid network devices needing to derive this information from other header fields. This could result in development and evolution of transport-independent tools around a common observable header, and permit transport protocols to also evolve independently of this ossified header . On the other hand, protocols and implementations might be designed to avoid consistently exposing external information that reflects the actual internal information used by the protocol itself. An endpoint/protocol could choose to expose transport header information to optimise the benefit it gets from the network . The value of this information would be enhanced if the exposed information could be verified to match the protocol's observed behavior. The motivation to reflect actual transport header information and the implications of network devices using this information has to be considered when proposing such a method. RFC 8558 summarises this as "When signals from endpoints to the path are independent from the signals used by endpoints to manage the flow's state mechanics, they may be falsified by an endpoint without affecting the peer's understanding of the flow's state. For encrypted flows, this divergence is not detectable by on-path devices." .

If the transport headers are encrypted, on-path devices can make measurements by utilising additional protocol headers carrying operations, administration and management (OAM) information in an additional packet header. Using network-layer approaches to reveal information has the potential that the same method (and hence same observation and analysis tools) can be consistently used by multiple transport protocols. This approach also could be applied to methods beyond OAM (see ). There can also be less desirable implications from separating the operation of the transport protocol from the measurement framework.

OAM information can be added at the ingress to a maintenance domain (e.g., an Ethernet protocol header with timestamps and sequence number information using a method such as 802.11ag or in-situ OAM , or as a part of encapsulation protocol). The additional header information is typically removed the at the egress of the maintenance domain. Although some types of measurements are supported, this approach does not cover the entire range of measurements described in this document. In some cases, it can be difficult to position measurement tools at the appropriate segments/nodes and there can be challenges in correlating the downstream/upstream information when in-band OAM data is inserted by an on-path device.

OAM information can also be added at the network layer as an IPv6 extension header or an IPv4 option. This information can be used across multiple network segments, or between the transport endpoints. One example is the IPv6 Performance and Diagnostic Metrics (PDM) destination option . 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 has to be explicitly enabled at the sender.

Header encryption and strong integrity checks are being incorporated into new transport protocols and have important benefits. The pace of development of transports using the WebRTC data channel, and the rapid deployment of the QUIC transport protocol, can both be attributed to using the combination of UDP as a substrate while providing confidentiality and authentication of the encapsulated transport headers and payload. This document has described some current practises, and the implications for some stakeholders, when transport layer header encryption is used. It does not judge whether these practises are necessary, or endorse the use of any specific practise. Rather, the intent is to highlight operational tools and practises to consider when designing and modifying transport protocols, so protocol designers can make informed choice about what transport header fields to encrypt, and whether it might be beneficial to make an explicit choice to expose certain fields to the network. In making such a decision, it is important to balance: User Privacy: The less transport header information that is exposed to the network, the lower the risk of leaking metadata that might have privacy implications for the users. Transports that chose to expose some header fields need to make a privacy assessment to understand the privacy cost versus benefit trade-off in making that information available. The process used to define and expose the QUIC spin bit to the network is an example of such an analysis. Transport Ossification: Unencrypted transport header fields are likely to ossify rapidly, as network devices come to rely on their presence, making it difficult to change the transport in future. This argues that the choice to expose information to the network is made deliberately and with care, since it is essentially defining a stable interface between the transport and the network. Some protocols will want to make that interface as limited as possible; other protocols might find value in exposing certain information to signal to the network, or in allowing the network to change certain header fields as signals to the transport. The visible wire image of a protocol should be explicitly designed. Network Ossification: While encryption can reduce ossification of the transport protocol, it does not itself prevent ossification of the network service. People seeking to understand network traffic could still come to rely on pattern inferences and other heuristics or machine learning to derive measurement data and as the basis for network forwarding decisions . This can also create dependencies on the transport protocol, or the patterns of traffic it can generate, also in time resulting in ossification of the service. Impact on Operational Practice: The network operations community has long relied on being able to understand Internet traffic patterns, both in aggregate and at the flow level, to support network management, traffic engineering, and troubleshooting. Operational practice has developed based on the information available from unencrypted transport headers. The IETF has supported this practice by developing operations and management specifications, interface specifications, and associated Best Current Practises. Widespread deployment of transport protocols that encrypt their information will impact network operations, unless operators can develop alternative practises that work without access to the transport header. Pace of Evolution: Removing obstacles to change can enable an increased pace of evolution. If a protocol changes its transport header format (wire image) or their transport behaviour, this can result in the currently deployed tools and methods becoming no longer relevant. Where this needs to be accompanied by development of appropriate operational support functions and procedures, it can incur a cost in new tooling to catch-up with each change. Protocols that consistently expose observable data do not require such development, but can suffer from ossification and need to consider if the exposed protocol metadata has privacy implications. There is no single deployment context, and therefore designers need to consider the diversity of operational networks (ISPs, enterprises, Distributed DoS (DDoS) mitigation and firewall maintainers, etc.). Supporting Common Specifications: Common, open, specifications can stimulate engagement by developers, users, researchers, and the broader community. Increased protocol diversity can be beneficial in meeting new requirements, but the ability to innovate without public scrutiny risks point solutions that optimise for specific cases, but that can accidentally disrupt operations of/in different parts of the network. The social contract that maintains the stability of the Internet relies on accepting common interworking specifications, and on it being possible to detect violations. It is important to find new ways of maintaining that community trust as increased use of transport header encryption limits visibility into transport behaviour. Impact on Benchmarking and Understanding Feature Interactions: An appropriate vantage point for observation, coupled with timing information about traffic flows, provides a valuable tool for benchmarking network devices, endpoint stacks, functions, and/or configurations. This can also help with understanding complex feature interactions. An inability to observe transport header information can make it harder to diagnose and explore interactions between features at different protocol layers, a side-effect of not allowing a choice of vantage point from which this information is observed. New approaches might have to be developed. Impact on Research and Development: Hiding transport header information can impede independent research into new mechanisms, measurement of behaviour, and development initiatives. Experience shows that transport protocols are complicated to design and complex to deploy, and that individual mechanisms have to be evaluated while considering other mechanisms, across a broad range of network topologies and with attention to the impact on traffic sharing the capacity. If increased use of transport header encryption results in reduced availability of open data, it could eliminate the independent checks to the standardisation process that have previously been in place from research and academic contributors (e.g., the role of the IRTF Internet Congestion Control Research Group (ICCRG) and research publications in reviewing new transport mechanisms and assessing the impact of their deployment). Observable transport header information might be useful to various stakeholders. Other sets of stakeholders have incentives to limit what can be observed. This document does not make recommendations about what information ought to be exposed, to whom it ought to be observable, or how this will be achieved. There are also design choices about where observable fields are placed. For example, one location could be a part of the transport header outside of the encryption envelope, another alternative is to carry the information in a network-layer option or extension header. New transport protocol designs ought to explicitly identify any fields that are intended to be observed, consider if there are alternative ways of providing the information, and reflect on the implications of observable fields being used by network devices, and how this might impact user privacy and protocol evolution when these fields become ossified. As notes, "Making networks unmanageable to mitigate (pervasive monitoring) is not an acceptable outcome, but ignoring (pervasive monitoring) would go against the consensus documented here." Providing explicit information can help avoid traffic being inappropriately classified, impacting application performance. An appropriate balance will emerge over time as real instances of this tension are analysed . This balance between information exposed and information hidden ought to be carefully considered when specifying new transport protocols.

This document is about design and deployment considerations for transport protocols. Issues relating to security are discussed throughout this document. Authentication, confidentiality protection, and integrity protection are identified as Transport Features by . 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 design of a transport protocol or to modify an existing transport. 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 . Preventing observation of headers provides an opportunity for greater freedom to update the protocols and can ease experimentation with new techniques and their final deployment in endpoints. A protocol specification needs to weigh the costs of ossifying common headers, versus the potential benefits of exposing specific information that could be observed along the network path to provide tools to manage new variants of protocols. Header encryption can provide confidentiality of some or all of the transport header information. This prevents an on-path device from knowledge of the header field. It therefore prevents mechanisms being built that directly rely on the information or seeks to infer semantics of an exposed header field. Reduced visibility into transport metadata can limit the ability to measure and characterise traffic, and conversely can provide privacy benefits. Extending the transport payload security context to also include the transport protocol header protects both information with the same key. A privacy concern would arise if this key was shared with a third party, e.g., providing access to transport header information to debug a performance issue, would also result in exposing the transport payload data to the same third party. Such risks would be mitigated using a layered security design that provides one domain of protection and associated keys for the transport payload and encrypted transport headers; and a separate domain of protection and associated keys for any observable transport header fields. Exposed transport headers are sometimes utilised as a part of the information to detect anomalies in network traffic. "While PM is an attack, other forms of monitoring that might fit the definition of PM can be beneficial and not part of any attack, e.g., network management functions monitor packets or flows and anti-spam mechanisms need to see mail message content." . This can be used as the first line of defence to identify potential threats from DoS or malware and redirect suspect traffic to dedicated nodes responsible for DoS analysis, malware detection, or to perform packet "scrubbing" (the normalisation of packets so that there are no ambiguities in interpretation by the ultimate destination of the packet). These techniques are currently used by some operators to also defend from distributed DoS attacks. Exposed transport header fields can also form a part of the information used by the receiver of a transport protocol to protect the transport layer from data injection by an attacker. In evaluating this use of exposed header information, it is important to consider whether it introduces a significant DoS threat. For example, an attacker could construct a DoS attack by sending packets with a sequence number that falls within the currently accepted range of sequence numbers at the receiving endpoint, this would then introduce additional work at the receiving endpoint, even though the data in the attacking packet might not finally be delivered by the transport layer. This is sometimes known as a “shadowing attack”. An attack can, for example, disrupt receiver processing, trigger loss and retransmission, or make a receiving endpoint perform unproductive decryption of packets that cannot be successfully decrypted (forcing a receiver to commit decryption resources, or to update and then restore protocol state). One mitigation to off-path attack is to deny knowledge of what header information is accepted by a receiver or obfuscate the accepted header information, e.g., setting a non-predictable initial value for a sequence number during a protocol handshake, as in and , or a port value that cannot be predicted (see Section 5.1 of ). A receiver could also require additional information to be used as a part of a validation check before accepting packets at the transport layer (e.g., utilising a part of the sequence number space that is encrypted; or by verifying an encrypted token not visible to an attacker). This would also mitigate against on-path attacks. An additional processing cost can be incurred when decryption has 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 header information. Open data, and accessibility to tools that can help understand trends in application deployment, network traffic and usage patterns can all contribute to understanding security challenges. The Security and Privacy Considerations in the Framework for Large-Scale Measurement of Broadband Performance (LMAP) contain considerations for Active and Passive measurement techniques and supporting material on measurement context. Addition of observable transport information to the path increases the information available to an observer and may, when this information can be linked to a node or user, reduce the privacy of the user. See the security considerations of .

This memo includes no request to IANA.

The authors would like to thank Mohamed Boucadair, Spencer Dawkins, Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris Wood, Thomas Fossati, Mohamed Boucadair, Martin Thomson, David Black, Martin Duke, and other members of TSVWG for their comments and feedback. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688421, and the EU Stand ICT Call 4. The opinions expressed and arguments employed reflect only the authors' view. The European Commission is not responsible for any use that might be made of that information. This work has received funding from the UK Engineering and Physical Sciences Research Council under grant EP/R04144X/1.

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-00 This is an individual draft for the IETF community. -01 This draft was a result of walking away from the text for a few days and then reorganising the content. -02 This draft fixes textual errors. -03 This draft follows feedback from people reading this draft. -04 This adds an additional contributor and includes significant reworking to ready this for review by the wider IETF community Colin Perkins joined the author list. Comments from the community are welcome on the text and recommendations. -05 Corrections received and helpful inputs from Mohamed Boucadair. -06 Updated following comments from Stephen Farrell, and feedback via email. Added a draft conclusion section to sketch some strawman scenarios that could emerge. -07 Updated following comments from Al Morton, Chris Seal, and other feedback via email. -08 Updated to address comments sent to the TSVWG mailing list by Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on 11/05/2018, and Spencer Dawkins. -09 Updated security considerations. -10 Updated references, split the Introduction, and added a paragraph giving some examples of why ossification has been an issue. -01 This resolved some reference issues. Updated section on observation by devices on the path. -02 Comments received from Kyle Rose, Spencer Dawkins and Tom Herbert. The network-layer information has also been re-organised after comments at IETF-103. -03 Added a section on header compression and rewriting of sections referring to RTP transport. This version contains author editorial work and removed duplicate section. -04 Revised following SecDir Review Added some text on TLS story (additional input sought on relevant considerations). Section 2, paragraph 8 - changed to be clearer, in particular, added "Encryption with secure key distribution prevents" Flow label description rewritten based on PS/BCP RFCs. Clarify requirements from RFCs concerning the IPv6 flow label and highlight ways it can be used with encryption. (section 3.1.3) Add text on the explicit spin-bit work in the QUIC DT. Added greasing of spin-bit. (Section 6.1) Updated section 6 and added more explanation of impact on operators. Other comments addressed. -05 Editorial pass and minor corrections noted on TSVWG list. -06 Updated conclusions and minor corrections. Responded to request to add OAM discussion to Section 6.1. -07 Addressed feedback from Ruediger and Thomas. Section 2 deserved some work to make it easier to read and avoid repetition. This edit finally gets to this, and eliminates some duplication. This also moves some of the material from section 2 to reform a clearer conclusion. The scope remains focussed on the usage of transport headers and the implications of encryption - not on proposals for new techniques/specifications to be developed. -08 Addressed feedback and completed editorial work, including updating the text referring to RFC7872, in preparation for a WGLC. -09 Updated following WGLC. In particular, thanks to Joe Touch (specific comments and commentary on style and tone); Dimitri Tikonov (editorial); Christian Huitema (various); David Black (various). Amended privacy considerations based on SECDIR review. Emile Stephan (inputs on operations measurement); Various others. Added summary text and refs to key sections. Note to editors: The section numbers are hard-linked. -10 Updated following additional feedback from 1st WGLC. Comments from David Black; Tommy Pauly; Ian Swett; Mirja Kuehlewind; Peter Gutmann; Ekr; and many others via the TSVWG list. Some people thought that "needed" and "need" could represent requirements in the document, etc. this has been clarified. -11 Updated following additional feedback from Martin Thomson, and corrections from other reviewers. -12 Updated following additional feedback from reviewers. -13 Updated following 2nd WGLC with comments from D.L.Black; T. Herbert; Ekr; and other reviewers. -14 Update to resolve feedback to rev -13. This moves the general discussion of adding fields to transport packets to section 6, and discusses with reference to material in RFC8558. -15 Feedback from D.L. Black, T. Herbert, J. Touch, S. Dawkins and M. Duke. Update to add reference to RFC7605. Clarify a focus on immutable transport fields, rather than modifying middleboxes with Tom H. Clarified Header Compression discussion only provides a list of examples of HC methods for transport. Clarified port usage with Tom H/Joe T. Removed some duplicated sentences, and minor edits. Added NULL-ESP. Improved after initial feedback from Martin Duke. -16 Editorial comments from Mohamed Boucadair. Added DTLS 1.3. -17 Revised to satisfy ID-NITs and updates REFs to latest rev, updated HC REFs; cited IAB guidance on security and privacy within IETF specs. -18 Revised based on AD review.