| TSVWG | G. Fairhurst |
| Internet-Draft | University of Aberdeen |
| Intended status: Informational | C. Perkins |
| Expires: May 6, 2020 | University of Glasgow |
| November 3, 2019 |
Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols
draft-ietf-tsvwg-transport-encrypt-09
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, while also protecting 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, to account for network operations, prevent network ossification, and enable transport evolution, while still respecting user privacy.
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Transport protocols have supported end-to-end encryption of payload data for many years. Examples include Transport Layer Security (TLS) over TCP, Datagram TLS (DTLS) over UDP, and their corresponding usage guidelines, Secure RTP, and TCPcrypt which permits opportunistic encryption of the TCP transport payload. Some of these also provide integrity protection of all or part of the transport header.
This end-to-end transport payload encryption brings many benefits in terms of providing confidentiality and protecting user privacy. Such benefits have been widely discussed [RFC7258] [RFC7624]. This document strongly supports and encourages increased use of end-to-end payload encryption in transport protocols. The implications of protecting the transport payload data are therefore not further discussed in this document.
A further level of protection can be achieved by encrypting the entire network layer payload, including both the transport layer headers and the payload. This method provides confidentiality for the entire transport packet. It therefore does not expose any transport information to devices in the network, and prevents modification along a network path. An example of encryption at the network layer is the IPsec Encapsulating Security Payload (ESP) [RFC4303] in tunnel mode. Some Virtual Private Network (VPN) methods also encrypt these headers. This form of encryption is not further discussed in this document.
There is also a middle ground, comprising transport protocols that encrypt some, or all, of their transport layer header information, in addition to the payload. An example of such a protocol, that is seeing widespread interest and deployment, is the QUIC transport protocol [I-D.ietf-quic-transport]. Encryption and authentication of the transport header information can prevent unwanted modification of transport headers by middleboxes. It also reduces the amount of metadata about the progress of the transport connection that is visible to the network.
As discussed in [RFC7258], Pervasive Monitoring (PM) nis a technical attack that needs to be mitigated in the design of IETF protocols. This document supports that conclusion and the use of transport header encryption to protect against pervasive monitoring. RFC 7258 also notes, though, that "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".
The following sections further considers some of the costs and changes to network management and research that are implied by widespread use of transport protocols that encrypt the transport header information. It reviews the implications of developing transport protocols that use end-to-end encryption to provide confidentiality of their transport layer headers, and considers the effect of such changes on transport protocol design and network operations. It also considers some anticipated implications on transport and application evolution. That is, it considers the issues in designing transport protocols that both protect their header information and respect user privacy.
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, using higher-layer protocols running on the end systems (transport endpoints).
This simple architectural view hides one of the core functions of the transport: to discover and adapt to 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 the results of measurements and the insights of the network operations community to understand the trade-offs, and to inform selection of appropriate mechanisms to ensure a safe, reliable, and robust Internet (e.g., [RFC1273]). In turn, the network operator and access provider communities have relied on being able to understand the pattern and requirements of traffic passing over the Internet, both in aggregate and at the flow level. The widespread use of transport header encryption could change this.
Encryption is expected to form a core part of future transport protocol designs. This can be in the form of encrypted transport protocols (i.e., transport protocols that use encryption to provide confidentiality of some or all of the transport-layer header information), and/or the encryption of transport payloads (i.e., confidentiality of the payload data). There are many motivations for deploying such transports. Increasing public concerns about interference with Internet traffic [RFC7624] have led to a rapidly expanding deployment of encrypted transport protocols such as QUIC [I-D.ietf-quic-transport].
Using encryption to provide confidentiality of the transport layer therefore brings some well-known privacy and security benefits.
In-network measurement of transport flow characteristics can be used to enhance performance, and control cost and service reliability. To support network operations and enhance performance, some operators have deployed functionality that utilises on-path observations of the transport headers of packets passing through their network. These devices can rely on the presence and semantics of specific header information, which leads to ossification where an endpoint has to supply a specific header to receive the network service that it desires.
In some cases, network-layer use of transport header information can be benign or advantageous to the protocol (e.g., recognising the start of a TCP connection, providing header compression for a Secure RTP flow, or explicitly using exposed protocol information to provide consistent decisions by on-path devices). However, in other cases, ossification can frustrate the evolution of the transport protocol. A mechanism implemented in a network device, such as a firewall, that requires a header field to have only a specific known set of values (i.e., that regards the field as invariant) can prevent the device from forwarding packets using a different version of a protocol that introduces a feature that changes the value of the observed field.
An example of such ossification was observed in the development of Transport Layer Security (TLS) 1.3 [RFC8446]. This necessitated a design that recognised that deployed middleboxes relied on the presence of certain header fields exposed in TLS 1.2, and failed if those headers were changed.
The design of 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 reset connections when the window does not grow as expected. Similarly, Issues have been reported with TCP Fast Open using middleboxes that modify the transport header of packets by removing unknown TCP options, that drop segments with unknown TCP options, drop segments that contain data and have the SYN bit set, drop packets with SYN/ACK that acknowledge data, or that disrupt connections that send data before the three-way handshake completes. Other examples of ossification have included middleboxes that rewrite TCP sequence and acknowledgement numbers, but are unaware of the (newer) TCP selective acknowledgement (SACK) Option and therefore fail to correctly rewrite the selective acknowledgement header information to match the changes that were made to the fixed TCP header.
In all these cases, the issue was caused by middleboxes that had a hard-coded understanding of transport behaviour, and that interacted poorly with transport protocols when the transport behaviour changed. Many protocol specifications had also failed to clearly indicate the invariant parts of the transport header and were designed without thought for how header information could be used within the network.
Transport header encryption can help reduce such ossification of the transport layer. A protocol design that uses header encryption with secure key distribution can provide confidentiality for some, or all, of the protocol header information. This prevents an on-path device from observing the transport headers, and stops mechanisms being built that directly rely on transport header information, or that seek to infer semantics of exposed header fields. This encryption is normally combined with authentication of the protected information. RFC 8546 summarises this, 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" [RFC8546].
While encryption can hide transport header information and therefore help to reduce ossification of the transport protocol, it does not prevent ossification of the network service. People seeking to understand network traffic could come to rely on pattern inferences and other heuristics as the basis for network decision and to derive measurement data. This can create new dependencies on the transport protocol, or the patterns of traffic it can generate. This use of machine-learning methods usually demands large data sets, presenting it own requirements for collecting and distributing the data.
The design of a transport protocol needs to determine whether to encrypt all or a part of the transport information. It is possible that on-path devices could develop mechanisms that rely on the presence of any non-encrypted field, or a known value in the field. Section 4 of RFC8558 goes further, to state: "Anything exposed to the path should be done with the intent that it be used by the network elements on the path" [RFC8558]. In this context, specification of a non-encrypted transport header field explicitly allows protocol designers to make the certain header information observable by the network. This supports use of this information by on-path devices, but at the same time this can lead to ossification of the exposed part of a transport header. That is, network forwarding could evolve to depend on the presence and/or value of these fields (even if the header is not modified by the in-network device).
New protocol designs will make use of authentication to provide a cryptographic integrity check for the transport header fields. Transport header information that is authenticated, but not encrypted, permits inspection of the non-encrypted header fields by devices on the path, but does prevent undetected manipulation by network devices.
Sometimes a protocol design employs a header field that is not encrypted, but it is desired to avoid unwanted inspection restricting the choice of usable values in the field (and the resulting potential for undesirable ossification). In this case, the protocol designers can choose to intentionally vary the format and/or value of exposed header fields to reduce the chance of ossification (see Section 4 and [I-D.ietf-tls-grease]).
Transport headers have end-to-end meaning, but are often observed by equipment within the network. The decision about which transport headers fields are made observable offers trade-offs around header confidentiality versus header observability (including non-encrypted but authenticated header fields) for network operations and management, and the implications for ossification and user privacy. The impact differs depending on the activity, as discussed below and developed in the remainder of this document:
There are architectural challenges and considerations in the way transport protocols are designed, and the ability to characterise and compare different transport solutions [Measure]. Different parties will view the relative importance of these differently. For some, the benefits of encrypting the transport headers could outweigh the impact of doing so; others might make a different trade-off.
In response to pervasive monitoring [RFC7624] revelations and the IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258], efforts are underway to increase encryption of Internet traffic. Applying confidentiality to transport header fields affects how protocol information is used [RFC8404], requiring consideration of the trade-offs discussed in Section 2.3. To understand the implications, it is necessary to understand how transport layer headers are currently observed and/or modified by middleboxes within the network.
This section reviews some current usage. This review does not consider the 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 [RFC6269].
If in-network observation of transport protocol headers is needed, this requires knowledge of the format of the transport header:
The following subsections describe various ways that observable transport information has been utilised.
Flow/Session identification [RFC8558] is a common function. For example, performed by measurement activities, QoS classification, firewalls, Denial of Service, DOS, prevention.
Observable transport header information, together with information in the network header, has been used to identify flows and their connection state, together with the 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, a low-numbered (well-known) transport port number can identify the protocol. However, port information alone is not sufficient to guarantee identification when applications can use arbitrary ports, multiple sessions can be multiplexed on a single port, and ports can be re-used by subsequent sessions. UDP-based protocols often do not use well-known port numbers. Some flows can instead be identified by observing signalling protocol data (e.g., [RFC3261], [I-D.ietf-rtcweb-overview]) or through the use of magic numbers placed in the first byte(s) of the datagram payload [RFC7983].
Concealing transport header information can remove information used to classify flows by passive observers along the path, so operators will be unable to use this information directly. Operators could turn to more ambitious ways to collect, estimate, or infer that data, 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 practices aimed at inferring transport parameters are out of scope for this document, and are only mentioned here to recognize that encryption does not prevent operators from attempting to apply practices that were used with unencrypted transport headers.
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 characterizing 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:
This information can support network operations, inform capacity planning, and assist in determining the need for equipment and/or configuration changes by network operators. It can also inform Internet engineering activities by informing the development of new protocols, methodologies, and procedures.
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 to implement access rules (see also section 2.2.2 of [RFC8404]). 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, serving as a replacement/addition to the exposed transport information [RFC8558]. This can provide information to 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 hide 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 network needs of individual sub-flows. There are several ways this could be done:
When transport headers are concealed, operators will be unable to use this information directly. Careful use of the network layer features can help address provide similar information in the case where the network is unable to inspect transport protocol headers. Section Section 5 describes use of network extension headers.
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 the transport layer, until the emergence of transport protocols performing header encryption, with the obvious exception of VPNs and IPsec.
When encryption conceals more layers in each packet, people seeking understanding of the network operation rely more on pattern inference and other heuristics. It remains to be seen whether more complex inferences can be mastered to produce the same monitoring accuracy (see section 2.1.1 of [RFC8404]).
When measurement datasets are made available by servers or client endpoints, additional metadata, such as the state of the network, is often 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/device under evaluation [RFC7799].
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, 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 [RFC7567], 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 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 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.
A network operator supporting traffic that uses transport header encryption might 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 might 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 to inform operational practice.
While active measurements (see section 3.4 of [RFC7799]) could be used within a network, passive measurements (see section 3.6 of [RFC7799]) can have advantages in terms of eliminating unproductive test traffic, reducing the influence of test traffic on the overall traffic mix, and the ability to choose the point of observation (see Section 3.2.1). Passive measurements can rely on observing transport headers, which is not possible if those headers are encrypted, but could utilise information about traffic volumes or patterns of interaction to deduce metrics.
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., under severe congestion) by deploying rate-limiters, traffic shaping or network transport circuit breakers [RFC8084].
Transport header information can be useful for a variety of operational tasks [RFC8404]: to diagnose network problems, assess network provider performance, evaluate equipment/protocol performance, capacity planning, management of security threats (including denial of service), and responding to user performance questions. Section 3.1.2 and Section 5 of [RFC8404] provide further examples. These tasks seldom involve the need to determine the contents of the transport payload, or other application details. The use of payload encryption has the desirable effect of preventing unintended observation of the user data.
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 might 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).
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 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 some cases, measurements could involve active injection of test traffic to perform a measurement. 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 [RFC7799] (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 added while traffic traverses an operational network to make the measurements. These measurements do not require the cooperation of an endpoint.
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 from 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, 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.
A flow that conceals its transport header information could imply "don't touch" to some operators. This could 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. Header compression has been specified for use with TCP/IP and RTP/UDP/IP flows [RFC2507], [RFC2508], [RFC4995].
While it is possible to compress only the network layer headers, significant savings can be made if both the network and transport layer headers are compressed together as a single unit. The Secure RTP extensions [RFC3711] were explicitly designed to leave the transport protocol headers unencrypted, but authenticated, since support for header compression was considered important. Encrypting the transport protocol headers does not break such header compression, but does cause a fall back to compressing only the network layer headers, with a significant reduction in efficiency.
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.
There are several motivations for encryption:
Encryption methods do not 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. The use of transport layer authentication and encryption exposes a tussle between middlebox vendors, operators, applications developers and users:
Whatever the motives, a decision to use pervasive transport header encryption will have implications on the way in which design and evaluation is performed. This can, in turn, impact the direction of evolution of the transport protocol stack. While the IETF can specify protocols, the success in actual deployment is often determined by many factors [RFC5218] that are not always clear at the time when protocols are being defined.
The following briefly reviews some security design options for transport protocols. A Survey of Transport Security Protocols [I-D.ietf-taps-transport-security] provides more details concerning commonly used encryption methods at the transport layer.
As seen, different transports use encryption to protect their header information to varying degrees. There is, however, a trend towards increased protection with newer transport protocols.
An on-path device 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 [RFC8558]. There could also be less desirable implications of 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 [I-D.ietf-ippm-ioam-data], 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 [RFC8250]. This allows a sender to optionally include a destination option that caries header fields that can be used to observe timestamps and packet sequence numbers. This information could be authenticated by receiving transport endpoints when the information is added at the sender and visible at the receiving endpoint, although methods to do this have not currently been proposed. This method needs to be explicitly enabled at the sender.
Current measurement results suggest that it could currently be undesirable to rely on methods requiring end to end support of network options or extension headers across the Internet. IPv4 network options are often not supported (or are carried on a slower processing path) and some IPv6 networks have been observed to drop packets that set an IPv6 header extension (e.g., results from 2016 in [RFC7872]).
Another potential issue is that protocols that separately expose header information do not necessarily have an incentive to expose the actual information that is utilised by the protocol itself and could therefore manipulate the exposed header information to gain an advantage from the network. Where the information is provided by an endpoint, the incentive to reflect actual transport information needs to be considered when proposing a method.
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. This approach provides incentives to not reveal any information that is not necessary for the end-to-end communication. 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 if the transport community is to maintain an accurate understanding of the network. Encrypting transport header encryption changes the ability 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 for loss detection and recovery, congestion detection and control. Others need to work only end-to-end (e.g., parameter negotiation, flow-control).
The majority of present Internet applications use two well-known transport protocols, TCP and UDP. Although TCP represents the majority of current traffic, many 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.
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, 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 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 information represents the actual state of the endpoints.
When encryption is used to conceal 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 be designed, for example, to provide summary data regarding its performance, congestion control state, etc., or to make an explicit measurement signal available. 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 [I-D.ietf-quic-transport]).
When providing or using such information, it becomes 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 measurement signals 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. Some operational uses need the information to contain sufficient detail to understand, and possibly reconstruct, the network traffic pattern for further testing; such operators need to gain the trust of transport protocol implementers if they are to correctly reveal such information.
Operations, Administration, and Maintenance (OAM) data records [I-D.ietf-ippm-ioam-data] 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 conceal some or all of the transport headers, analysis will require 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 a standardised endpoint-based logging format (e.g., based on Quic-Trace). Such information will have 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. Logs collected at endpoints could be shared (after appropriate annoymisation) to help understand performance and pathologies. Measurements based on logging will need to establish the validity and provenance of the logged information to establish how and when traces were captured.
However, endpoint logs do not provide equivalent information to in-network measurements. In particular, endpoint logs contain only a part of the information needed to understand the operation of network devices and identify issues such as link performance or capacity sharing between multiple flows. Additional information is needed to determine which equipment/links are used and the configuration of equipment along the network paths being measured.
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, 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 could 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, as discussed in Section 3.2.4). Equally, operators could use analysis of transport headers and transport flow state to demonstrate that they are not providing differential treatment to certain flows. Obfuscating or hiding this information using encryption 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 [RFC8084], or blocking of uncharacterised traffic), and this needs to be considered when evaluating the impact of designs for transport encryption [RFC5218].
Many network operators currently utilise observed transport information as a part of their operational practice, and have developed tools and operational practices based around currently deployed transports and their applications. Encryption of the transport information prevents tools from directly observing this information. A variety of open source and commercial tools have been deployed that utilise this information for a variety of short and long term measurements.
The network will not break just because transport headers are encrypted, although alternative diagnostic and troubleshooting tools would need to be developed and deployed. Introducing a new protocol or application can require these tool chains and practice to be updated, and could in turn impact operational mechanisms, and policies. Each change can introduce associated costs, including the cost of collecting data, and the tooling needed to handle multiple formats (possibly as these co-exist in the network, when measurements need to span time periods during which changes are deployed, or to compare with historical data). These costs are incurred by an operator to manage the service and debug network issues.
At the time of writing, the additional operational cost of using encrypted transports is not yet well understood. Design trade-offs could mitigate these costs by explicitly choosing to expose selected information (e.g., header invariants and the spin-bit in QUIC [I-D.ietf-quic-transport]), the specification of common log formats, and development of alternative approaches.
Evolution and the ability to understand (measure) the impact need to proceed hand-in-hand. Observable transport headers can 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 the interactions between cooperating protocols and network mechanism, the implications of sharing capacity with other traffic and the impact of different patterns of usage. The ability of other stake holders to review transport header traces helps develop insight into performance and traffic contribution of specific variants of a protocol.
In development of new transport protocol mechanisms, attention needs to be paid to the expected scale of deployment. Whatever the mechanism, experience has shown that it is often difficult to correctly implement combinations of mechanisms [RFC8085]. 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.
Concealing 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 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 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 over 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 information.
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.
To achieve stable Internet operations, the IETF transport community has, to date, relied heavily on measurement and insights of the network operations community to understand the trade-offs, and to inform selection of appropriate mechanisms, to ensure a safe, reliable, and robust Internet (e.g., [RFC1273],[RFC2914]).
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 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 levels (ISPs, enterprises, firewall maintainer, etc.) to identify appropriate operational support functions and procedures. Protocols that change their transport header format (wire image) or their behaviour (e.g., algorithms that are needed to classify and characterise the protocol), will require new network tooling to be developed to catch-up with each change. If a protocol changes so that the currently deployed tools and methods are no longer relevant, then these tools can not be used to 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 as a result of incorrect classification or monitoring).
There are benefits in exposing consistent information to the network that avoids traffic being inappropriately 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. Other information might also be useful to various stakeholders, however 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 trade-offs and implications of increased use of transport header encryption when designing a protocol. Transport 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 header information. This lack of visibility of transport header information can be expected to impact the ways that protocols are deployed, standardised, and their operational support. The impact of hiding transport headers therefore needs to be considered in the specification and development of protocols and standards. This has a potential impact on the way in which the IRTF and IETF develop new protocols, specifications, and guidelines: [RFC7258] notes, "Making networks unmanageable to mitigate (pervasive monitoring) is not an acceptable outcome, but ignoring (pervasive monitoring) would go against the consensus documented here."
The design of future transport protocols needs to consider encryption of their transport headers to satisfy security and privacy concerns. This choice to encrypt all, or part, of the transport layer protocol headers needs to also take into account the impact on operations, standards, and research. As
As part of a protocol’s design, the community 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 ensure network operators, researchers and other stakeholders have appropriate tools to manage their networks and enable stable operation of the Internet as new protocols are deployed. An appropriate balance will emerge over time as real instances of this tension are analysed [RFC7258]. This balance between information exposed and information concealed 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 [RFC8095]. As currently deployed in the Internet, these features are generally provided by a protocol or layer on top of the transport protocol [I-D.ietf-taps-transport-security].
Confidentiality and strong integrity checks have properties that can also be incorporated into the design 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 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.
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 reduces visibility into transport metadata, and can limit the ability to measure and characterise traffic. It can also provide privacy benefits in some cases.
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 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 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 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 [RFC3550] and [RFC6056], or a port value that can not be predicted (see section 5.1 of [RFC8085]). A receiver could also require additional information to be used as a part of 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 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.
The Security and Privacy Considerations in the Framework for Large-Scale Measurement of Broadband Performance (LMAP) [RFC7594] contain considerations for Active and Passive measurement techniques and supporting material on measurement context.
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, Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris Wood, Thomas Fossati, 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, 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.
-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
-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 commentry on style and tone); Dimitri Tikonov (editorial); Christian Huitema (various) David Black (various). Ammended 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.