| Internet-Draft | Transport Encryption | September 2017 |
| Fairhurst & Perkins | Expires 31 March 2018 | [Page] |
This document describes implications of applying end-to-end encryption at the transport layer. It identifies some in-network uses of transport layer header information that can be used with a transport header integrity check. It reviews the implication of developing encrypted end-to-end transport protocols and examines the implication of developing and deploying encrypted end-to-end transport protocols. Since transport measurement and analysis of the impact of network characteristics have been important to the design of current transport protocols, it also considers some anticipated implications on transport and application evolution.¶
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This document discusses the implications of end-to-end encryption applied at the transport layer, and examines the impact on transport protocol design, usage, and network operations and management. It also considers anticipated implications on transport and application evolution.¶
The transport layer provides the first end-to-end interactions across the Internet. Transport protocols layer directly over the network-layer service and are sent in the payload of network-layer packets. They support end-to-end communication between applications, supported by higher-layer protocols, running on the end systems (or transport endpoint). This simple architectural view hides one of the core functions of the transport, however - to discover and adapt to the properties of the Internet path that is currently being used. The design of Internet transport protocols is as much about trying to avoid the unwanted side effects of congestion on a flow and other capacity-sharing flows, avoiding congestion collapse, adapting to changes in the path characteristics, etc., as it is about end-to-end feature negotiation, flow control and optimising for performance of a specific application.¶
To achieve stable Internet operations the IETF transport community has to date relied heavily on measurement and insights of the network operations community to understand the trade-offs, and to inform selection of select appropriate mechanisms, to ensure a safe, reliable and robust Internet. In turn, the network operations community relies on being able to understand the traffic passing over the Internet, both in aggregate and at the flow level -- inspecting transport layer headers to help understand traffic dynamics.¶
There are many motivations for deploying encrypted transports, and encryption of transport payloads. The increasing public concerns about the interference with Internet traffic have led to a rapidly expanding deployment of encryption to protect end-user privacy, in protocols like QUIC. At the same time, network operators and access providers, especially in mobile networks, have come to rely on the in-network measurement of transport properties and the functionality provided by middleboxes to both support network operations and enhance performance.¶
This document considers some implications of working with encrypted transport protocols, and discusses trade-offs around authentication, encryption of transport protocol headers. It describes some of the architectural challenges and considerations in the way transport protocols are designed when using encryption [Measure].¶
Encryption of the transport layer brings some well-known privacy and security benefits, but also introduces various costs that need to be considered. Specifically, it can impact the following activities that rely on measurement and analysis of traffic flows:¶
The last point leads us to consider the impact of encrypting all the transport headers the specification and development of protocols and standards. It has potential impact on:¶
Pervasive use of transport header encryption can impact the ways that protocols are designed, standardised, deployed, and operated. The choice of whether future transport protocols encrypt their protocol headers therefore needs to be taken based not solely on security and privacy considerations, but also taking into account the impact on operations, standards, and research. A network that is secure but unusable due to persistent congestion collapse is not an improvement, and while that would be an extreme outcome proposals that impose high costs for very limited benefits need to be considered carefully, to ensure the benefits outweigh the costs.¶
The transport layer is the first end-to-end layer in the network stack. Despite headers having end-to-end meaning, some transport headers have come to be used in various ways within the Internet. In response to pervasive monitoring [RFC7624] revelations and the IETF consensus that "Pervasive Monitoring is an Attack" [RFC7258], efforts are underway to increase encryption of Internet traffic, which would prevent visibility of transport headers. This affects on how network protocols are designed and used [I-D.mm-wg-effect-encrypt]. To understand these implications, it is first necessary to understand how transport layer headers are currently observed and/or modified by middleboxes within the network.¶
Transport protocols can be designed to encrypt or authenticate transport header fields. Authentication methods at the transport layer can be sued to detect any changes to an immutable header field that were made by a network device along a path. The intentional modification of transport headers by middleboxes (such as Network Address Translation with Protocol Translation, NAT-PT, or Firewalls) is not considered.¶
In-network observation of transport protocol headers requires knowledge of the format of the transport header:¶
The following subsections describe various ways that observable transport information may be utilised.¶
Transport protocol header information can identify a flow and the connection state of the flow, together with the protocol options being used. In some usages, a low-numbered (well-known ) port can identify a protocol (although port information alone is not sufficient to guarantee identification of a protocol). Transport protocols, such as TCP and Stream Control Transport Protocol (SCTP) specify a standard base header that includes sequence number information and other data, with the possibility to negotiate additional headers at connection setup, identified by an option number in the transport header. UDP-based protocols can use, but sometimes do not use, well-known ports. Some can instead be identified by signalling protocols or through the use of magic numbers placed in the first byte(s) of the datagram payload.¶
Some actors have a need to characterise the performance of link/network segments. Passive monitoring uses observed traffic to makes inferences from transport headers to derive these measurements. A variety of open source and commercial tools have been deployed that utilise this information. The following metrics can be derived from transport header information:¶
Some protocols provide in-built monitoring and reporting functions. Transport fields in the RTP header [RFC3550] [RFC4585] can be observed to derive traffic volume measurements and provide information on the progress and quality of a session using RTP. Key performance indicators are retransmission rate, packet drop rate, sector utilization level, a measure of reordering, peak rate, the CE-marking rate, etc. Metadata is often important to understand the context under which the data was collected, including the time, observation point, and way in which metrics were accumulated. The RTCP protocol directly reports some of this information in a form that can be directly visible in the network. A user of summary measurement data needs to trust the source of this data and the method used to generate the summary information.¶
When encryption conceals information in packet headers, measurements need to rely on pattern inferences and other heuristics grows, and accuracy suffers [I-D.mm-wg-effect-encrypt].¶
Some transport information is made visible in the network-layer protocol header. These header fields are not encrypted and can be used to make flow observations.¶
The common language between network operators and application/content providers/users is packet transfer performance at a layer that all can view and analyse. For most packets, this has been transport layer, until the emergence of QUIC, with the obvious exception of VPNs and IPsec. When encryption conceals more layers in a packet, people seeking understanding of the network operation need to rely more on pattern inferences and other heuristics. The accuracy of measurements therefore suffers, as does the ability to investigate and troubleshoot interactions between different anomalies. For example, the traffic patterns between a web server and a browser are dependent on browser supplier and version, even use of the application (e.g., web e-mail access). Even when measurement datasets are made available (e.g., from endpoints) additional metadata, such as the state of the network, is often required to interpret the data. Collecting and coordinating such metadata is more difficult when the observation point is at a different location to the bottleneck/device under evaluation.¶
Packet sampling techniques can be used to scale the processing involved in observing packets on high rate links. This exports only the packet header information of (randomly) selected packets. The utility of these measurements depends on the type of bearer and number of mechanisms used by network devices. Simple routers are relatively easy to manage, a device with more complexity demands understanding of the choice of many system parameters. This level of complexity exists when several network methods are combined.¶
This section discusses topics concerning observation of transport flows, with a focus on transport measurement.¶
Often measurements can only be understood in the context of the other flows that share a bottleneck. A simple example is monitoring of AQM. For example, FQ-CODEL [I-D.ietf-aqm-fq-codel], combines sub queues (statistically assigned per flow), management of the queue length (CODEL), flow-scheduling, and a starvation prevention mechanism. Usually such algorithms are designed to be self-tuning, but current methods typically employ heuristics that can result in more loss under certain path conditions (e.g., large RTT, effects of multiple bottlenecks [RFC7567]).¶
In-network measurements can distinguish between upstream and downstream metrics with respect to the measurement point. These are particularly useful for locating the source of problems or to assess the performance of a network segment or a particular device configuration.¶
By correlating observations at multiple points along the path (e.g., at the ingress and egress of a network segment), an observer can determine the contribution of a portion of the path to an observed metric (to locate a source of delay, jitter, loss, reordering, congestion marking, etc.).¶
Traffic measurements (e.g., traffic volume, loss, latency) is used by operators to help plan deployment of new equipment and configurations in their networks. Data is also important to equipment vendors who need to understand traffic trends traffic and patterns of usage as inputs to decisions about planning products and provisioning for new deployments. This measurement information can also be correlated with billing information when this is also collected by an operator.¶
A network operator supporting traffic that uses transport header encryption may not have access to per-flow measurement data. Trends in aggregate traffic can be observed and can be related this to the endpoint addresses being used, but it may not be possible to correlate patterns in measurements with changes in transport protocols (e.g., the impact of changes in introducing a new transport protocol mechanism). This increases the dependency on other indirect sources of information to inform planning and provisioning.¶
Traffic measurements (e.g., traffic volume, loss, latency) can be used by various actors to help analyse the performance available to users of a network segment, and inform operational practice. While active measurements may be used in-network passive measurements can have advantages in terms of eliminating unproductive traffic, reducing the influence of test traffic on the overall traffic mix, and the ability to choose the point of measurement Section 1.1.2.1.¶
Information provided by tools observing transport headers can help determine whether mechanisms are needed in the network to prevent flows from acquiring excessive network capacity. Operators can implement operational practices to manage traffic flows (e.g., to prevent flows from acquiring excessive network capacity under severe congestion) by deploying rate-limiters, traffic shaping or network transport circuit breakers [RFC8084].¶
Transport header information is useful for a variety of operational tasks [I-D.mm-wg-effect-encrypt]: to diagnose network problems, assess performance, capacity planning, management of denial of service threats, and responding to user performance questions. These tasks seldom involve the need to determine the contents of the transport payload, or other application details.¶
A network operator supporting traffic that uses transport header encryption can see only encrypted transport headers. This prevents deployment of performance measurement tools that rely on transport protocol information. Choosing to encrypt all information may be expected to reduce the ability for networks to "help" (e.g., in response to tracing issues, making appropriate Quality of Service, QoS, decisions). For some this will be blessing, for others it may be a curse. For example, operational performance data about encrypted flows needs to be determined by traffic pattern analysis, rather than relying on traditional tools. This can impact the ability of the operator to respond to faults, it could require reliance on endpoint diagnostic tools or user involvement in diagnosing and troubleshooting unusual use cases or non-trivial problems. A key need here is that tools need to provide useful information during network anomalies (e.g., significant reordering, high or intermittent loss). Although many network operators utilise transport information as a part of their operational practice, the network will not break because transport headers are encrypted.¶
Information from the transport protocol can be used by a multi-field classifier as a part of policy framework. Policies are commonly used for QoS management for resource-constrained networks and by firewalls that use the information to implement access rules. Traffic that cannot be classified, will typically receive a default treatment.¶
End-to-end encryption can be applied at various protocol layers. It can be applied above the transport to encrypt the transport payload. Encryption methods can hide information from an eavesdropper in the network. Encryption can also help protect the privacy of a user, by hiding data relating to user/device identity or location. Neither an integrity check nor encryption methods prevent traffic analysis, and usage needs to reflect that profiling of users, identification of location and fingerprinting of behaviour can take place even on encrypted traffic flows.¶
One motive to use encryption is a response to perceptions that the network has become ossified by over-reliance on middleboxes that prevent new protocols and mechanisms from being deployed. This has lead to a common perception that there is too much "manipulation" of protocol headers within the network, and that designing to deploy in such networks is preventing transport evolution. In the light of this, a method that authenticates transport headers may help improve the pace of transport development, by eliminating the need to always consider deployed middleboxes [I-D.trammell-plus-abstract-mech], or potentially to only explicitly enable middlebox use for particular paths with particular middleboxes that are deliberately deployed to realise a useful function for the network and/or users[RFC3135].¶
Another motivation stems from increased concerns about privacy and surveillance. Some Internet users have valued the ability to protect identity, user location, and defend against traffic analysis, and have used methods such as IPsec ESP and Tor [Tor]. Revelations about the use of pervasive surveillance [RFC7624] have, to some extent, eroded trust in the service offered by network operators, and following the Snowden revelation in the USA in 2013 has led to an increased desire for people to employ encryption to avoid unwanted "eavesdropping" on their communications. Whatever the reasons, there are now activities in the IETF to design new protocols that may include some form of transport header encryption (e.g., QUIC [I-D.ietf-quic-transport]).¶
Authentication methods (that provide integrity checks of protocols fields) have also been specified at the network layer, and this also protects transport header fields. The network layer itself carries protocol header fields that are increasingly used to help forwarding decisions reflect the need of transport protocols, such the IPv6 Flow Label [RFC6437], the Differentiated Services Code Point (DSCP) [RFC2474] and Explicit Congestion Notification (ECN) [RFC3168].¶
The use of transport layer authentication and encryption exposes a tussle between middlebox vendors, operators, applications developers and users.¶
Whatever the motives, a decision to use pervasive of transport header encryption will have implications on the way in which design and evaluation is performed, and which can in turn impact the direction of evolution of the TCP/IP stack.¶
The next subsections briefly review some security design options for transport protocols.¶
Transport layer header information can be authenticated. An integrity check that protects the immutable transport header fields, but can still expose the transport protocol header information in the clear, allowing in-network devices to observes these fields. An integrity check can not prevent in-network modification, but can avoid a receiving accepting changes and avoid impact on the transport protocol operation.¶
An example transport authentication mechanism is TCP-Authentication (TCP-AO) [RFC5925]. This TCP option authenticates TCP segments, including the IP pseudo header, TCP header, and TCP data. TCP-AO protects the transport layer, preventing attacks from disabling the TCP connection itself. TCP-AO may interact with middleboxes, depending on their behaviour [RFC3234].¶
The IPsec Authentication Header (AH) [RFC4302] works at the network layer and authenticates the IP payload. This therefore also authenticates all transport headers, and verifies their integrity at the receiver, preventing in-network modification.¶
The transport layer payload can be encrypted to protect the content of transport segments. This leaves transport protocol header information in the clear. The integrity of immutable transport header fields could be protected by combining this with an integrity check (Section 2.1).¶
Examples of encrypting the payload include Transport Layer Security (TLS) over TCP [RFC5246] [RFC7525] or Datagram TLS (DTLS) over UDP [RFC6347] [RFC7525].¶
The network layer payload could be encrypted (including the entire transport header and payload). This method does not expose any transport information to devices in the network, which also prevents modification along the network path.¶
The IPsec Encapsulating Security Payload (ESP) [RFC4303] is an example of encryption at the network layer, it encrypts and authenticates all transport headers, preventing visibility of the headers by in-network devices. Some Virtual Private Network (VPN) methods also encrypt these headers.¶
A transport protocol design can encrypt selected header fields, while also choosing to authenticate fields in the transport header. This allows specific transport header fields to be made observable by network devices. End-to end integrity checks can prevent an endpoint from undetected modification of the immutable transport headers.¶
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. On the one hand, security work typically employs a design technique that seeks to expose only what is needed. On the other hand, there may be performance and operational benefits in exposing selected information to network tools.¶
Mutable fields in the transport header provide opportunities for middleboxes to modify the transport behaviour (e.g., the extended headers described in [I-D.trammell-plus-abstract-mech]). This considers only immutable fields in the transport headers, that is, fields that may be authenticated end-to-end across a path.¶
An example of a method that encrypts some, but not all, transport information is GRE-in-UDP [RFC8086] when used with GRE encryption.¶
The transport information can be made visible in a network-layer header. This has the advantage that this information can then be observed by in-network devices. This has the advantage that a single header can support all transport protocols, but there may also be less desirable implications of separating the operation of the transport protocol from the measurement framework.¶
Some measurements may be made by adding additional protocol headers carrying operations, administration and management (OAM) information to packets at the ingress to a maintenance domain (e.g., an Ethernet protocol header with timestamps and sequence number information using a method such as 802.11ag) and removing the additional header at the egress of the maintenance domain. This approach enables some types of measurements, but does not cover the entire range of measurements described in this document.¶
Another example of a network-layer approach is the IPv6 Performance and Diagnostic Metrics (PDM) Destination Option [I-D.ietf-ippm-6man-pdm-option]. This allows a sender to optionally include a destination option that caries header fields that can be used to observe timestamps and packet sequence numbers. This information could be authenticated by receiving transport endpoints when the information is added at the sender and visible at the receiving endpoint, although methods to do this have not currently been proposed. This method needs to be explicitly enabled at the sender.¶
A drawback of using extension headers is that IPv4 network options are often not supported (or are carried on a slower processing path) and some IPv6 networks are also known to drop packets that set an IPv6 header extension. Another disadvantage is that protocols that separately expose header information do not necessarily have an advantage to expose the information that is utilised by the protocol itself, and could manipulate this header information to gain an advantage from the network.¶
This section explores key implications of working with encrypted transport protocols.¶
Independent observation by multiple actors is important for scientific analysis. Encrypting transport header encryption changes the ability for other actors to collect and independently analyse data. Internet transport protocols employ a set of mechanisms. Some of these need to work in cooperation with the network layer - loss detection and recovery, congestion detection and congestion control, some of these need to work only end-to-end (e.g., parameter negotiation, flow-control).¶
When encryption conceals information in the transport header, it could be possible for an applications to provide summary data on performance and usage of the network. This data could be made available to other actors. However, this data needs to contain sufficient detail to understand (and possibly reconstruct the network traffic pattern for further testing) and to be correlated with the configuration of the network paths being measured. Sharing information between actors needs also to consider the privacy of the user and the incentives for providing accurate and detailed information. Protocols that expose the state information used by the transport protocol in their header information (e.g., timestamps used to calculate the RTT, packet numbers used to asses congestion and requests for retransmission) provide an incentive for the sending endpoint to provide correct information, increasing confidence that the observer understands the transport interaction with the network. This becomes important when considering changes to transport protocols, changes in network infrastructure, or the emergence of new traffic patterns.¶
The patterns and types of traffic that share Internet capacity changes with time as networked applications, usage patterns and protocols continue to evolve.¶
If "unknown" or "uncharacterised" traffic patterns form a small part of the traffic aggregate passing through a network device or segment of the network the path, the dynamics of the uncharacterised traffic may not have a significant collateral impact on the performance of other traffic that shares this network segment. Once the proportion of this traffic increases, the need to monitor the traffic and determine if appropriate safety measures need to be put in place.¶
Tracking the impact of new mechanisms and protocols requires traffic volume to be measured and new transport behaviours to be identified. This is especially true of protocols operating over a UDP substrate. The level and style of encryption needs to be considered in determining how this activity is performed. On a shorter timescale, information may also need to be collected to manage denial of service attacks against the infrastructure.¶
Information provided by tools observing transport headers can help determine whether mechanisms are needed in the network to prevent flows from acquiring excessive network capacity, and where needed to deploy appropriate tools Section 1.1.2.4. Obfuscating or hiding this information using encryption is expected to lead operators and maintainers of middleboxes (firewalls, etc.) to seek other methods to classify and mechanisms to condition network traffic. A lack of data seems likely to reduce the level of precision with which these mechanisms are applied, and this needs to be considered when evaluating the impact of designs for transport encryption.¶
Measurement data is increasingly being used to inform design decisions in networking research, during development of new mechanisms and protocols and in standardisation. Measurement has a critical role in the design of transport protocol mechanisms and their acceptance by the wider community (e.g., as a method to judge the safety for Internet deployment). Observation of pathologies are also important in understanding the interactions between cooperating protocols and network mechanism, the implications of sharing capacity with other traffic and the impact of different patterns of usage.¶
Attention needs to be paid to the expected scale of deployment of new protocols and protocol mechanisms. Whatever the mechanism, experience has shown that it is often difficult to correctly implement combination of mechanisms [RFC8085]. These mechanisms therefore typically evolve as a protocol matures, or in response to changes in network conditions, changes in network traffic or changes to application usage.¶
The growth and diversity of applications and protocols using the Internet continues to expand - and 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 [I-D.ietf-tcpm-dctcp], and methods proposed for Low Latency Low Loss Scalable throughput, L4S). For each new method it is desirable to build a body of data reflecting its behaviour under a wide range of deployment scenarios, traffic load, and interactions with other deployed/candidate methods.¶
Open standards motivate a desire for this evaluation to include independent observation and evaluation of performance data, which in turn suggests control over where and when measurement samples are collected. This requires consideration of the appropriate balance between encrypting all and no transport information.¶
The author would like to thank all who have talked to him face-to-face or via email. ...¶
This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 688421.The opinions expressed and arguments employed reflect only the authors' view. The European Commission is not responsible for any use that may be made of that information.¶
This document is about design and deployment considerations for transport protocols. 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; no current full-featured standards-track transport protocol provides these features on its own. Therefore, these features are not considered in this document, with the exception of native authentication capabilities of TCP and SCTP for which the security considerations in RFC4895.¶
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. Standard protocols and understanding of the interactions between mechanisms and traffic patterns can also provide valuable insight into appropriate security design. Like congestion control mechanisms, security mechanisms are difficult to design and implement correctly. It is hence recommended that applications employ well-known standard security mechanisms such as DTLS, TLS or IPsec, rather than inventing their own.¶
XX RFC ED - PLEASE REMOVE THIS SECTION XXX¶
This memo includes no request to IANA.¶
-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 contributore 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.¶