TSVWG G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational May 29, 2017
Expires: November 30, 2017

The Impact of Transport Header Encryption on Operation and Evolution of the Internet


This document describes the 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 transport header authentication,. 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.

Status of This Memo

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This Internet-Draft will expire on November 30, 2017.

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Table of Contents

1. Introduction

This document discusses the implications of end-to-end encryption applied at the transport layer, and examines the impact on transport protocol design, transport use, and network operations and management. It also considers some anticipated implications on transport and application evolution.

The transport layer is the first end-to-end layer in the network stack. Despite headers having end-to-end meaning, 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 prvents visibility of transport headers, and have implications on how network protocols are designed and used (e.g., [I-D.mm-wg-effect-encrypt]).

Transport information that is sent without end-to-end authentication could be modified by "middleboxes" - defined as any intermediary box performing functions apart from normal, standard functions of an IP router on the data path between a source host and destination host [RFC3234]. When transport headers are modified by network devices on the path this can change the end-to-end protocol transport protocol behaviour in a way that may benefit the user or may hinder transport performance and application experience. Whatever the outcome, modification of packets by a middlebox was not usually intended when the protocol was specified and is usually not known by the sender or receiver.

Middleboxes have been deployed for a variety of reasons [RFC3234], including middlebox protocol enhancement, proxy-based methods, such as Protocol Enhancing Proxies (PEPs) [RFC3135], TCP acknowledgement (ACK) enhancement [RFC3449], use of application protocol caches [I-D.mm-wg-effect-encrypt], application layer gateways [I-D.mm-wg-effect-encrypt], etc. [I-D.dolson-plus-middlebox-benefits] summarizes some of the functions provided to the Internet by such middleboxes, and the benefits that may arise when they are used in a number of deployment scenarios. Methods that involve in-network modification of transport headers are not further discussed.

This document notes that transport protocols can be designed to encrypt or authenticate transport header fields. Authentication methods can be used at the transport layer to detect any changes to an immutable header field that were made by a network device along a path. These methods do not require encryption of the header fields and hence these fields may remain visible to network devices. End-to-end authentication allows the receiving transport endpoint to avoid accepting modified protocol headers. This document therefore considers the case where protocol fields in the transport header are not altered as a packet traverses the network path.

Authentication methods have also been specified at the network layer, and cover fields not protected by a transport authentication header. Network layer header fields can convey codepoints that are increasingly being 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].

Encryption methods can help to 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 authentication nor encryption methods prevent traffic analysis, and usage needs to reflect that profiling of users and fingerprinting of behaviour can take place even on encrypted traffic flows.

This document seeks to identify the implications of various approaches to transport protocol authentication and encryption.

2. Internet Transports and Pervasive Encryption

End-to-end encryption can be applied at various protocol layers. It can be applied above the transport to encrypt the transport payload. 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 [RFC3135].

Another perspective stems from increased concerns about privacy and surveillance . Some Internet users have valued the ability to protect identity and defend against traffic analysis, and have used methods such as IPsec ESP and Tor [Tor]. Revelations about the use of pervaisive 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]).

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 canin turn impact the direction of evolution of the TCP/IP stack.

The next subsections briefly review some security design options for transport protocols.

2.1. Authenticating the Transport Protocol Header

Transport layer header information can be authenticated. An authentication method protects the integrity of immutable transport header fields, but can still expose the transport protocol header information in the clear, allowing in-network devices to observes these fields. Authentication can not prevent in-network modification, but can avoid 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 behavior [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.

2.2. Encrypting the Transport Payload

The transport layer payload can be encrypted to protect the content of transport segments. This leaves transport protocol header information in the clear. The integrity of immutable transport header fields could be protected by combining this with authentication methods (Section 2.1).

Examples of encrypting the payload include TLS over TCP [RFC5246] [RFC7525] or DTLS over UDP [RFC6347] [RFC7525].

2.3. Encrypting the Transport Header

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.

2.4. Authenticating Transport Information and Selectively Encrypting the Transport Header

A transport protocol design can encrypt selected header fields, while also choosing to authenticate some or all of other fields in the transport header. This allows only specific transport header fields to be observable by network devices. End-to end authentication 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 the use of immutable fields in the transport headers, that is, fields that could be authenticated end-to-end across a transport path.

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

2.5. Adding transport information to network-layer Protocol Headers

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 packet headers carrying operations, administration and management (OAM) information to packets at the ingress to a maintenance domain (e.g., adding 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 measurments 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 cariies header fields that can be used to observe timestamps and packet sequence numbers. Transmission of the packets with thsi option can be impacted by destination-options processing by network devices. 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 seprately 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.

3. Use of Transport Headers in the Network

This section identifies ways that observable (non-encrypted) transport header fields can be used by devices in the network. There are a number of actors who can benefit from observing this information. These include:

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 server and browser are dependent on browser supplier and version, even when the sessions use the same server application (e.g., web e-mail access). Even when measurment 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 such metadata is more difficult when the observation point is at a different location to the bottleneck/equipment under evaluation.

To observe protocol headers requires knowledge of the format of the transport header. In-network observation of transport protocol headers requires:

If there is more than one format for visible headers, the observer needs to know the protocol that is used. As protocols evolve over time and there mau be a need to introduce new transport headers.This may require interpretation of protocol version information.TCP and SCTP specify a standard base header that includes sequence number information and other data. TCP and SCTP options may be negotiated to indicate the presence of new (negotiated) features, the size and function of each option is identified by an option number in the transport header.

Protocols that expose header information that is utilised by the protocol itself provide an incentive for the endpoints to provide correct information.

Packet sampling techniques can be used to scale processing involved in observing apckets on high rate links. This only exports 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.

The following subsections describe various ways that observable transport information may be utilised.

3.1. Use to Identify Flows

Transport protocol header infromation can identify the connection state of a flow, and identify separate flows operating over a path.

Connection information can assist a firewall in deciding which flows are permitted through a security gateway [I-D.trammell-plus-statefulness], or to help maintain the network address translation (NAT) bindings in a NAPT or application layer gateway. This information may also find use in load balancers, where visibility of the 5-tuple and meaningful use could be used as a method for determining forwarding or selecting a server [I-D.mm-wg-effect-encrypt].

The use of UDP as a substrate protocol is discussed further in Section 4.1.2, and the implications of mobility bindings in Section 4.1.3.

3.2. Use to derive Traffic Statistics

Passive monitoring uses observed traffic to makes inferences frok transport headers to derive measurements. A variety of open source and commecial tools exists that can utilise the information in RTP and RTCP headers to derive traffic volume measurements and provide infromation on the progress and quality of a session using RTP.

Any Internet transport or application could report data to the network, by sending status packets or by providing access to measurement data. However, to be useful a user of measurement data needs to trust the source of this data and importantly require metadata to understand the context under which the data was collected, including the time, observation point, and way in which metrics were accumulated.

When encryption conceals information in packet headers, measurments need to rely on pattern inferences and other heuristics grows, and accuracy suffers [I-D.mm-wg-effect-encrypt].

3.2.1. Use to Characterise Traffic Rate and Volume

Operators can measure per-subscriber information about the volume and pattern of network usage. Transport headers may be observed on a per-application (or per endpoints) basis. Capacity usage ican be valuable for capacity planning (providing more detail of trends rather than the volume per subscriber). This can also be used for measurement-based traffic shaping and to implement QoS support within the network and lower layers.

3.2.2. Use of the Network-Layer DSCP

Applications can expose their delivery expectations to the network allowing endpoints to encode in the Differentiated Services Code Point (DSCP) field of IPv4 and IPv6 packets. Setting this field provides explicit information that can be used in place of inferring traffic requirements (e.g., by inferring QoS requirements from port information via a multi-field classifier). This information can be collected by measurement campaigns, but does not directly provide any performance data.

3.2.3. Measuring Loss rate and Loss Pattern

Various actors have a need to characterise link/network segments and derive key performance indicators (retransmission rate, packet drop rate, sector utilization Level, a measure of reordering, peak rate, the CE-marking rate, etc.). The quality of a transport path may be assessed using dedicated tools that generate test traffic. However such tools need to be run from an endpoint, and most operators do not have access to this equipment. There also may be costs associated with running such tests. (e.g., the implications of bandwidth tests in a mobile network are obvious.) An alternative is to use in-network techniques that observe visible transport packet sequence numbers to determine transport flow statistics.

The design tradeoffs for radio networks are often very different to those of wired networks. A radio-based network (e.g., cellular mobile, enterprise WiFi, satellite access/backhaul, 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 pattern of loss and congestion, impact of different traffic types, correlation with propagation and interference measures can all have significant impact on the cost and performance of providing a service. The need for this type of information is expected to increase as operators seek to bring together heterogeneous types of network equipment and seek to deploy opportunistic methods to access radio spectrum.

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. Often impact is only understood in the context of the other flows that share a bottleneck. In summary, the common language between network operators and application/content providers/users is packet transfer performance at a layer that all can view and analyze. For most packets, this has been transport layer, until the emergence of QUIC, with the obvious exception of VPNs and IPsec.

A simple example is the monitoring of Active Queue Management (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]).

3.2.4. Measuring Throughput and Goodput

The throughput observed by a flow can be determined even when a flow is encrypted, providing the individual flow can be identified. Goodput [RFC7928] is a measure of useful data exchanged (the ratio of useful/total volume of traffic sent by a flow), which requires ability to differentiate the different ways packets are used at the remote endpoint (e.g., by observing duplicate packet sequence numbers in TCP).

3.2.5. Measuring Latency (Network Transit Delay and Jitter)

Latency is a key performance metric that impacts application response time and user perceived response time. This often indirectly impacts throughput and flow completion time. It also determines the reaction time of the transport protocol itself, impacting flow setup, congestion control, loss recovery, and other transport mechanisms. The overall latency can have many components [Latency], but of these unnecessary/unwanted queuing in network buffers has often been observed as a significant factor. Once the cause of unwanted latency has been identified, this can often be eliminated, and determining latency metrics is a key driver in the deployment of AQM [RFC7567], DiffServ [RFC2474], and ECN [RFC3168] [RFC8087].

To measure latency across a part of the path, an observation point has to measure the experienced round-trip time (RTT). This can be achieved using packet sequence numbers, and acknowledgement points. An observation point in the network is able to determine not only the path RTT, but also to measure the upstream and downstream RTT, respectively to the sending and receiving endpoints. This may be used to locate a source of latency, e.g., by observing cases where the ratio of median to minimum RTT is large for a part of a path.

An example usage of this method could be to identify excessive buffers and to deploy or configure Active Queue Management (AQM) [RFC7567] [RFC7928]. Operators deploying such tools can effectively eliminate unnecessary queuing in routers and other devices. AQM methods need to be deployed at the capacity bottleneck, but are often deployed in combination with other techniques, such as scheduling [RFC7567] [I-D.ietf-aqm-fq-codel] and although parameter-less methods are desired [RFC7567], current methods [I-D.ietf-aqm-fq-codel] [I-D.ietf-aqm-codel] [I-D.ietf-aqm-pie] often cannot scale across all possible deployment scenarios. The service offered by operators can therefore benefit from latency information to understand the impact of deployment and tune deployed services.

Some network applications are sensitive to packet jitter, and to support this type of application, it can be useful to monitor the jitter observed along a portion of the path. The requirements to measure jitter resemble those for the measurement of latency.

3.2.6. Measuring Flow Reordering

Significant flow reordering can impact time-critical applications and can be interpreted as loss by reliable transports. Many transport protocols (e.g., TCP) therefore use technqiues that are impacted reordering. Packet reordering can occur for many reasons (from equipment design to misconfiguration of forwarding rules). As in the drive to reduce network latency, there is a need for operational tools to be able to detect misordered packet flows and quantify the degree or reordering. Techniques for measuring reordering typically observe packet sequence numbers. Metrics have been defined that evaluate whether a network has maintained packet order on a packet-by-packet basis [RFC4737] and [RFC5236].

There has been initiatives in the IETF transport area to reduce the impact of reordering withing a transport flow, possibly leading to reduced the requirements for ordering. These have promise to simplify network equipment design as well as the potential to improve robustness of the transport service. Measurements of reordering can help understand the level of reordering within deployed infrastructure, and inform decisions about how to progress such mechanisms.

3.3. Network-Layer Header Information

Some network-layer information is closely tied to transport protocol operation.

3.3.1. Use of IPv6 Network-Layer Flow Label

Endpoints should expose flow information in the IPv6 Flow Label field of the network-layer header (e..g. [RFC8085]). This can be used to inform network-layer queuing, forwarding (e.g., for equal cost multi-path (ECMP) routing, and Link Aggregation (LAG)). This can provide useful information to assign packets to flows in the data collected by measurement campaigns, but does not directly provide any performance data.

3.3.2. Use Network-Layer Differentiated Services Code Point Point (DSCP)

Application should expose their delivery expectations to the network allowing endpoints to encode in the Differentiated Services Code Point (DSCP) field of IPv4 and IPv6 packets. This can be used to inform network-layer queuing and forwarding, and can also provide information on the relative importance of packet information collected by measurement campaigns, but does not directly provide any performance data.

Setting this field provides explicit information that can be used in place of inferring traffic requirements (e.g., by inferring QoS requirements from port information via a multi-field classifier).

3.3.3. Use of Explicit Congestion Marking

Explicit Congestion Notification (ECN)[RFC3168] uses a codepoint in the network-layer header. This exposes the presence of congestion on a network path to the transport and network layer. Use of ECN can offer gains in terms of increased throughput, reduced delay, and other benefits when used over a path that includes equipment that supports an AQM method that performs Congestion Experienced (CE) marking of IP packets [RFC8087].

ECN CE-marks are visible to in-network devices on the transport path. The reception of CE-marked packets can therefore be used to monitor the presence and estimate the level of incipient congestion on the upstream portion of the path from the point of observation (Section 2.5 of [RFC8087]). Because ECN marks carried in the IP protocol header, measuring ECN can be much easier than metering packet loss. However, interpretting the marking behaviour (i.e., assessing congestion and diagnosing faults) requires context from the transport layer (path RTT, visibility of loss - that could be due to queue overflow, congestion response, etc) [RFC7567].

Some proposed ECN-capable network devices provide richer (more frequent and fine-grained) indication of their congestion state. setting congestion marks proportional to the level of congestion (e.g., DCTP [I-D.ietf-tcpm-dctcp], and L4S [I-D.ietf-tsvwg-l4s-arch]).

AQM and ECN can use and combine a range of algorithms and configuration options, it is therefore important for tools to be available to network operators and researchers to understand the implication of configuration choices and transport behaviour as use of ECN increases and new methods emerge [RFC7567] [RFC8087]. ECN-monitoring is expected to become important as AQM is deployed that supports ECN [RFC8087]

Section Section 4.4 describes the transport layer feedback information that accompanies the use of ECN.

3.4. Use by Operators to Plan and Provision Networks

Traffic measurements can and is used by operators to help plan deployment of new equipment and configurations in their networks. Data is also important to equipment vendors who also need to understand trends in the volume of traffic and the patterns of usage as inputs to decisions about planning and provisioning.

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

3.5. Use for Network Diagnostics and Troubleshooting

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.

In-network measurements that can distinguish between upstream and downstream metrics with respect to the measurement point are particularly useful to locating the source of problems or to asses the performance of a network segment.

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. Although many network operators utilise transport information as a part of their operational practice, the network will not break because transport headers are encrypted.

3.6. Verification of Acceptable Response to Congestion

Many network operators implicitly accept that TCP traffic to conform to a behaviour that is acceptable for use in the shared Internet. TCP algorithms have been continuously improved over decades, and they have reached a level of efficiency and correctness that custom application-layer mechanisms will struggle to easily duplicate [RFC8085]. A standards-compliant TCP stack provides congestion control that is therefore 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 TCP transports.

Tools exist that can interpret the transport protocol header information to help understand the impact of specific transport protocols (or protocol mechanisms) on other traffic that shares their network. An observation in the network can gain understanding of the dynamics of a flow and its congestion control behaviour, by observing TCP sequence numbers to show how a flow shares available capacity, deduce its congestion dynamics, etc. (e.g., it is common to visualise plots of TCP sequence numbers versus time [Osterman]). Analysing packet sequence numbers can be used to help understand whether an application flow backs-off its share of the network load in the face of persistent congestion, and hence to understand whether the behaviour is appropriate for sharing limited network capacity.

The User Datagram Protocol (UDP) provides a minimal message-passing 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 an Internet transport must employ mechanisms to prevent congestion collapse, avoid unacceptable contributions to jitter/latency, and to establish an acceptable share of capacity with concurrent traffic [RFC8085]. A network operator has no way of knowing the specific methods used by UDP applications, and an operator may need to deploy methods such as rate-limited, transport circuit breakers or other methods to enforce acceptable usage.

UDP flows can also expose a well-known header by specifying the format of header fields. This information 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 the RTP and RTCP header information of real-time flows (see Section 3.2).

Independent observation by multiple actors is important for scientific analysis, and ability to validate the behaviour in-situ within a network is important. Transport header encryption changes the ability for other actors to collect and independently analyse data. This is important when considering transport protocols (e.g., changes to transport mechanisms, changes in network infrastructure, and changes in the transport use).

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), BBR, the introduction of active queue management (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.

This has implications:

3.6.1. Impact on Network Operations

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).

Information provided by tools can help determine whether mechanisms are needed in the network to prevent flows from acquiring excessive network capacity. Operators can 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].

3.6.2. Accountability and the Evolution of Internet Transport

One often used premise is to "trust but verify" the behaviours of protocol using the network.

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. 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.

Measurement have 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. Open standards suggest that such evaluation needs to include independent observation and evaluation of performance data.

4. The Effect of Encrypting Transport Header Fields

This section examines implications of encrypting specific transport header fields.

4.1. Flow Identifier

To measure and analyse flow traffic, a measurement tool needs to be able to identify traffic flows. Aggregation of sessions, and persistent use of established transport flows by multiple sessions means that a flow at the transport layer is not necessarily the same as a flow seen at the application layer. This is usually not a consequence, and data is measured for the aggregate transport flow.

If flow information is observed from transport headers, then there needs to be a way to identify the format of the header - such as observing parameter negotiation at connection setup, identifying the protocol version from other data (e.g., a "magic" number embedded in the header). This allows an observer to determine the presence, size and position of any observable header fields fro protocol decapsulation (decoding).

Some measurement methods sample traffic, rather than collecting all packets passing through a measurement point. These methods still require a way to determine the presence, size and position of any observable header fields.

4.1.1. Identification by a well-known Transport Port

All IETF-defined transport protocols include a transport port field in their transport header. Observation of a well-known port value may be indicative of the protocol being encapsulated, but there is no way to enforce this usage. This can be used to configure decapsulation. This is not the necessarily case, e.g., RTP traffic may utilise ephemeral ports, requiring measurement tools to include additional methods to determine the protocol being used.

4.1.2. Use of a Transport as a Substrate

When a transport is used as a substrate, the transport provides an encapsulation that allows another transport flow to be within the payload of a transport flow. The transported protocol header may provide additional information for multiplexing multiple flows over the same 5-tuple. The UDP Guidelines [RFC8085] provides some guidance on using UDP as a substrate protocol. If there is no additional information about the protocol transported by the substrate, this may be viewed as an opaque traffic aggregate.

Examples include GRE-in-UDP, SCTP-in-UDP. GRE-in-UDP may include an encryped payload, but does not encrypt the GRE protocol header.

4.1.3. Mobility and Flow Migration

With the proliferation of mobile connected devices, there is a stated need for connection-oriented protocols to maintain connections after a network migration by an endpoint. The ability and desirability of in-network devices to track such migration depends on the context. On the one hand, a load-balancer device in front of server may find it useful to map a migrated connection to the same server endpoint. On the other hand, a user performing migration to avoid detection may prefer the network not to be able to correlate the different parts of a migrating session. Care must then be exercised to make sure that the information encoded by the endpoints is not sufficient to identify unique flows and facilitate a persistent surveillance attack vector [I-D.mm-wg-effect-encrypt].

The impact of flow migration on measurement activities depends on the data being measured, rate of migration and level of encryption that is employed.

Requirements for load balancing and mobility can lead to complex protocol interactions.

QUIC is an example of a transport protocol designed to provide mobolity, which is in development by the IETF.

4.1.4. IPv6 Network-Layer Flow Label

Endpoints should expose flow information in the IPv6 Flow Label field of the network-layer header (e..g. [RFC8085]). This can be used to inform network-layer queuing, forwarding (e.g., for equal cost multi-path (ECMP) routing, and Link Aggregation (LAG)). This can provide useful information to assign packets to flows in the data collected by measurement campaigns, but does not directly provide any performance data.

4.1.5. Flow Start and Stop Indicator

Transports can expose that start and end of flows in a transport header field (e.g., TCP SYN, FIN, RST). This can also help measurement devices identify the start of flows, or to remove stale flow information. This use resembles the use by in-network devices such as firewalls and NAPTs. It provides supplemental information - flows can start and end at any time, the Internet network layer provides only a best effort service that allows alternate routing, reordering, loss, etc, so a network measurement tool can not rely upon observing these indicators. The time to complete a protoocl connection and/or session setup can be measured as a peformance metric.

One consequence of encrypting transport headers, is that this information is not visible to forwarding devices (such as a NAPT or Firewall). This may impact the network service. For example, UDP-based middlebox traversal usually relies on timeouts to remove old state, since middleboxes are unaware when a particular flow ceases to be used by an application [RFC8085]. This can often lead to the state table entries not being kept as long as those for which the flows are identifiable.

4.2. Use of Transport Sequence Number

The TCP or RTP sequence number can be observed in one direction (the path that carries data segments). An authenticated header prevents this field being modified or terminated/split [RFC3135] by a network device, but allows this to be used to observe progress of the network flow.

An incrementing sequence number enables detection of loss (either by correlating ingress and egress value, or when assuming that all packets follow a single path), duplication and reordering (with understanding that not necessarily all packets of a flow follow the same path, and reordering can complicate processing of observations). Tools are widely available to interpret RTP and TCP sequence numbers - ranging from open source tools to dedicated commercial packages.

4.3. Use of Transport Sequence Acknowledgment Number

Acknowledgement (ACK) data provides information about the path from the network device to the remote endpoint. The information can help identify packet loss (or the point of loss), RTT, and other network-related performance parameters (e.g., throughput, jitter, reordering). Unless this information is correlated with other data there is no way to disambiguate the cause of impairments (congestion loss, link transmission loss, equipment failure).

An in-network device must not modify the flow of end-to-end ACK data when using an authenticated protocol. That is, must not use the in-network methods described in [RFC3449]. This can impact the performance and/or efficiency (e.g., cost) of using paths where the return capacity is limited or has implications on the overall design (e.g., using TCP with cellular mobile uplinks, DOCSIS uplinks).

The TCP stream can be observed by correlating the stream of TCP ACKs that flow from a receiver in the return direction. Although these ACKs are cumulative, and are not necessarily sent on the same path as the forward data, when visible, their sequence can confirm successful transmission and the path RTT. In the case of TCP they may also indicate packet loss (duplicate ACKs).

An RTP session can provide RTCP [RFC3605] [RFC4585] feedback using the RTP framework. This reception information and can be observed by in-network measurement devices and can be interpreted to provide a variety of quality of experience information for the related RTP flow, as well as basic network performance data (RTT, loss, jitter, etc).

4.4. Use of ECN Transport Feedback Information

Transport protocols that use ECN Section 3.3.3 need to provide ECN feedback information in the transport header to inform the sender whether packets have been received with an ECN CE-mark [RFC3819]. This information can be in the form of feedback once each RTT [RFC3819] or more frequent. The latter may involve sending a detailed list of all ECN-marked packets (e.g., [I-D.ietf-tcpm-accurate-ecn] and [RFC6679]). The detailed information can provide detail about the pattern and rate of marking. The information provided in these protocol headers can help a network operator to understand the congestion status of the forward path and the impact of marking algorithms on the traffic that is carried [RFC8087].

IETF specifications for Congestion Exposure (CONEX) [RFC7713] and Per-congestion Notification (PCN) [RFC5559] are examples of frameworks that monitor reception reports for CE-marked packets to support network operations.

4.5. Interpretation of Transport Header Fields

Understanding and analysing transport protocol behaviour typically demands tracking changes to the protocol state at the transport endpoints. Although protocols communicate state information in their protocol headers, a protocol implementation typically also contains internal state that is not directly visible from observing transport protocol headers. Effective measurement tools need to consider that not all packets may be observed (due to drops at the capture tap or because packets take an alternate route that does not pass the tap). Some flows of packets may also be encapsulatedmaintenance domain in other protocols, which further complicates analysis.

Some examples of using network measurements of transport headers to infer internal TCP state information include:

5. Implications on Evolution of the Internet Transport

Architectural, the transport layer provides the first end-to-end interactions across the Internet. The transport protocols are layered directly over the network service and are sent in the payload of network-layer packets. However, this simple architecural view hides one of the core functions of the transport, 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 avoiding the unwanted side effects of congestion, avoiiding 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 for a specific application. The IETF transport community has to date relied heavily on measurement and insight provided from the wider community to understand the trade-offs and to inform selection of select appropriate mechansims to ensure a safe, reliable and robust Internet.

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 functionality provided by middleboxes both to enhance performance and support network operations. This presents a need for architectural changes and new approaches to the way network transport protocols are designed [Measure].

There are many motivations for deploying encrypted transports, and encryption of transport payloads. This document has expanded upon the expected implications on operational practices when working with encrypted transport protocols, and offers insight into the potential benefit of authentication, encryption and techniques that require in-network devices to interpret specific protocol header fields.

The use of encryption to protect individual privacy may reasonably be considered a choice that users may make. This comes with implications that need to be considered:

Troubleshooting and diagnostics.
Encrypting all transport information eliminates the incentive for operators to troubleshoot what they cannot interpret: one flow experiencing packet loss looks like any other. When transport header encryption prevents decoding the transport header (if sequence numbers and flow ID are obscured), and hence understanding the impact on a particular flow or flows that share a common network segment. Encrypted traffic therefore implies "don't touch", and a likely first response will be "can't help, no trouble found", or the implication that this complexity comes with an additional operational cost [I-D.mm-wg-effect-encrypt].

Open verifyable data
The use of transport header encryption may reduce the range of parties who can capture useful measurement data. This may restricts the information sources of available to the Internet community to understand the operation of the network and transport protocols that use this to inform design decisions for new protocols, new equipment and operational practice. This could mean that key information is only available at endpoints: i.e., at user devices and within service platforms. While these devices could be designed to offer data about the network paths that they use, this can not be independently captured - and therefore a new level of trust is required between these actors and those that use this data.

Operational practice
Published transport specifications can bring assurance to those operating networks that they have sufficient understanding to not deploy complex techniques to not routinely monitor and to not need to routinely manage TCP/IP traffic flows (e.g. Avoiding the capital and operational costs of deploying flow rate-limiting and network circuit-breaker methods). This should continue when encrypted transport headers are used, providing the traffic produced conforms to the expectations of the operator. However, operators will need to establish this is the case.

Traffic analysis
The use of encryption makes it harder to determine which transport methods are being used across a network segment and the trends in usage. This could impact the ability for an operator to anticipate the need for network upgrades and roll-out. It can also impact on-going traffic engineering activities. Although the impact in many case may be small, there are cases where operators directly support services (e.g., in radio environments) and the more complex the underlying infrastructure the more important this impact.

Interactions between mechanisms
Encryption restricts the ability to explore interactions between functions at different protocol layers. This is a side-effect of not allowing a choice of the vantage point from which this information is observed, an important issue in examining collateral impact of flows sharing a bottleneck, or where the intention is to understand the interaction between a layer 2 function (e.g., radio resource management policy, a channel impairment, an AQM configuration, a PHB or scheduling method) and a transport protocol. An appropriate vantage point, coupled with timing information for the flow (fine-grained timestamps) is a valuable tool in benchmarking equipment/configurations and understanding non-trivial interactions.

Common specifications
Since the introduction of congestion control, TCP has continued to contribute the predominate transport, with a range but consistent approach to avoiding congestion collapse. There is also a risk that the diversity of transport mechanisms could also increase, with incentives to use a wide range of methods, this is not in itself a problem, nor is this a direct result of encryption. However, when encryption is used, this risk needs to be weighed against the reduced visibility to network operators. Especially, if a development cycle focused on specific protocols/applications could for instance incentivise optimisations (e.g., expectations of capacity, expectations of RTT, loss rate, level of multiplexing, etc) that may prove suboptimal for users or operators that utilise a network segments with different characteristics than targeted by the developer. Encryption places the onus on validation in the hands of developers. While there is little to doubt that developers will seek to produce high quality code for their target use, it is not clear whether there is sufficient incentive to ensure good practice that benefits the wide diversity of requirements from the Internet community as a whole.

Restricting research and development
The use of encryption may impede independent research and development initiatives. Experience shows that high quality transport protocols are complicated to design and complex to deploy, and that individual mechanisms need to be evaluated while considering other mechanism, across a broad range of network topologies and with attention to the impact on traffic sharing the capacity. This could eliminate the independent self-checks that have previously been in place from research and academic contributors (e.g., the role of ICCRG, and research publications in reviewing new transport mechanisms and assessing the impact of their experimental deployment).

Pervasive use of transport header encryption can impact the ways that future protocols are designed and deployed. The choice of whether candidate transport designs should encrypt their protocol headers therefore needs to be taken based not just on security considerations, but also on the impact on operating networks and the constrictions this may place on evolution of Internet protocols. While encryption of all transport information can help reduce ossification of the transport layer, it could result in ossification of the network service. There can be advantages in providing a level of ossification of the header in terms of providing a set of open specified header fields that are observable from in-network devices.

6. Acknowledgements

The author would like to thank all who have talked to him face-to-face or via email. ...

7. IANA Considerations


This memo includes no request to IANA.

8. Security Considerations

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.

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.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

9.2. Informative References

[I-D.dolson-plus-middlebox-benefits] Dolson, D., Snellman, J., Boucadair, M. and C. Jacquenet, "Beneficial Functions of Middleboxes", Internet-Draft draft-dolson-plus-middlebox-benefits-03, March 2017.
[I-D.ietf-aqm-codel] Nichols, K., Jacobson, V., McGregor, A. and J. Jana, "Controlled Delay Active Queue Management", Internet-Draft draft-ietf-aqm-codel-00, October 2014.
[I-D.ietf-aqm-fq-codel] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys, J. and E. Dumazet, "FlowQueue-Codel", Internet-Draft draft-ietf-aqm-fq-codel-00, January 2015.
[I-D.ietf-aqm-pie] Pan, R., Natarajan, P., Baker, F. and G. White, "PIE: A Lightweight Control Scheme To Address the Bufferbloat Problem", Internet-Draft draft-ietf-aqm-pie-00, October 2014.
[I-D.ietf-ippm-6man-pdm-option] Elkins, N., Hamilton, R. and m. mackermann@bcbsm.com, "IPv6 Performance and Diagnostic Metrics (PDM) Destination Option", Internet-Draft draft-ietf-ippm-6man-pdm-option-10, May 2017.
[I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-03, May 2017.
[I-D.ietf-tcpm-accurate-ecn] Briscoe, B., KĂźhlewind, M. and R. Scheffenegger, "More Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-tcpm-accurate-ecn-00, December 2015.
[I-D.ietf-tcpm-dctcp] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L. and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters", Internet-Draft draft-ietf-tcpm-dctcp-06, May 2017.
[I-D.ietf-tsvwg-l4s-arch] Briscoe, B., Schepper, K. and M. Bagnulo, "Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture", Internet-Draft draft-ietf-tsvwg-l4s-arch-00, May 2017.
[I-D.mm-wg-effect-encrypt] Moriarty, K. and A. Morton, "Effect of Pervasive Encryption on Operators", Internet-Draft draft-mm-wg-effect-encrypt-11, April 2017.
[I-D.trammell-plus-abstract-mech] Trammell, B., "Abstract Mechanisms for a Cooperative Path Layer under Endpoint Control", Internet-Draft draft-trammell-plus-abstract-mech-00, September 2016.
[I-D.trammell-plus-statefulness] Kuehlewind, M., Trammell, B. and J. Hildebrand, "Transport-Independent Path Layer State Management", Internet-Draft draft-trammell-plus-statefulness-02, December 2016.
[Latency] Briscoe, B., "Reducing Internet Latency: A Survey of Techniques and Their Merits", November 2014.
[Measure] Fairhurst, G., Kuehlewind, M. and D. Lopez, "Measurement-based Protocol Design", June 2017.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, DOI 10.17487/RFC3135, June 2001.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G. and M. Sooriyabandara, "TCP Performance Implications of Network Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449, December 2002.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, DOI 10.17487/RFC3605, October 2003.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, DOI 10.17487/RFC4302, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C. and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI 10.17487/RFC4585, July 2006.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, S. and J. Perser, "Packet Reordering Metrics", RFC 4737, DOI 10.17487/RFC4737, November 2006.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A. and R. Whitner, "Improved Packet Reordering Metrics", RFC 5236, DOI 10.17487/RFC5236, June 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN) Architecture", RFC 5559, DOI 10.17487/RFC5559, June 2009.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/RFC6437, November 2011.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P. and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 2012.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.
[RFC7567] Baker, F. and G. Fairhurst, "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., Trammell, B., Huitema, C. and D. Borkmann, "Confidentiality in the Face of Pervasive Surveillance: A Threat Model and Problem Statement", RFC 7624, DOI 10.17487/RFC7624, August 2015.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx) Concepts, Abstract Mechanism, and Requirements", RFC 7713, DOI 10.17487/RFC7713, December 2015.
[RFC7928] Kuhn, N., Natarajan, P., Khademi, N. and D. Ros, "Characterization Guidelines for Active Queue Management (AQM)", RFC 7928, DOI 10.17487/RFC7928, July 2016.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017.
[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017.
[RFC8086] Yong, L., Crabbe, E., Xu, X. and T. Herbert, "GRE-in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, March 2017.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using Explicit Congestion Notification (ECN)", RFC 8087, DOI 10.17487/RFC8087, March 2017.
[Tor] The Tor Project, , "https://www.torproject.org", June 2017.

Appendix A. Revision information

-00 This is an individual draft for the IETF community

Author's Address

Godred Fairhurst University of Aberdeen Department of Engineering Fraser Noble Building Aberdeen, AB24 3UE Scotland EMail: gorry@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk/