1. RFC 8517
Independent Submission                                    D. Dolson, Ed.
Request for Comments: 8517
Category: Informational                                      J. Snellman
ISSN: 2070-1721
                                                       M. Boucadair, Ed.
                                                            C. Jacquenet
                                                           February 2019

  An Inventory of Transport-Centric Functions Provided by Middleboxes:
                        An Operator Perspective


   This document summarizes an operator's perception of the benefits
   that may be provided by intermediary devices that execute functions
   beyond normal IP forwarding.  Such intermediary devices are often
   called "middleboxes".

   RFC 3234 defines a taxonomy of middleboxes and issues in the
   Internet.  Most of those middleboxes utilize or modify application-
   layer data.  This document primarily focuses on devices that observe
   and act on information carried in the transport layer, and especially
   information carried in TCP packets.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Operator Perspective  . . . . . . . . . . . . . . . . . .   3
     1.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Measurements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Packet Loss . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Round-Trip Times  . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Measuring Packet Reordering . . . . . . . . . . . . . . .   6
     2.4.  Throughput and Bottleneck Identification  . . . . . . . .   7
     2.5.  Congestion Responsiveness . . . . . . . . . . . . . . . .   7
     2.6.  Attack Detection  . . . . . . . . . . . . . . . . . . . .   8
     2.7.  Packet Corruption . . . . . . . . . . . . . . . . . . . .   8
     2.8.  Application-Layer Measurements  . . . . . . . . . . . . .   9
   3.  Functions beyond Measurement: A Few Examples  . . . . . . . .   9
     3.1.  NAT . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Firewall  . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  DDoS Scrubbing  . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  Implicit Identification . . . . . . . . . . . . . . . . .  11
     3.5.  Performance-Enhancing Proxies . . . . . . . . . . . . . .  11
     3.6.  Network Coding  . . . . . . . . . . . . . . . . . . . . .  12
     3.7.  Network-Assisted Bandwidth Aggregation  . . . . . . . . .  13
     3.8.  Prioritization and Differentiated Services  . . . . . . .  13
     3.9.  Measurement-Based Shaping . . . . . . . . . . . . . . . .  14
     3.10. Fairness to End-User Quota  . . . . . . . . . . . . . . .  14
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     5.1.  Confidentiality and Privacy . . . . . . . . . . . . . . .  14
     5.2.  Active On-Path Attacks  . . . . . . . . . . . . . . . . .  15
     5.3.  Improved Security . . . . . . . . . . . . . . . . . . . .  15
   6.  Informative References  . . . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   From [RFC3234], "A middlebox is defined as any intermediary device
   performing functions other than the normal, standard functions of an
   IP router on the datagram path between a source host and destination

   Middleboxes are usually (but not exclusively) deployed at locations
   permitting observation of bidirectional traffic flows.  Such
   locations are typically points where leaf networks connect to the
   Internet, for example:

   o  Where a residential or business customer connects to its service
      provider(s), which may include multihoming;

   o  On the Gi interface where a Gateway General Packet Radio Service
      (GPRS) Support Node (GGSN) connects to a Packet Data Network (PDN)
      (Section 3.1 of [RFC6459]).

   For the purposes of this document (and to be consistent with the
   definition in RFC 3234), middlebox functions may be found in routers
   and switches in addition to dedicated devices.

   This document itemizes a variety of features provided by middleboxes
   and by ad hoc analysis performed by operators using packet analyzers.

   Many of the techniques described in this document require stateful
   analysis of transport streams.  A generic state machine is described
   in [PATH-STATE].

   This document summarizes an operator's perception of the benefits
   that may be provided by middleboxes.  A primary goal is to provide
   information to the Internet community to aid understanding of what
   might be gained or lost by decisions that may affect (or be affected
   by) middlebox operation in the design of new transport protocols.
   See Section 1.1 for more details.

1.1.  Operator Perspective

   Network operators are often the ones first called upon when
   applications fail to function properly, often with user reports about
   application behaviors (not about packet behaviors).  Therefore, it
   isn't surprising that operators (wanting to be helpful) desire some
   visibility into flow information to identify how well the problem
   flows are progressing and how well other flows are progressing.

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   Advanced service functions (e.g., Network Address Translators (NATs),
   firewalls, etc.)  [RFC7498] are widely used to achieve various
   objectives such as IP address sharing, firewalling, avoiding covert
   channels, detecting and protecting against ever-increasing
   Distributed Denial of Service (DDoS) attacks, etc.  For example,
   environment-specific designs may require a number of service
   functions, such as those needed at the Gi interface of a mobile
   infrastructure [USE-CASE].

   These sophisticated service functions are located in the network but
   also in service platforms or intermediate entities (e.g., Content
   Delivery Networks (CDNs)).  Network maintenance and diagnostic
   operations are complicated, particularly when responsibility is
   shared among various players.

   Network Providers are challenged to deliver differentiated services
   as a competitive business advantage while mastering the complexity of
   the applications, (continuously) evaluating the impacts on
   middleboxes, and enhancing customers' quality of experience.

   Despite the complexity, removing all those service functions is not
   an option because they are used to address constraints that are often
   typical of the current (and changing) Internet.  Operators must deal
   with constraints such as global IPv4 address depletion and support a
   plethora of services with different requirements for QoS, security,
   robustness, etc.

1.2.  Scope

   Although many middleboxes observe and manipulate application-layer
   content (e.g., session boarder controllers [RFC5853]), they are out
   of scope for this document, the aim being to describe middleboxes
   using transport-layer features.  [RFC8404] describes the impact of
   pervasive encryption of application-layer data on network monitoring,
   protecting, and troubleshooting.

   The current document is not intended to recommend (or prohibit)
   middlebox deployment.  Many operators have found the value provided
   by middleboxes to outweigh the added cost and complexity; this
   document attempts to provide that perspective as a reference in
   discussion of protocol design trade-offs.

   This document is not intended to discuss issues related to
   middleboxes.  These issues are well known and are recorded in
   existing documents such as [RFC3234] and [RFC6269].  This document
   aims to elaborate on the motivations leading operators to enable such
   functions in spite of complications.

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   This document takes an operator perspective that measurement and
   management of transport connections is of benefit to both parties:
   the end user receives a better quality of experience, and the network
   operator improves resource usage, the former being a consequence of
   the latter.

   This document does not discuss whether exposing some data to on-path
   devices for network-assistance purposes can be achieved by using
   in-band or out-of-band mechanisms.

2.  Measurements

   A number of measurements can be made by network devices that are
   either on-path or off-path.  These measurements can be used either by
   automated systems or for manual network troubleshooting purposes
   (e.g., using packet-analysis tools).  The automated systems can
   further be classified into two types: 1) monitoring systems that
   compute performance indicators for single connections or aggregates
   of connections and generate aggregated reports from them; and 2)
   active systems that make decisions also on how to handle packet flows
   based on these performance indicators.

   Long-term trends in these measurements can aid an operator in
   capacity planning.

   Short-term anomalies revealed by these measurements can identify
   network breakages, attacks in progress, or misbehaving devices/

2.1.  Packet Loss

   It is very useful for an operator to be able to detect and isolate
   packet loss in a network.

   Network problems and underprovisioning can be detected if packet loss
   is measurable.  TCP packet loss can be detected by observing gaps in
   sequence numbers, retransmitted sequence numbers, and selective
   acknowledgement (SACK) options [RFC2018].  Packet loss can be
   detected per direction.

   Gaps indicate loss upstream of the traffic observation point;
   retransmissions indicate loss downstream of the traffic observation
   point.  SACKs can be used to detect either upstream or downstream
   packet loss (although some care needs to be taken to avoid
   misidentifying packet reordering as packet loss) and to distinguish
   between upstream versus downstream losses.

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   Packet-loss measurements on both sides of the measurement point are
   an important component in precisely diagnosing insufficiently
   dimensioned devices or links in networks.  Additionally, packet
   losses are one of the two main ways for congestion to manifest, the
   others being queuing delay or Explicit Congestion Notification (ECN)
   [RFC3168]; therefore, packet loss is an important measurement for any
   middlebox that needs to make traffic handling decisions based on
   observed levels of congestion.

2.2.  Round-Trip Times

   The ability to measure partial-path round-trip times (RTTs) is
   valuable in diagnosing network issues (e.g., abnormal latency,
   abnormal packet loss).  Knowing if latency is poor on one side of the
   observation point or the other provides more information than is
   available at either endpoint, which can only observe full RTTs.

   For example, a TCP packet stream can be used to measure the RTT on
   each side of the measurement point.  During the connection handshake,
   the SYN, SYN/ACK, and ACK timings can be used to establish a baseline
   RTT in each direction.  Once the connection is established, the RTT
   between the server and the measurement point can only reliably be
   determined using TCP timestamps [RFC7323].  On the side between the
   measurement point and the client, the exact timing of data segments
   and ACKs can be used as an alternative.  For this latter method to be
   accurate when packet loss is present, the connection must use
   selective acknowledgements.

   In many networks, congestion will show up as increasing packet
   queuing, and congestion-induced packet loss will only happen in
   extreme cases.  RTTs will also show up as a much smoother signal than
   the discrete packet-loss events.  This makes RTTs a good way to
   identify individual subscribers for whom the network is a bottleneck
   at a given time or geographical sites (such as cellular towers) that
   are experiencing large-scale congestion.

   The main limit of RTT measurement as a congestion signal is the
   difficulty of reliably distinguishing between the data segments being
   queued versus the ACKs being queued.

2.3.  Measuring Packet Reordering

   If a network is reordering packets of transport connections, caused
   perhaps by Equal-Cost Multipath (ECMP) misconfiguration (described in
   [RFC2991] and [RFC7690], for example), the endpoints may react as if
   packet loss is occurring and retransmit packets or reduce forwarding
   rates.  Therefore, a network operator desires the ability to diagnose
   packet reordering.

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   For TCP, packet reordering can be detected by observing TCP sequence
   numbers per direction.  See, for example, a number of standard
   packet-reordering metrics in [RFC4737] and informational metrics in

2.4.  Throughput and Bottleneck Identification

   Although throughput to or from an IP address can be measured without
   transport-layer measurements, the transport layer provides clues
   about what the endpoints were attempting to do.

   One way of quickly excluding the network as the bottleneck during
   troubleshooting is to check whether the speed is limited by the
   endpoints.  For example, the connection speed might instead be
   limited by suboptimal TCP options, the sender's congestion window,
   the sender temporarily running out of data to send, the sender
   waiting for the receiver to send another request, or the receiver
   closing the receive window.

   This data is also useful for middleboxes used to measure network
   quality of service.  Connections, or portions of connections, that
   are limited by the endpoints do not provide an accurate measure of
   the network's speed and can be discounted or completely excluded in
   such analyses.

2.5.  Congestion Responsiveness

   Congestion control mechanisms continue to evolve.  Tools exist that
   can interpret protocol sequence numbers (e.g., from TCP or RTP) to
   infer the congestion response of a flow.  Such tools can be used by
   operators to help understand the impact of specific transport
   protocols on other traffic that shares their network.  For example,
   packet sequence numbers can be analyzed to help understand whether an
   application flow backs off its load in the face of persistent
   congestion (as TCP does) and, hence, whether the behavior is
   appropriate for sharing limited network capacity.

   These tools can also be used to determine whether mechanisms are
   needed in the network to prevent flows from acquiring excessive
   network capacity under severe congestion (e.g., by deploying rate
   limiters or network transport circuit breakers [RFC8084]).

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2.6.  Attack Detection

   When an application or network resource is under attack, it is useful
   to identify this situation from the network perspective, upstream of
   the attacked resource.

   Although detection methods tend to be proprietary, attack detection
   from within the network may comprise:

   o  Identifying uncharacteristic traffic volumes or sources;

   o  Identifying congestion, possibly using techniques in Sections 2.1
      and 2.2;

   o  Identifying incomplete connections or transactions, from attacks
      that attempt to exhaust server resources;

   o  Fingerprinting based on whatever available fields are determined
      to be useful in discriminating an attack from desirable traffic.

   Two trends in protocol design will make attack detection more

   o  The desire to encrypt transport-layer fields;

   o  The desire to avoid statistical fingerprinting by adding entropy
      in various forms.

   While improving privacy, those approaches may hinder attack

2.7.  Packet Corruption

   One notable source of packet loss is packet corruption.  This
   corruption will generally not be detected until the checksums are
   validated by the endpoint and the packet is dropped.  This means that
   detecting the exact location where packets are lost is not sufficient
   when troubleshooting networks.  An operator would like to find out
   where packets are being corrupted.  IP and TCP checksum verification
   allows a measurement device to correctly distinguish between upstream
   packet corruption and normal downstream packet loss.

   Transport protocol designers should consider whether a middlebox will
   be able to detect corrupted or tampered packets.

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2.8.  Application-Layer Measurements

   Information about network health may also be gleaned from
   application-layer diagnosis, such as:

   o  DNS response times and retransmissions calculated by correlating
      answers to queries;

   o  Various protocol-aware voice and video quality analyses.

   Could this type of information be provided in a transport layer?

3.  Functions beyond Measurement: A Few Examples

   This section describes features provided by on-path devices that go
   beyond measurement by modifying, discarding, delaying, or
   prioritizing traffic.

3.1.  NAT

   Network Address Translators (NATs) allow multiple devices to share a
   public address by dividing the transport-layer port space among the

   NAT behavior recommendations are found for UDP in BCP 127 [RFC4787]
   and for TCP in BCP 142 [RFC7857].

   To support NAT, there must be transport-layer port numbers that can
   be modified by the NAT.  Note that required fields (e.g., port
   numbers) are visible in all IETF-defined transport protocols.

   The application layer must not assume the port number was left
   unchanged (e.g., by including it in a checksum or signing it).

   Address sharing is also used in the context of IPv6 transition.  For
   example, DS-Lite Address Family Transition Router (AFTR) [RFC6333],
   NAT64 [RFC6146], or A+P [RFC7596][RFC7597] are features that are
   enabled in the network to allow for IPv4 service continuity over an
   IPv6 network.

   Further, because of some multihoming considerations, IPv6 prefix
   translation may be enabled by some enterprises by means of IPv6-to-
   IPv6 Network Prefix Translation (NPTv6) [RFC6296].

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3.2.  Firewall

   Firewalls are pervasive and essential components that inspect
   incoming and outgoing traffic.  Firewalls are usually the cornerstone
   of a security policy that is enforced in end-user premises and other
   locations to provide strict guarantees about traffic that may be
   authorized to enter/leave the said premises, as well as end users who
   may be assigned different clearance levels regarding which networks
   and portions of the Internet they access.

   An important aspect of a firewall policy is differentiating
   internally initiated from externally initiated communications.

      For TCP, this is easily done by tracking the TCP state machine.
      Furthermore, the ending of a TCP connection is indicated by RST or
      FIN flags.

      For UDP, the firewall can be opened if the first packet comes from
      an internal user, but the closing is generally done by an idle
      timer of arbitrary duration, which might not match the
      expectations of the application.

   Simple IPv6 firewall capabilities for customer premises equipment
   (both stateless and stateful) are described in [RFC6092].

   A firewall functions better when it can observe the protocol state
   machine, described generally by "Transport-Independent Path Layer
   State Management" [PATH-STATE].

3.3.  DDoS Scrubbing

   In the context of a DDoS attack, the purpose of a scrubber is to
   discard attack traffic while permitting useful traffic.  Such a
   mitigator is described in [DOTS].

   When attacks occur against constrained resources, some traffic will
   be discarded before reaching the intended destination.  A user
   receives better experience and a server runs more efficiently if a
   scrubber can discard attack traffic, leaving room for legitimate

   Scrubbing must be provided by an on-path network device, because
   neither endpoint of a legitimate connection has any control over the
   source of the attack traffic.

   Source-spoofed DDoS attacks can be mitigated at the source using BCP
   38 [RFC2827], but it is more difficult if source address filtering
   cannot be applied.

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   In contrast to devices in the core of the Internet, middleboxes
   statefully observing bidirectional transport connections can reject
   source-spoofed TCP traffic based on the inability to provide sensible
   acknowledgement numbers to complete the three-way handshake.
   Obviously, this requires middlebox visibility into a transport-layer
   state machine.

   Middleboxes may also scrub on the basis of statistical
   classification: testing how likely a given packet is to be
   legitimate.  As protocol designers add more entropy to headers and
   lengths, this test becomes less useful, and the best scrubbing
   strategy becomes random drop.

3.4.  Implicit Identification

   In order to enhance the end user's quality of experience, some
   operators deploy implicit identification features that rely upon the
   correlation of network-related information to access some local
   services.  For example, service portals operated by some operators
   may be accessed immediately by end users without any explicit
   identification for the sake of improved service availability.  This
   is doable thanks to on-path devices that inject appropriate metadata
   that can be used by the remote server to enforce per-subscriber
   policies.  The information can be injected at the application layer
   or at the transport layer (when an address-sharing mechanism is in

   An experimental implementation using a TCP option is described in

   For the intended use of implicit identification, it is more secure to
   have a trusted middlebox mark this traffic than to trust end-user

3.5.  Performance-Enhancing Proxies

   Performance-Enhancing Proxies (PEPs) can improve performance in some
   types of networks by improving packet spacing or generating local
   acknowledgements; they are most commonly used in satellite and
   cellular networks.  Transport-Layer PEPs are described in
   Section 2.1.1 of [RFC3135].

   PEPs allow central deployment of congestion control algorithms more
   suited to the specific network, most commonly for delay-based
   congestion control.  More advanced TCP PEPs deploy congestion control
   systems that treat all of a single end user's TCP connections as a
   single unit, improving fairness and allowing faster reaction to

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   changing network conditions.  For example, it was reported that
   splitting the TCP connections in some network accesses can result in
   a halved page downloading time [ICCRG].

   Local acknowledgements generated by PEPs speed up TCP slow start by
   splitting the effective latency, and they allow for retransmissions
   to be done from the PEP rather than from the actual sender.  Local
   acknowledgements will also allow a PEP to maintain a local buffer of
   data appropriate to the actual network conditions, whereas the actual
   endpoints would often send too much or too little.

   A PEP function requires transport-layer fields that allow chunks of
   data to be identified (e.g., TCP sequence numbers), acknowledgements
   to be identified (e.g., TCP ACK numbers), and acknowledgements to be
   created from the PEP.

   Note that PEPs are only useful in some types of networks.  In
   particular, PEPs are very useful in contexts wherein the congestion
   control strategies of the endpoints are inappropriate for the network
   connecting them.  That being said, poor design can result in degraded
   performances when PEPs are deployed.

3.6.  Network Coding

   Network Coding is a technique for improving transmission performance
   over low-bandwidth, long-latency links such as some satellite links.
   Coding may involve lossless compression and/or adding redundancy to
   headers and payload.  A Network Coding Taxonomy is provided by
   [RFC8406]; an example of end-to-end coding is FECFRAME [RFC6363].  It
   is typically deployed with network-coding gateways at each end of
   those links, with a network-coding tunnel between them via the
   slow/lossy/long-latency links.

   Network-coding implementations may be specific to TCP, taking
   advantage of known properties of the protocol.

   The network-coding gateways may employ some techniques of PEPs, such
   as creating acknowledgements of queued data, removing
   retransmissions, and pacing data rates to reduce queue oscillation.

   The interest in more network coding in some specific networks is
   discussed in [SATELLITES].

   Note: This is not to be confused with transcoding, which performs
   lossy compression on transmitted media streams and is not in scope
   for this document.

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3.7.  Network-Assisted Bandwidth Aggregation

   The Hybrid Access Aggregation Point is a middlebox that allows
   customers to aggregate the bandwidth of multiple access technologies.

   One of the approaches uses Multipath TCP (MPTCP) proxies
   [TCP-CONVERT] to forward traffic along multiple paths.  The MPTCP
   proxy operates at the transport layer while being located in the
   operator's network.

   The support of multipath transport capabilities by communicating
   hosts remains a privileged target design so that such hosts can
   directly use the available resources provided by a variety of access
   networks they can connect to.  Nevertheless, network operators do not
   control end hosts, whereas the support of MPTCP by content servers
   remains marginal.

   Network-assisted MPTCP deployment models are designed to facilitate
   the adoption of MPTCP for the establishment of multipath
   communications without making any assumption about the support of
   MPTCP capabilities by communicating peers.  Network-assisted MPTCP
   deployment models rely upon MPTCP Conversion Points (MCPs) that act
   on behalf of hosts so that they can take advantage of establishing
   communications over multiple paths [TCP-CONVERT].

   Note there are cases when end-to-end MPTCP cannot be used even though
   both client and server are MPTCP-compliant.  An MPTCP proxy can
   provide multipath utilization in these cases.  Examples of such cases
   are listed below:

   1.  The use of private IPv4 addresses in some access networks.
       Typically, additional subflows cannot be added to the MPTCP
       connection without the help of an MCP.

   2.  The assignment of IPv6 prefixes only by some networks.  If the
       server is IPv4-only, IPv6 subflows cannot be added to an MPTCP
       connection established with that server, by definition.

   3.  Subscription to some service offerings is subject to volume

3.8.  Prioritization and Differentiated Services

   Bulk traffic may be served with a higher latency than interactive
   traffic with no reduction in throughput.  This fact allows a
   middlebox function to improve response times in interactive
   applications by prioritizing, policing, or remarking interactive
   transport connections differently from bulk-traffic transport

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   connections.  For example, gaming traffic may be prioritized over
   email or software updates.  Configuration guidelines for Diffserv
   service classes are discussed in [RFC4594].

   Middleboxes may identify different classes of traffic by inspecting
   multiple layers of header and length of payload.

3.9.  Measurement-Based Shaping

   Basic traffic-shaping functionality requires no transport-layer
   information.  All that is needed is a way of mapping each packet to a
   traffic shaper quota.  For example, there may be a rate limit per
   5-tuple or per subscriber IP address.  However, such fixed traffic
   shaping rules are wasteful as they end up rate-limiting traffic even
   when the network has free resources available.

   More advanced traffic-shaping devices use transport-layer metrics
   described in Section 2 to detect congestion on either a per-site or a
   per-user level and use different traffic-shaping rules when
   congestion is detected [RFC3272].  This type of device can overcome
   limitations of downstream devices that behave poorly (e.g., by
   excessive buffering or suboptimally dropping packets).

3.10.  Fairness to End-User Quota

   Several service offerings rely upon a volume-based charging model
   (e.g., volume-based data plans offered by cellular providers).
   Operators may assist end users in conserving their data quota by
   deploying on-path functions that shape traffic that would otherwise
   be aggressively transferred.

   For example, a fast download of a video that won't be viewed
   completely by the subscriber may lead to quick exhaustion of the data
   quota.  Limiting the video download rate conserves quota for the
   benefit of the end user.  Also, discarding unsolicited incoming
   traffic prevents the user's quota from being unfairly exhausted.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

5.1.  Confidentiality and Privacy

   This document intentionally excludes middleboxes that observe or
   manipulate application-layer data.

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   The measurements and functions described in this document can all be
   implemented without violating confidentiality [RFC6973].  However,
   there is always the question of whether the fields and packet
   properties used to achieve operational benefits may also be used for

   In particular, the question is what confidentiality is lost by
   exposing transport-layer fields beyond what can be learned by
   observing IP-layer fields:

   o  Sequence numbers: an observer can learn how much data is

   o  Start/Stop indicators: an observer can count transactions for some

   o  Device fingerprinting: an observer may be more easily able to
      identify a device type when different devices use different
      default field values or options.

5.2.  Active On-Path Attacks

   An on-path attacker being able to observe sequence numbers or session
   identifiers may make it easier to modify or terminate a transport
   connection.  For example, observing TCP sequence numbers allows
   generation of a RST packet that terminates the connection.  However,
   signing transport fields softens this attack.  The attack and
   solution are described for the TCP authentication option [RFC5925].
   Still, an on-path attacker can also drop the traffic flow.

5.3.  Improved Security

   Network maintainability and security can be improved by providing
   firewalls and DDoS mechanisms with some information about transport
   connections.  In contrast, it would be very difficult to secure a
   network in which every packet appears unique and filled with random
   bits (from the perspective of an on-path device).

   Some features providing the ability to mitigate/filter attacks owing
   to a network-assisted mechanism will therefore improve security --
   e.g., by means of Distributed-Denial-of-Service Open Threat Signaling

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6.  Informative References

   [DOTS]     Mortensen, A., Andreasen, F., Reddy, T., Compton, R., and
              N. Teague, "Distributed-Denial-of-Service Open Threat
              Signaling (DOTS) Architecture", Work in Progress,
              draft-ietf-dots-architecture-07, September 2018.

              Reddy, T., Boucadair, M., Patil, P., Mortensen, A., and N.
              Teague, "Distributed Denial-of-Service Open Threat
              Signaling (DOTS) Signal Channel Specification", Work in
              Progress, draft-ietf-dots-signal-channel-25, September

   [ICCRG]    Kuhn, N., "MPTCP and BBR performance over Internet
              satellite paths", IETF 100, 2017,

              Kuehlewind, M., Trammell, B., and J. Hildebrand,
              "Transport-Independent Path Layer State Management", Work
              in Progress, draft-trammell-plus-statefulness-04, November

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991,
              DOI 10.17487/RFC2991, November 2000,

   [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,

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   [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,

   [RFC3272]  Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and
              X. Xiao, "Overview and Principles of Internet Traffic
              Engineering", RFC 3272, DOI 10.17487/RFC3272, May 2002,

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
              S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
              DOI 10.17487/RFC4737, November 2006,

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.

   [RFC5236]  Jayasumana, A., Piratla, N., Banka, T., Bare, A., and
              R. Whitner, "Improved Packet Reordering Metrics",
              RFC 5236, DOI 10.17487/RFC5236, June 2008,

   [RFC5853]  Hautakorpi, J., Ed., Camarillo, G., Penfield, R.,
              Hawrylyshen, A., and M. Bhatia, "Requirements from Session
              Initiation Protocol (SIP) Session Border Control (SBC)
              Deployments", RFC 5853, DOI 10.17487/RFC5853, April 2010,

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

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   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,

   [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", RFC 6363,
              DOI 10.17487/RFC6363, October 2011,

   [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
              T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, DOI 10.17487/RFC6459, January 2012,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and
              R. Scheffenegger, Ed., "TCP Extensions for High
              Performance", RFC 7323, DOI 10.17487/RFC7323, September
              2014, <https://www.rfc-editor.org/info/rfc7323>.

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   [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
              Service Function Chaining", RFC 7498,
              DOI 10.17487/RFC7498, April 2015,

   [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and
              I. Farrer, "Lightweight 4over6: An Extension to the Dual-
              Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
              July 2015, <https://www.rfc-editor.org/info/rfc7596>.

   [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, Ed., "Mapping of Address and
              Port with Encapsulation (MAP-E)", RFC 7597,
              DOI 10.17487/RFC7597, July 2015,

   [RFC7690]  Byerly, M., Hite, M., and J. Jaeggli, "Close Encounters of
              the ICMP Type 2 Kind (Near Misses with ICMPv6 Packet Too
              Big (PTB))", RFC 7690, DOI 10.17487/RFC7690, January 2016,

   [RFC7857]  Penno, R., Perreault, S., Boucadair, M., Ed.,
              Sivakumar, S., and K. Naito, "Updates to Network Address
              Translation (NAT) Behavioral Requirements", BCP 127,
              RFC 7857, DOI 10.17487/RFC7857, April 2016,

   [RFC7974]  Williams, B., Boucadair, M., and D. Wing, "An Experimental
              TCP Option for Host Identification", RFC 7974,
              DOI 10.17487/RFC7974, October 2016,

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [RFC8406]  Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
              F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
              Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
              S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
              Network Communications", RFC 8406, DOI 10.17487/RFC8406,
              June 2018, <https://www.rfc-editor.org/info/rfc8406>.

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              Kuhn, N. and E. Lochin, "Network coding and satellites",
              Work in Progress, draft-irtf-nwcrg-network-coding-
              satellites-02, November 2018.

              Bonaventure, O., Boucadair, M., Gundavelli, S., and S.
              Seo, "0-RTT TCP Convert Protocol", Work in Progress,
              draft-ietf-tcpm-converters-04, October 2018.

   [USE-CASE] Napper, J., Stiemerling, M., Lopez, D., and J. Uttaro,
              "Service Function Chaining Use Cases in Mobile Networks",
              Work in Progress, draft-ietf-sfc-use-case-mobility-08,
              May 2018.

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   The authors thank Brian Trammell, Brian Carpenter, Mirja Kuehlewind,
   Kathleen Moriarty, Gorry Fairhurst, Adrian Farrel, and Nicolas Kuhn
   for their review and suggestions.

Authors' Addresses

   David Dolson (editor)

   Email: ddolson@acm.org

   Juho Snellman

   Email: jsnell@iki.fi

   Mohamed Boucadair (editor)
   4 rue du Clos Courtel
   Rennes  35000

   Email: mohamed.boucadair@orange.com

   Christian Jacquenet
   4 rue du Clos Courtel
   Rennes  35000

   Email: christian.jacquenet@orange.com

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  1. RFC 8517