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An Inventory of Transport-centric Functions Provided by Middleboxes

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This is an older version of an Internet-Draft that was ultimately published as RFC 8517.
Authors David Dolson , Juho Snellman , Mohamed Boucadair , Christian Jacquenet
Last updated 2018-11-20 (Latest revision 2018-11-14)
Replaces draft-dolson-plus-middlebox-benefits
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Network Working Group                                          D. Dolson
Intended status: Informational                               J. Snellman
Expires: May 18, 2019
                                                            M. Boucadair
                                                            C. Jacquenet
                                                       November 14, 2018

  An Inventory of Transport-centric Functions Provided by Middleboxes


   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".

   RFC3234 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 18, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . . . . . . .   7
     2.7.  Packet Corruption . . . . . . . . . . . . . . . . . . . .   8
     2.8.  Application-Layer Measurements  . . . . . . . . . . . . .   8
   3.  Functions Beyond Measurement: A Few Examples  . . . . . . . .   9
     3.1.  NAT . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Firewall  . . . . . . . . . . . . . . . . . . . . . . . .   9
     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  . . . . . . . . .  12
     3.8.  Prioritization and Differentiated Services  . . . . . . .  13
     3.9.  Measurement-Based Shaping . . . . . . . . . . . . . . . .  13
     3.10. Fairness to End-User Quota  . . . . . . . . . . . . . . .  14
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     5.1.  Confidentiality & Privacy . . . . . . . . . . . . . . . .  14
     5.2.  Active On-Path Attacks  . . . . . . . . . . . . . . . . .  15
     5.3.  Improved Security . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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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; e.g.,:

   o  Where a residential or business customer connects to its service
      provider(s), which may include multi-homing.

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

   For the purposes of this document (and consistent with the RFC3234
   definition), 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 [I-D.trammell-plus-statefulness].

   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.

   Advanced service functions (e.g., Network Address Translators (NATs),
   firewalls, etc.)  [RFC7498] are widely used to achieve various

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   objectives such as IP address sharing, firewalling, avoiding covert
   channels, detecting and protect 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

   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 customer's 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 must
   support a plethora of services with different QoS, security,
   robustness, etc. requirements.

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.  An earlier document [RFC8404] describes
   the impact of pervasive encryption of application-layer data on
   network monitoring, protecting, and troubleshooting.

   This 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.

   This document takes an operator perspective that measurement and
   management of transport connections is of benefit to both parties:

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   for the end-user to receive better quality of experience, and for the
   network operator to improve 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 as monitoring systems that compute performance
   indicators for single or aggregates of connections and generate
   aggregated reports from them, and active systems that make decisions
   also on how to handle packet flows based on these performance

   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/applications.

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 under-provisioning can be detected if packet
   loss is measurable.  TCP packet loss can be detected by observing
   gaps in sequence numbers, retransmitted sequence numbers, and SACK
   options.  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.  Selective acknowledgements (SACKs, [RFC2018]) can be used to
   detect either upstream or downstream packet loss (although some care
   needs to be taken to avoid mis-identifying packet reordering as
   packet loss), and to distinguish between upstream vs. downstream

   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, since packet
   losses are one of the two main ways for congestion to manifest (the
   others being queueing delay or Explicit Congestion Notification (ECN,

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   [RFC3168])), 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 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 end-point, which can only observe full round-trip times.

   For example, a TCP packet stream can be used to measure the round-
   trip time 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 Round-Trip Time (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
   queueing, 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 vs. the ACKs being queued.

2.3.  Measuring Packet Reordering

   If a network is reordering packets of transport connections, caused
   perhaps by Equal-cost multi-path (ECMP) misconfiguration (e.g.,
   described in [RFC2991] and [RFC7690]), the end-points 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.

   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

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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 end-points 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
   network's speed, and can be discounted or completely excluded in such

2.5.  Congestion Responsiveness

   Congestion control mechanisms continue to evolve.  Tools exist that
   can interpret protocol sequence numbers (e.g., from TCP, 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,
   analysing packet sequence numbers can be used to help understand
   whether an application flow backs-off its load in the face of
   persistent congestion (as TCP does), and hence to understand whether
   the behaviour 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]).

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;

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

   o  Identifying incomplete connections or transactions, from attacks
      which 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.

2.8.  Application-Layer Measurements

   Network health may also be gleaned from application-layer diagnosis.

   o  DNS response times and retransmissions by correlating answers to

   o  Various protocol-aware voice and video quality analysis.

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

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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 AFTR [RFC6333], NAT64 [RFC6146], or MAP-* are
   features that are enabled in the network to allow for IPv4 service
   continuity over an IPv6 network.

   Further, because of some multi-homing considerations, IPv6 prefix
   translation may be enabled by some enterprises by means of NPTv6

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.

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      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 [I-D.trammell-plus-statefulness].

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.  E.g., such a
   mitigator is described in [I-D.ietf-dots-architecture].

   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 end-point 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.

   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 transport-layer
   state machine.

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

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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, and 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 use of 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
   changing network conditions.

   Local acknowledgements generated by PEPs speed up TCP slow start by
   splitting the effective latency, and allow for retransmissions to be
   done from the PEP rather than from the actual sender, saving downlink
   bandwidth on retransmissions.  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.

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   Note that PEPs are only useful in some types of networks, and poor
   design could make performance worse.

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.

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

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 MPTCP proxies [I-D.ietf-tcpm-converters]
   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 while 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 multi-path
   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 [I-D.ietf-tcpm-converters].

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   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 can not 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
   connections.  E.g., 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
   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 sub-optimally dropping packets).

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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.

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

5.1.  Confidentiality & Privacy

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

   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.

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5.2.  Active On-Path Attacks

   Being able to observe sequence numbers or session identifiers by an
   on-path attacker may make it easier to modify or terminate a
   transport connection.  E.g., 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 will improve security
   (e.g., by means of DOTS signalling [I-D.ietf-dots-signal-channel]).

6.  Acknowledgements

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

7.  Informative References

              Mortensen, A., Andreasen, F., K, R., Compton, R., and N.
              Teague, "Distributed-Denial-of-Service Open Threat
              Signaling (DOTS) Architecture", draft-ietf-dots-
              architecture-07 (work in progress), September 2018.

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

              Napper, J., Stiemerling, M., Lopez, D., and J. Uttaro,
              "Service Function Chaining Use Cases in Mobile Networks",
              draft-ietf-sfc-use-case-mobility-08 (work in progress),
              May 2018.

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

              Kuehlewind, M., Trammell, B., and J. Hildebrand,
              "Transport-Independent Path Layer State Management",
              draft-trammell-plus-statefulness-04 (work in progress),
              November 2017.

   [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, <>.

   [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,

   [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,

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   [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, <>.

   [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, <>.

   [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, <>.

   [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,

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   [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,

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

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   [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, <>.

Authors' Addresses

   David Dolson


   Juho Snellman


   Mohamed Boucadair
   4 rue du Clos Courtel
   Rennes  35000


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   Christian Jacquenet
   4 rue du Clos Courtel
   Rennes  35000


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