Independent Submission D. Dolson, Ed.

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

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

Abstract

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
https://www.rfc-editor.org/info/rfc8517.

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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
host."

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/
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 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
[RFC5236].

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
difficult:

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
detection.

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
devices.

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
traffic.

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
use).

An experimental implementation using a TCP option is described in
[RFC7974].

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

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
quota.

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
harm.

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
transferred.

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

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
(DOTS) [DOTS-SIGNAL].

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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.

[DOTS-SIGNAL]
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
2018.

[ICCRG] Kuhn, N., "MPTCP and BBR performance over Internet
satellite paths", IETF 100, 2017,
<https://datatracker.ietf.org/meeting/100/materials/
slides-100-iccrg-mptcp-and-bbr-performance-over-satcom-
links-00>.

[PATH-STATE]
Kuehlewind, M., Trammell, B., and J. Hildebrand,
"Transport-Independent Path Layer State Management", Work
in Progress, draft-trammell-plus-statefulness-04, November
2017.

[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.

[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,
<https://www.rfc-editor.org/info/rfc2991>.

[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,
<https://www.rfc-editor.org/info/rfc3135>.

Dolson, et al. Informational [Page 16]

RFC 8517 Transport-Centric Middlebox Functions February 2019

[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,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
<https://www.rfc-editor.org/info/rfc3234>.

[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,
<https://www.rfc-editor.org/info/rfc3272>.

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.

[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.

[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,
<https://www.rfc-editor.org/info/rfc5236>.

[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,
<https://www.rfc-editor.org/info/rfc5853>.

[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>.

Dolson, et al. Informational [Page 17]

RFC 8517 Transport-Centric Middlebox Functions February 2019

[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,
<https://www.rfc-editor.org/info/rfc6092>.

[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,
<https://www.rfc-editor.org/info/rfc6269>.

[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
<https://www.rfc-editor.org/info/rfc6296>.

[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,
<https://www.rfc-editor.org/info/rfc6333>.

[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.

[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,
<https://www.rfc-editor.org/info/rfc6459>.

[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,
<https://www.rfc-editor.org/info/rfc6973>.

[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>.

Dolson, et al. Informational [Page 18]

RFC 8517 Transport-Centric Middlebox Functions February 2019

[RFC7498] Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
Service Function Chaining", RFC 7498,
DOI 10.17487/RFC7498, April 2015,
<https://www.rfc-editor.org/info/rfc7498>.

[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,
<https://www.rfc-editor.org/info/rfc7597>.

[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,
<https://www.rfc-editor.org/info/rfc7690>.

[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,
<https://www.rfc-editor.org/info/rfc7857>.

[RFC7974] Williams, B., Boucadair, M., and D. Wing, "An Experimental
TCP Option for Host Identification", RFC 7974,
DOI 10.17487/RFC7974, October 2016,
<https://www.rfc-editor.org/info/rfc7974>.

[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.

[RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of
Pervasive Encryption on Operators", RFC 8404,
DOI 10.17487/RFC8404, July 2018,
<https://www.rfc-editor.org/info/rfc8404>.

[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>.

Dolson, et al. Informational [Page 19]

RFC 8517 Transport-Centric Middlebox Functions February 2019

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

[TCP-CONVERT]
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.

Dolson, et al. Informational [Page 20]

RFC 8517 Transport-Centric Middlebox Functions February 2019

Acknowledgements

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: [email protected]

Juho Snellman

Email: [email protected]

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

Email: [email protected]

Christian Jacquenet
Orange
4 rue du Clos Courtel
Rennes 35000
France

Email: [email protected]

Dolson, et al. Informational [Page 21]