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Internet Architecture Board (IAB) T. Hardie, Ed.
Request for Comments: 8558 April 2019
Category: Informational
ISSN: 2070-1721
Transport Protocol Path Signals
Abstract
This document discusses the nature of signals seen by on-path
elements examining transport protocols, contrasting implicit and
explicit signals. For example, TCP's state machine uses a series of
well-known messages that are exchanged in the clear. Because these
are visible to network elements on the path between the two nodes
setting up the transport connection, they are often used as signals
by those network elements. In transports that do not exchange these
messages in the clear, on-path network elements lack those signals.
Often, the removal of those signals is intended by those moving the
messages to confidential channels. Where the endpoints desire that
network elements along the path receive these signals, this document
recommends explicit signals be used.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. It represents the consensus of the
Internet Architecture Board (IAB). Documents approved for
publication by the IAB 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/rfc8558.
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RFC 8558 Transport Protocol Path Signals April 2019
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
2. Signal Types Inferred ...........................................4
2.1. Session Establishment ......................................4
2.1.1. Session Identity ....................................4
2.1.2. Routability and Intent ..............................4
2.1.3. Flow Stability ......................................5
2.1.4. Resource Requirements ...............................5
2.2. Network Measurement ........................................5
2.2.1. Path Latency ........................................5
2.2.2. Path Reliability and Consistency ....................5
3. Options .........................................................5
3.1. Do Not Restore These Signals ...............................6
3.2. Replace These with Network-Layer Signals ...................6
3.3. Replace These with Per-Transport Signals ...................6
3.4. Create a Set of Signals Common to Multiple Transports ......6
4. Recommendation ..................................................7
5. IANA Considerations .............................................8
6. Security Considerations .........................................8
7. Informative References ..........................................9
IAB Members at the Time of Approval ...............................10
Acknowledgements ..................................................10
Author's Address ..................................................10
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RFC 8558 Transport Protocol Path Signals April 2019
1. Introduction
This document discusses the nature of signals seen by on-path
elements examining transport protocols, contrasting implicit and
explicit signals. For example, TCP's state machine uses a series of
well-known messages that are exchanged in the clear. Because these
are visible to network elements on the path between the two nodes
setting up the transport connection, they are often used as signals
by those network elements. While the architecture of the Internet
may be best realized by end-to-end protocols [RFC1958], there are
cases such as the use of Network Address Translators [RFC3234] where
some functions are commonly provided by on-path network elements. In
transports that do not exchange these messages in the clear, on-path
network elements lack those signals. Often, the removal of those
signals is intended by those moving the messages to confidential
channels. Where the endpoints desire that network elements along the
path receive these signals, this document recommends explicit signals
be used.
The interpretation of TCP [RFC0793] by on-path elements is an example
of implicit signal usage. It uses cleartext handshake messages to
establish, maintain, and close connections. While these are
primarily intended to create state between two communicating nodes,
these handshake messages are visible to network elements along the
path between them. It is common for certain network elements to
treat the exchanged messages as signals that relate to their own
functions.
A firewall may, for example, create a rule that allows traffic from a
specific host and port to enter its network when the connection was
initiated by a host already within the network. It may subsequently
remove that rule when the communication has ceased. In the context
of TCP handshake, it sets up the pinhole rule on seeing the initial
TCP SYN acknowledgement and then removes it upon seeing a RST or FIN
and ACK exchange. Note that in this case, it does nothing to rewrite
any portion of the TCP packet; it simply enables a return path that
would otherwise have been blocked.
When a transport encrypts the fields it uses for state mechanics,
these signals are no longer accessible to path elements. The
behavior of path elements will then depend on which signal is not
available, on the default behavior configured by the path element
administrator, and by the security posture of the network as a whole.
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2. Signal Types Inferred
The following list of signals that may be inferred from transport
state messages includes those that may be exchanged during session
establishment and those that derive from the ongoing flow.
Some of these signals are derived from the direct examination of
packet sequences, such as using a sequence number gap pattern to
infer network reliability; others are derived from association, such
as inferring network latency by timing a flow's packet inter-arrival
times.
This list is not exhaustive, and it is not the full set of effects
due to encrypting data and metadata in flight. Note as well that
because these are derived from inference, they do not include any
path signals that would not be relevant to the endpoint state
machines; indeed, an inference-based system cannot send such signals.
2.1. Session Establishment
One of the most basic inferences made by examination of transport
state is that a packet will be part of an ongoing flow; that is, an
established session will continue until messages are received that
terminate it. Path elements may then make subsidiary inferences
related to the session.
2.1.1. Session Identity
Path elements that track session establishment will typically create
a session identity for the flow, commonly using a tuple of the
visible information in the packet headers. This is then used to
associate other information with the flow.
2.1.2. Routability and Intent
A second common inference that session establishment provides is that
the communicating pair of hosts can each reach each other and are
interested in continuing communication. The firewall example given
above is a consequence of that inference; because the internal host
initiates the connection, it is presumed to want to receive return
traffic. That, in turn, justifies the pinhole.
Some other on-path elements assume that a host that asked to
communicate with a remote address has authorized receiving incoming
communications from any other host (e.g., Endpoint-Independent
Mapping or Endpoint-Independent Filtering [RFC7857]). This is, for
example, the default behavior in Network Address and Protocol
Translation from IPv6 Clients to IPv4 Servers (NAT64).
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2.1.3. Flow Stability
Some on-path devices that are responsible for load-sharing or load-
balancing may be instructed to preserve the same path for a given
flow rather than dispatching packets belonging to the same flow on
multiple paths as this may cause packets in the flow to be delivered
out of order.
2.1.4. Resource Requirements
An additional common inference is that network resources will be
required for the session. These may be requirements within the
network element itself, such as table entry space for a firewall or
NAT; they may also be communicated by the network element to other
systems. For networks that use resource reservations, this might
result in reservation of radio air time, energy, or network capacity.
2.2. Network Measurement
Some network elements will also observe transport messages to engage
in measurement of the paths that are used by flows on their network.
The list of measurements below is illustrative, not exhaustive.
2.2.1. Path Latency
There are several ways in which a network element may measure path
latency using transport messages, but two common ones are examining
exposed timestamps and associating sequence numbers with a local
timer. These measurements are necessarily limited to measuring only
the portion of the path between the system that assigned the
timestamp or sequence number and the network element.
2.2.2. Path Reliability and Consistency
A network element may also measure the reliability of a particular
path by examining sessions that expose sequence numbers;
retransmissions and gaps are then associated with the path segments
on which they might have occurred.
3. Options
The set of options below are alternatives that optimize very
different things. Though it comes to a preliminary conclusion, this
document intends to foster a discussion of those trade-offs, and any
discussion of them must be understood as preliminary.
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3.1. Do Not Restore These Signals
It is possible, of course, to do nothing. The transport messages
were not necessarily intended for consumption by on-path network
elements, and encrypting them so they are not visible may be taken by
some as a benefit. Each network element would then treat packets
without these visible elements according to its own defaults. While
our experience of that is not extensive, one consequence has been
that state tables for flows of this type are generally not kept as
long as those for which sessions are identifiable. The result is
that heartbeat traffic must be maintained to keep any bindings (e.g.,
NAT or firewall) from early expiry. When those bindings are not
kept, methods like a QUIC connection-id [QUIC] may be necessary to
allow load balancers or other systems to continue to maintain a
flow's path to the appropriate peer.
3.2. Replace These with Network-Layer Signals
It would be possible to replace these implicit signals with explicit
signals at the network layer. Though IPv4 has relatively few
facilities for this, IPv6 hop-by-hop headers [RFC7045] might suit
this purpose. Further examination of the deployability of these
headers may be required.
3.3. Replace These with Per-Transport Signals
It is possible to replace these implicit signals with signals that
are tailored to specific transports, just as the initial signals are
derived primarily from TCP. There is a risk here that the first
transport that develops these will be reused for many purposes
outside its stated purpose, simply because it traverses NATs and
firewalls better than other traffic. If done with an explicit intent
to reuse the elements of the solution in other transports, the risk
of ossification might be slightly lower.
3.4. Create a Set of Signals Common to Multiple Transports
Several proposals use UDP [RFC0768] as a demux layer, onto which new
transport semantics are layered. For those transports, it may be
possible to build a common signaling mechanism and set of signals,
such as that proposed in "Transport-Independent Path Layer State
Management" [PLUS].
This may be taken as a variant of the reuse of common elements
mentioned in the section above, but it has a greater chance of
avoiding the ossification of the solution into the first moving
protocol.
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4. Recommendation
The IAB urges protocol designers to design for confidential operation
by default. We strongly encourage developers to include encryption
in their implementations and to make them encrypted by default. We
similarly encourage network and service operators to deploy
encryption where it is not yet deployed, and we urge firewall policy
administrators to permit encrypted traffic. One of the consequences
of the change will be the loss of implicit signals.
Fundamentally, this document recommends that implicit signals should
be avoided and that an implicit signal should be replaced with an
explicit signal only when the signal's originator intends that it be
used by the network elements on the path. For many flows, this may
result in the signal being absent but allows it to be present when
needed.
Discussion of the appropriate mechanism(s) for these signals is
continuing, but at a minimum, any method should aim to adhere to
these basic principles:
o The portion of protocol signaling that is intended for end-system
state machines should be protected by confidentiality and
integrity protection such that it is only available to those end
systems.
o Anything exposed to the path should be done with the intent that
it be used by the network elements on the path. This information
should be integrity protected, so that end systems can detect if
path elements have made changes in flight.
o Signals exposed to the path should be decoupled from signals that
drive the protocol state machines in endpoints. This avoids
creating opportunities for additional inference.
o Intermediate path elements should not add visible signals that
identify the user, origin node, or origin network [RFC8164]. Note
that if integrity protection is provided as suggested above, any
signals added by intermediate path elements will be clearly
distinguishable from those added by endpoints, as they will not be
within the integrity-protected portion of the packet.
The IAB notes that methods for allowing on-path actors to verify
integrity protection are not available unless those actors have
shared keys with the end systems or share a common set of trust
points. As a result, integrity protection can generally be reliably
applied by and verified only by endpoints.
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Verifying the authenticity of signals generated by on-path actors is
similarly difficult. Endpoints that consume signals generated by
on-path actors, particularly where those signals are unauthenticated,
need to fully consider the implications of doing so. Managing the
authentication of on-path signals is an area of active research, and
defining or recommending methods for it is outside the scope of this
document.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
Path-visible signals allow network elements along the path to act
based on the signaled information, whether the signal is implicit or
explicit. If the network element is controlled by an attacker, those
actions can include dropping, delaying, or mishandling the
constituent packets of a flow. An attacker may also characterize the
flow or attempt to fingerprint the communicating nodes based on the
pattern of signals.
Note that actions that do not benefit the flow or the network may be
perceived as an attack even if they are conducted by a responsible
network element. Designing a system that minimizes the ability to
act on signals at all by removing as many signals as possible may
reduce this possibility. This approach also comes with risks,
principally that the actions will continue to take place on an
arbitrary set of flows.
Addition of visible signals to the path also increases the
information available to an observer and may, when the information
can be linked to a node or user, reduce the privacy of the user.
When signals from endpoints to the path are independent from the
signals used by endpoints to manage the flow's state mechanics, they
may be falsified by an endpoint without affecting the peer's
understanding of the flow's state. For encrypted flows, this
divergence is not detectable by on-path devices. The intent of this
practice may be to garner improved treatment from the network or to
avoid strictures. Protocol designers should be cautious when
introducing explicit signals to consider how falsified signals would
impact protocol operation and deployment. Similarly, operators
should be cautious in deployments to be sure that default operation
without these signals does not encourage gaming the system by
providing false signals.
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7. Informative References
[PLUS] Kuehlewind, M., Trammell, B., and J. Hildebrand,
"Transport-Independent Path Layer State Management", Work
in Progress, draft-trammell-plus-statefulness-04, November
2017.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", Work in Progress,
draft-ietf-quic-transport-19, March 2019.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1958] Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
<https://www.rfc-editor.org/info/rfc1958>.
[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>.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045,
DOI 10.17487/RFC7045, December 2013,
<https://www.rfc-editor.org/info/rfc7045>.
[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>.
[RFC8164] Nottingham, M. and M. Thomson, "Opportunistic Security for
HTTP/2", RFC 8164, DOI 10.17487/RFC8164, May 2017,
<https://www.rfc-editor.org/info/rfc8164>.
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IAB Members at the Time of Approval
Jari Arkko
Alissa Cooper
Ted Hardie
Christian Huitema
Gabriel Montenegro
Erik Nordmark
Mark Nottingham
Melinda Shore
Robert Sparks
Jeff Tantsura
Martin Thomson
Brian Trammell
Suzanne Woolf
Acknowledgements
In addition to the editor listed in the header, this document
incorporates contributions from Brian Trammell, Mirja Kuehlewind,
Martin Thomson, Aaron Falk, Mohamed Boucadair, and Joe Hildebrand.
These ideas were also discussed at the PLUS BoF, sponsored by Spencer
Dawkins. The ideas around the use of IPv6 hop-by-hop headers as a
network-layer signal benefited from discussions with Tom Herbert.
The description of UDP as a demuxing protocol comes from Stuart
Cheshire. Mark Smith, Kazuho Oku, Stephen Farrell, and Eliot Lear
provided valuable comments on earlier draft versions of this
document.
All errors are those of the editor.
Author's Address
Ted Hardie (editor)
Email: ted.ietf@gmail.com
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