A Vocabulary of Path Properties
draft-enghardt-panrg-path-properties-01

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PANRG                                                        T. Enghardt
Internet-Draft                                                 TU Berlin
Intended status: Informational                           C. Kraehenbuehl
Expires: September 12, 2019                                  ETH Zuerich
                                                          March 11, 2019

                    A Vocabulary of Path Properties
                draft-enghardt-panrg-path-properties-01

Abstract

   This document defines and categorizes information about Internet
   paths that an entity, such as an endpoint, might have or want to
   have.  This information is expressed as properties of paths between
   two endpoints.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Domain Properties . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Backbone Properties . . . . . . . . . . . . . . . . . . . . .   6
   5.  Dynamic Properties  . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Because the current Internet provides an IP-based best-effort bit
   pipe, endpoints have little information about paths to other
   endpoints.  A Path Aware Network exposes information about one or
   multiple paths through the network to endpoints or the network
   infrastructure.

   It is impossible to provide an exhaustive list of path properties, as
   with every new technology and protocol, novel properties might become
   relevant.  In this document, we specify a set of path properties
   which might be useful in the following use cases: Traffic policies,
   network monitoring, and path selection.

   o  Traffic policies: Entities such as network operators or end users
      may want to define traffic policies leveraging path awareness.
      Such policies can allow or disallow sending traffic over specific
      networks or nodes, select an appropriate protocol depending on the
      capabilities of the on-path devices, or adjust protocol parameters
      to an existing path.  An example of a traffic policy is a video
      streaming application choosing an (initial) video quality based on
      the achievable data rate, or the monetary cost of the link using a
      volume-based or flat-rate cost model.  Another example is an
      enterprise network where all traffic has to go through a firewall,
      in which case the endpoint needs to be aware of on-path firewalls.

   o  Network monitoring: Network operators can use path properties
      (e.g., measured by on-path devices), to observe Quality of Service
      (QoS) characteristics of recent end-user traffic, and identify
      potential problems with their network early on, before the end-
      user complains.

   o  Path selection: In some cases, entities can choose to use a
      certain path (or subset of paths) from a set of paths to achieve a
      specific goal.  As the possible benefits of a well chosen path

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      varies based on the goal, as a baseline, a path selection
      algorithm should aim to not perform worse than the default case
      most of the time.  Depending on the goal, an entity may prefer
      paths with different properties, e.g., retrieving a small webpage
      as quickly as possible requires low latency paths, or retrieving a
      large file in a peer-to-peer network requires paths with high
      achievable data rate.  Additionally, there may be trade-offs
      between path properties (e.g., latency and data rate), and
      entities may influence these trade-offs with their choices.  A
      network (e.g., an AS) can adjust its path selection for internal
      or external routing based on the path properties.  In BGP, the
      Multi Exit Discriminator (MED) attribute decides which path to
      choose if other attributes are equal; in a path aware network,
      instead of using this single MED value, other properties such as
      maximum or available/expected data rate could additionally be used
      to improve load balancing.  An endpoint might be able to select
      between a set of paths, either if there are several paths to the
      same destination (e.g., if the endpoint is a mobile device with
      two wireless interfaces, both providing a path), or if there are
      several destinations, and thus several paths, providing the same
      service (e.g., Application-Layer Traffic Optimization (ALTO)
      [RFC5693], an application layer peer-to-peer protocol allowing
      endpoints a better-than-random peer selection).  Care needs to be
      taken when selecting paths based on path properties, as path
      properties that were previously measured may have become outdated
      and, thus, useless to predict the path properties of packets sent
      now.

   Such path properties may be relatively dynamic, e.g. current Round
   Trip Time, close to the origin, e.g. nature of the access technology
   on the first hop, or far from the origin, e.g. list of ASes
   traversed.

   Usefulness over time is fundamentally different for dynamic and non-
   dynamic properties.  The merit of a momentary measurement of a
   dynamic path property diminishes greatly as time goes on, e.g. the
   merit of an RTT measurement from a few seconds ago is quite small,
   while a non-dynamic path property might stay relevant, e.g. a NAT can
   be assumed to stay on a path during the lifetime of a connection, as
   the removal of the NAT would break the connection.

   Non-dynamic properties are further separated into (local) domain
   properties related to the first few hops of the connection, and
   backbone properties related to the remaining hops.  Domain properties
   expose a high amount of information to endpoints and strongly
   influence the connection behavior while there is little influence and
   information about backbone properties.

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   Dynamic properties are not separated into domain and backbone
   properties, since most of these properties are defined for a complete
   path and it is difficult and seldom useful to define them on part of
   the path.  There are exceptions such as dynamic wireless access
   properties, but these do not justify separation into different
   categories.

   This document addresses the first of the questions in Path-Aware
   Networking [I-D.irtf-panrg-questions], which is a product of the
   PANRG in the IRTF.

2.  Terminology

   Path element:  A path element is a device (including the endpoints),
      or link used to connect two devices and transmit information on a
      specific layer.  Path elements may exist on multiple layers (e.g.,
      the endpoint corresponds to a path element on every layer), may be
      hidden on higher layers (e.g., a layer 2 switch in the local
      network), or a path element may be an aggregation of several path
      elements on a lower layer (e.g., the link connecting the endpoints
      on the transport layer being an aggregation of all network layer
      path elements).

   Path segment:  A path segment is an ordered set of path elements at
      the network layer that can be traversed by a packet.

   Path:  A path is defined as an ordered set of path elements at the
      network layer between two endpoints.  A path can be traversed by a
      packet.

   Flow:  Several packets traversing the same path elements can be
      combined into a flow (e.g., all packets sent within a UDP session
      which traverse the same path elements).  As a special case, a flow
      can consist of just one packet.

   Property:  A property describes a trait of a set of path elements
      (e.g., capacity of a link, is device X a firewall, one-way maximum
      data rate which is the minimum of all links' maximum data rates),
      or a trait of a flow being sent on a set of path elements (e.g.,
      RTT, one-way delay).  A property is thus described by a tuple
      containing the ordered set of path elements, the set of packets
      traversing the path (the flow) or an empty set if no packets are
      relevant for the property, the name of the trait (e.g., maximum
      data rate), and the value of the trait (e.g., 100mbps).

   Aggregated Property:  A property can be aggregated over a set of path
      elements (e.g., MTU in the network backbone as the minimum MTU of
      the individual path elements), or over a set of packets (e.g.,

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      median one-way latency of all packets during the last second), or
      over both (e.g., average time a packets spends in buffers outside
      the local network).  Aggregation can be numerical (average, sum,
      min, ...), logical (true if all are true, true if at least X are
      true, ...), or an arbitrary function which maps a set of input
      properties to an output property.

   Measured & Potential Property:  A property can be classified by
      timescale into a measured property, based on concrete previous and
      current measurements, and a potential property, which is a
      property with predicted characteristics, possibly including the
      reliability of such predictions.  An example of a potential
      property with a high reliability is the maximum data rate of an
      ethernet link in the local network during the next day, while a
      potential property with a lower reliability is the expected one-
      way latency of packets sent to an endpoint on the other side of
      the planet during the next second.  The notion of reliability
      depends on the property, it might be the confidence level and
      interval for numerical properties or the likelihood that a
      property holds for non-numerical properties.

3.  Domain Properties

   Domain path properties relate to path elements within the first hop
   or the first few hops, which are usually in the same administrative
   domain as an endpoint considering them.

   Due to the potential physical proximity and pre-existing trust or
   contractual relationships between endpoints and path elements within
   the same domain, domain properties may be more accessible to the
   endpoint than other properties.

   Furthermore, endpoints may be able to influence both which domain
   they are in and which path elements in this domain to connect to, and
   they may be able to influence the properties of path elements within
   this domain.  For example, a user might select between multiple
   potential adjacent path elements by selecting between multiple
   available WiFi Access Points.  Or when connected to an Access Point,
   the user may move closer to enable their device to use a different
   access technology, potentially increasing the data rate available to
   the device.  Another example is a user changing their data plan to
   reduce the Monetary Cost to transmit a given amount of data across a
   network.

   Access Technology:  The physical or link layer technology used for
      transmitting or receiving a flow on one or multiple path elements
      in the same domain.  The Access Technology may be given in an
      abstract way, e.g., as a WiFi, Wired Ethernet, or Cellular link.

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      It may also be given as a specific technology, e.g., as a 2G, 3G,
      4G, or 5G cellular link, or an 802.11a, b, g, n, or ac WiFi link.
      Other path elements relevant to the access technology may include
      on-path devices, such as elements of a cellular backbone network.
      Note that there is no common registry of possible values for this
      property.

   Monetary Cost:  The price to be paid to transmit a specific flow
      across a path segment.

4.  Backbone Properties

   Backbone path properties relate to path elements not within the same
   domain as an endpoint considering them, thus, in the backbone from
   the endpoint's point of view.

   Typically, backbone properties are less accessible to an endpoint
   than domain properties, due to the potential increased distance and
   the lack of pre-existing trust or contractual relationship.

   Additionally, endpoints are less likely to be able to influence which
   path elements form their path in the backbone, as well as their
   properties.

   Some path properties relate to the entire path, part of which often
   lies outside of an endpoint's domain.  Thus, such properties are
   listed as Backbone Properties.

   Presence of a certain network function on the path:  Indicates that a
      certain path element performs a certain network function on a
      flow, e.g., whether the path element acts as a proxy, as a
      firewall, or performs Network Address Translation (NAT).  This
      path element may be either in the same domain as the endpoint or
      in a different domain, i.e., the backbone.

   Administrative Entity:  The administrative entity, e.g., the AS, to
      which a path element or path segment belongs.

   Disjointness:  For a set of two paths, the number of shared path
      elements can be a measure of intersection (e.g., Jaccard
      coefficient, which is the number of shared elements divided by the
      total number of elements).  Conversely, the number of non-shared
      path elements can be a measure of disjointness (e.g., 1 - Jaccard
      coefficient).  A multipath protocol might use disjointness of
      paths as a metric to reduce the number of single points of
      failure.

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   Path MTU:  The maximum size, in octets, of an IP packet that can be
      transmitted without fragmentation on a path segment.

   Transport Protocols available:  Whether a specific transport protocol
      can be used to establish a connection over a path or path segment.
      An endpoint may cache its knowledge about recent successfully
      established connections using specific protocols, e.g., a QUIC
      connection, or an MPTCP subflow, over a specific path.

   Protocol Features available:  Whether a specific protocol feature is
      available over this path, e.g., Explicit Congestion Notification
      (ECN), or TCP Fast Open.

5.  Dynamic Properties

   Dynamic path properties relate to a path segment with respect to the
   transmission of an individual packet or of a flow over this path
   segment.  Properties related to a path element which constitutes a
   single layer 2 domain are abstracted from the used physical and link
   layer technology, similar to [RFC8175].

   Typically, Dynamic Properties can only be approximated and sampled,
   and might be made available in an aggregated form, such as averages
   or minimums.  Dynamic Path Properties can be measured by the endpoint
   itself or somethere in the network.  See [ANRW18-Metrics] for a
   discussion of how to measure some dynamic path properties at the
   endpoint.

   Some dynamic properties are defined in different directions for the
   same path element, e.g., for transmitting and receiving packets.

   Maximum Data Rate (Transmit/Receive):  The theoretical maximum data
      rate, in bits per second, that can be achieved on a link, path
      segment, or path, for receiving or transmitting traffic.

   Current Data Rate (Transmit/Receive):  The data rate, in bits per
      second, at which a link is currently receiving or transmitting
      traffic.

   Latency:  The time delay between sending a packet on a path element
      and receiving the same packet on a different path element.

   Latency variation:  The variation of the Latency within a flow.

   Packet Loss:  The percentage of packets within a flow which are sent
      by one path element, but not received by a different path element.

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   Congestion:  Whether a protocol feature such as ECN has provided
      information that there currently is congestion on a path.

6.  Security Considerations

   If devices are basing policy or path selection decisions on path
   properties, they need to rely on the accuracy of path properties that
   other devices communicate to them.  In order to be able to trust such
   path properties, devices may need to establish a trust relationship
   or be able to verify the authenticity, integrity, and correctness of
   path properties received from another device.

7.  IANA Considerations

   This document has no IANA actions.

8.  Informative References

   [ANRW18-Metrics]
              Enghardt, T., Tiesel, P., and A. Feldmann, "Metrics for
              access network selection", Proceedings of the Applied
              Networking Research Workshop on - ANRW '18,
              DOI 10.1145/3232755.3232764, 2018.

   [I-D.irtf-panrg-questions]
              Trammell, B., "Open Questions in Path Aware Networking",
              draft-irtf-panrg-questions-01 (work in progress), October
              2018.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              DOI 10.17487/RFC5693, October 2009,
              <https://www.rfc-editor.org/info/rfc5693>.

   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,
              <https://www.rfc-editor.org/info/rfc8175>.

Acknowledgments

   Thanks to the Path-Aware Networking Research Group for the discussion
   and feedback.  Thanks to Adrian Perrig for the feedback.  Thanks to
   Paul Hoffman for the editorial changes.

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Authors' Addresses

   Theresa Enghardt
   TU Berlin

   Email: theresa@inet.tu-berlin.de

   Cyrill Kraehenbuehl
   ETH Zuerich

   Email: cyrill.kraehenbuehl@inf.ethz.ch

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