PANRG                                                        T. Enghardt
Internet-Draft                                                 TU Berlin
Intended status: Informational                           C. Kraehenbuehl
Expires: May 7, 2020                                         ETH Zuerich
                                                       November 04, 2019

                    A Vocabulary of Path Properties


   Path properties express information about paths across a network and
   the services provided via such paths.  In a path-aware network, path
   properties may be fully or partially available to entities such as
   hosts.  This document defines and categorizes path properties.
   Furthermore, the document specifies several path properties which
   might be useful to hosts or other entities.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Use Cases for Path Properties . . . . . . . . . . . . . . . .   4
     3.1.  Performance Monitoring and Enhancement  . . . . . . . . .   4
     3.2.  Path Selection  . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Traffic Configuration . . . . . . . . . . . . . . . . . .   5
   4.  Examples of Path Properties . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In the current Internet architecture, hosts generally do not have
   information about forwarding paths through the network and about
   services associated with these paths.  A path-aware network, as
   introduced in [I-D.irtf-panrg-questions], exposes information about
   paths to hosts or to other entities.  This document defines such
   information as path properties, addressing the first of the questions
   in path-aware networking [I-D.irtf-panrg-questions].

   As terms related to paths have different meanings in different areas
   of networking, first, this document provides a common terminology to
   define paths, path elements, and path properties.  Then, this
   document provides some examples for use cases for path properties.
   Finally, the document lists several path properties that may be
   useful for the mentioned use cases.

2.  Terminology

   Node:  An entity which processes packets, e.g., sends, receives,
      forwards, or modifies them.  A node may be physical or virtual,
      e.g., a machine or a service function.  A node may also be the
      collection of multiple entities which, as a collection, processes
      packets, e.g., an entire Autonomous System (AS).

   Host:  A node that generally executes application programs on behalf
      of user(s), employing network and/or Internet communication
      services in support of this function, as defined in [RFC1122].

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   Link:  A medium or communication facility that connects two or more
      nodes with each other.  A link enables a node to send packets to
      other nodes.  Links can be physical, e.g., a WiFi network which
      connects an Access Point to stations, or virtual, e.g., a virtual
      switch which connects two virtual machines hosted on the same
      physical machine.  A link is unidirectional and bidirectional
      communication can be modeled as two links between the same nodes
      in opposite directions.

   Path element:  Either a node or a link.

   Path:  A sequence of adjacent path elements over which a packet can
      be transmitted, starting and ending with a node.  Paths are time-
      dependent, i.e., the sequence of path elements over which packets
      are sent from one node to another may change frequently.  A path
      is defined between two nodes.  For multicast or broadcast, a
      packet may be sent by one node and received by multiple nodes.  In
      this case, the packet is sent over multiple paths at once, one
      path for each combination of sending and receiving node.  Note
      that an entity may have only partial visibility of the path
      elements that comprise a path, and entities may treat path
      elements at different levels of abstraction.

   Subpath:  Given a path, a subpath is a sequence of adjacent path
      elements of this path.

   Flow:  An entity made of packets to which the traits of a path or set
      of subpaths may be applied in a functional sense.  For example, a
      flow can consist of all packets sent within a TCP session with the
      same five-tuple between two hosts, or it can consist of all
      packets sent on the same physical link.

   Property:  A trait of one or a sequence of path elements, or a trait
      of a flow with respect to one or a sequence of path elements.  An
      example of a link property is the maximum data rate that can be
      sent over the link.  An example of a node property is the
      administrative domain that the node belongs to.  An example of a
      property of a flow with respect to a subpath is the aggregated
      one-way delay of the flow being sent from one node to another node
      over this subpath.  A property is thus described by a tuple
      containing the path element(s), the flow or an empty set if no
      packets are relevant for the property, the name of the property
      (e.g., maximum data rate), and the value of the property (e.g.,

   Aggregated property:  A collection of multiple values of a property
      into a single value, according to a function.  A property can be
      aggregated over multiple path elements (i.e., a path), e.g., the

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      MTU of a path as the minimum MTU of all links on the path, over
      multiple packets (i.e., a flow), e.g., the median one-way latency
      of all packets between two nodes, or over both, e.g., the mean of
      the queueing delays of a flow on all nodes along a path.  The
      aggregation function can be numerical, e.g., median, sum, minimum,
      it can be logical, e.g., "true if all are true", "true if at least
      50\% of values are true", or an arbitrary function which maps
      multiple input values to an output value.

   Observed property:  A property that is observed for a specific path
      element or path, e.g., using measurements.  For example, the one-
      way delay of a specific packet transmitted from one node to
      another node can be measured.

   Assessed property:  An approximate calculation or assessment of the
      value of a property.  An assessed property includes the
      reliability of the calculation or assessment.  The notion of
      reliability depends on the property.  For example, a path property
      based on an approximate calculation may describe the expected
      median one-way latency of packets sent on a path within the next
      second, including the confidence level and interval.  A non-
      numerical assessment may instead include the likelihood that the
      property holds.

3.  Use Cases for Path Properties

   When a path-aware network exposes path properties to hosts or other
   entities, these entities may use this information to achieve
   different goals.  This section lists several use cases for path
   properties.  Note that this is not an exhaustive list, as with every
   new technology and protocol, novel use cases may emerge, and new path
   properties may become relevant.

3.1.  Performance Monitoring and Enhancement

   Network operators can observe path properties (e.g., measured by on-
   path devices), to monitor Quality of Service (QoS) characteristics of
   recent end-user traffic on a path or subpath through their network.
   Such properties may help identify potential performance problems or
   trigger countermeasures to enhance performance.

3.2.  Path Selection

   Entities can choose what traffic to send over which path or subset of
   paths.  Entities may select their paths to fulfill a specific goal,
   e.g., related to security or performance.  As an example of security-
   related path selection, an entity may allow or disallow sending
   traffic over paths involving specific networks or nodes to enforce

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   traffic policies.  In an enterprise network where all traffic has to
   go through a specific firewall, a path-aware host can implement this
   policy using path selection, in which case the host needs to be aware
   of paths involving that firewall.  As an example of performance-
   related path selection, an entity may prefer paths with performance
   properties that best match its traffic, e.g., retrieving a small
   webpage as quickly as possible over a path with short One-Way Delays
   in both directions, or retrieving a large file over a path with high
   Link Capacities on all links.  Note, there may be trade-offs between
   path properties (e.g., One-Way Delay and Link Capacity), and entities
   may influence these trade-offs with their choices.  As a baseline, a
   path selection algorithm should aim to not perform worse than the
   default case most of the time.

   Path selection can be done both by hosts and by entities within the
   network: 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.  A host might be able to select between a set
   of paths, either if there are several paths to the same destination
   (e.g., if the host 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 hosts 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.

3.3.  Traffic Configuration

   When sending traffic over a specific path, entities can adjust this
   traffic based on the properties of the path.  For example, an entity
   may select an appropriate protocol depending on the capabilities of
   the on-path devices, or adjust protocol parameters to an existing
   path.  An example of traffic configuration is a video streaming
   application choosing an (initial) video quality based on the
   achievable data rate, or the monetary cost to send data across a
   network, eventually on a given path, using a volume-based or flat-
   rate cost model.

   Conversely, the selection of a protocol may influence the devices
   that will be involved in a path.  For example, a 0-RTT Transport
   Converter [I-D.ietf-tcpm-converters] will be involved in a path only

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   when invoked by a host; such invocation will lead to the use of MPTCP
   or TCPinc capabilities while such use is not supported via the
   default forwarding path.  Another example of traffic policies is a
   connection which may be composed of multiple streams; each stream
   with specific service requirements.  A host may decide to invoke a
   given service function (e.g., transcoding) only for some streams
   while others are not processed by that service function.

4.  Examples of Path Properties

   This Section gives some examples of Path Properties which may be
   useful, e.g., for the use cases described in Section 3.

   Path properties may be relatively dynamic, e.g., the one-way delay of
   a packet sent over a specific path, or non-dynamic, e.g., the MTU of
   an ethernet link which only changes infrequently.  Usefulness over
   time differs depending on how dynamic a property is: 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 for a longer period of time, e.g. a NAT typically stays
   on a specific path during the lifetime of a connection involving
   packets sent over this path.

   From the point of view of a host, path properties may relate to path
   elements close to the host, i.e., within the first few hops, or they
   may include path elements far from the host, e.g. list of ASes
   traversed.  The visibility of path properties to a specific entity
   may depend on factors such as the physical or network distance or the
   existence of trust or contractual relationships between the entity
   and the path element(s).

   Furthermore, entities may or may not be able to influence the path
   elements on their path and their path properties.  For example, a
   user might select between multiple potential adjacent links 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.
      The Access Technology may be given in an abstract way, e.g., as a
      WiFi, Wired Ethernet, or Cellular link.  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

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      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 network to which one or multiple path elements belong.

   Service function:  A service function that a path element applies to
      a flow, see [RFC7665].  Examples of abstract service functions
      include firewalls, Network Address Translation (NAT), and TCP

   Administrative Domain:  The administrative domain, e.g., the ICP
      area, AS, or Service provider network to which a path element or
      subpath 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

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

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

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

   Some path properties express the performance of the transmission of a
   packet or flow over a link or subpath.  Such transmission performance
   properties can be measured or approximated, e.g., by hosts or by path
   elements on the path.  They might be made available in an aggregated
   form, such as averages or minimums.  See [ANRW18-Metrics] for a
   discussion of how to measure some transmission performance properties
   at the host.  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].

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   Link Capacity:  The link capacity is the maximum data rate at which
      data that was sent over a link can correctly be received at the
      node adjacent to the link.  This property is analogous to the link
      capacity defined in [RFC5136] but not restricted to IP-layer

   Link Usage:  The link usage is the actual data rate at which data
      that was sent over a link is correctly received at the node
      adjacent to the link.  This property is analogous to the link
      usage defined in [RFC5136] but not restricted to IP-layer traffic.

   One-Way Delay:  The one-way delay is the delay between a node sending
      a packet and another node on the same path receiving the packet.
      This property is analogous to the one-way delay defined in
      [RFC7679] but not restricted to IP-layer traffic.

   One-Way Delay Variation:  The variation of the one-way delays within
      a flow.  This property is similar to the one-way delay variation
      defined in [RFC3393] but not restricted to IP-layer traffic and
      defined for packets on the same flow instead of packets sent
      between a source and destination IP address.

   One-Way Packet Loss:  Packets sent by a node but not received by
      another node on the same path after a certain time interval are
      considered lost.  This property is analogous to the one-way loss
      defined in [RFC7680] but not restricted to IP-layer traffic.
      Metrics such as loss patterns [RFC3357] and loss episodes
      [RFC6534] can be expressed as aggregated properties.

5.  Security Considerations

   If nodes 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, nodes may need to establish a trust relationship or
   be able to verify the authenticity, integrity, and correctness of
   path properties received from another node.

6.  IANA Considerations

   This document has no IANA actions.

7.  Informative References

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              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.

              Bonaventure, O., Boucadair, M., Gundavelli, S., Seo, S.,
              and B. Hesmans, "0-RTT TCP Convert Protocol", draft-ietf-
              tcpm-converters-13 (work in progress), October 2019.

              Trammell, B., "Current Open Questions in Path Aware
              Networking", draft-irtf-panrg-questions-03 (work in
              progress), October 2019.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC3357]  Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample
              Metrics", RFC 3357, DOI 10.17487/RFC3357, August 2002,

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,

   [RFC5136]  Chimento, P. and J. Ishac, "Defining Network Capacity",
              RFC 5136, DOI 10.17487/RFC5136, February 2008,

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              DOI 10.17487/RFC5693, October 2009,

   [RFC6534]  Duffield, N., Morton, A., and J. Sommers, "Loss Episode
              Metrics for IP Performance Metrics (IPPM)", RFC 6534,
              DOI 10.17487/RFC6534, May 2012,

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

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   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <>.

   [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,


   Thanks to the Path-Aware Networking Research Group for the discussion
   and feedback.  Specifically, thanks to Mohamed Boudacair for the
   detailed review and various text suggestions, thanks to Brian
   Trammell for suggesting the flow definition, and thanks to Adrian
   Perrig and Matthias Rost for the detailed feedback.  Thanks to Paul
   Hoffman for the editorial changes.

Authors' Addresses

   Theresa Enghardt
   TU Berlin


   Cyrill Kraehenbuehl
   ETH Zuerich


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