PANRG T. Enghardt
Internet-Draft TU Berlin
Intended status: Informational C. Kraehenbuehl
Expires: January 9, 2020 ETH Zuerich
July 08, 2019
A Vocabulary of Path Properties
draft-enghardt-panrg-path-properties-02
Abstract
This document defines and categorizes information about Internet
paths that an entity, such as a host, might have or want to have.
This information is expressed as properties of paths between two
hosts.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Because the current Internet provides an IP-based best-effort bit
pipe, hosts have little information about paths to other hosts. A
Path Aware Network exposes information about one or multiple paths
through the network to hosts 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 host 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
varies based on the goal, as a baseline, a path selection
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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. 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.
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 hosts 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
Node: An entity which processes packets, e.g., sends, receives,
forwards, or modifies them.
Host: A node that processes packets that are explicitly addressed to
itself.
Router: A node that processes packets that are not explicitly
addressed to itself.
Link: A medium or communication facility that connects two or more
nodes with each other and enables them to exchange packets. A
link 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.
Path element: Either a node or a link.
Path: A sequence of adjacent path elements, alternating between
nodes and links, starting and ending with a host. A path can be
viewed as an abstraction on a specific layer, omitting lower layer
path elements. For example, a router implementing IPv6 may be a
path element on a path when considering the network layer. If
this router does not implement transport layer functionality, it
is hidden when a higher layer, such as the transport or
application layer, is considered. In the case of multicast or
broadcast, a single packet may be sent over multiple paths at once
- one path for each combination of sending and receiving host.
Subpath: Given a path, a subpath is a sequence of adjacent path
elements of this path, starting and ending with a node.
Flow: One or multiple packets which are traversing the same subpath
or path. 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.
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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 a subpath. A property is thus described by a tuple
containing the sequence of path elements, 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., 1Gbps).
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
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.
Measured 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 can be measured.
Estimated property: An approximate calculation or judgment of the
value of a property. For example, an estimated property may
describe the expected median one-way latency of packets sent on a
path within the next second. An estimated property includes the
reliability of the estimate. The notion of reliability depends on
the property. For example, it may 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 a host considering them.
Due to the potential physical proximity and pre-existing trust or
contractual relationships between hosts and path elements within the
same domain, domain properties may be more accessible to the host
than other properties.
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Furthermore, hosts 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 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
in the same domain. 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 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 subpath.
4. Backbone Properties
Backbone path properties relate to path elements not within the same
domain as a host considering them, thus, in the backbone from the
host's point of view.
Typically, backbone properties are less accessible to a host than
domain properties, due to the potential increased distance and the
lack of pre-existing trust or contractual relationship.
Additionally, hosts 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 or to subpaths, part
of which often lies outside of a host's domain. Thus, such
properties are listed as Backbone Properties.
Presence of a certain network function on the path: Indicates that a
node performs a certain network function on a flow, e.g., whether
the node acts as a proxy, as a firewall, or performs Network
Address Translation (NAT). This node may be either in the same
domain as the host or in a different domain, i.e., the backbone.
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Administrative Entity: The administrative entity, e.g., the AS, 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
failure.
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.
5. Dynamic Properties
Dynamic path properties relate to the transmission of an individual
packet or of a flow over a subpath. 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 be measured or approximated, and
might be made available in an aggregated form, such as averages or
minimums. Dynamic Path Properties can be measured by the host itself
or by a different entity. See [ANRW18-Metrics] for a discussion of
how to measure some dynamic path properties at the host.
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, subpath,
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.
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Latency: The time delay between a node sending a packet and a
different node on the same path receiving the same packet.
Latency variation: The variation of the latency within a flow.
Packet Loss: The percentage of packets within a flow which are sent
by one node, but not received by a different node.
Congestion: Whether a protocol feature such as ECN has provided
information that there currently is congestion on a path.
6. 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.
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-02 (work in progress), May
2019.
[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>.
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Acknowledgments
Thanks to the Path-Aware Networking Research Group for the discussion
and feedback. Thanks to Adrian Perrig and Matthias Rost for the
feedback. Thanks to Paul Hoffman for the editorial changes.
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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