Internet DRAFT - draft-tiesel-taps-communitgrany
draft-tiesel-taps-communitgrany
TAPS Working Group P. Tiesel
Internet-Draft T. Enghardt
Intended status: Informational TU Berlin
Expires: May 7, 2019 November 03, 2018
Communication Units Granularity Considerations for Multi-Path Aware
Transport Selection
draft-tiesel-taps-communitgrany-03
Abstract
This document provides an approach how to reason about the
composition of multi-path aware transport stacks. It discusses how
to compose the functionality needed by stacking existing internet
protocols and the fundamental mechanisms that are used in multi-path
systems and the consequences of applying them to different
granularities of communication units, e.g, on a message or stream
granularity. This document is targeted as guidance for automation of
destination selection, path selection, and transport protocol
selection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 7, 2019.
Copyright Notice
Copyright (c) 2018 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
Tiesel & Enghardt Expires May 7, 2019 [Page 1]
Internet-Draft Communication Units Granularity November 2018
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Communication Units vs. Layering . . . . . . . . . . . . 3
2. Abstract Hierarchy of Communication Units . . . . . . . . . . 4
2.1. Message . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Stream . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Association / Connection Group . . . . . . . . . . . . . 5
2.4. Association Set / Flow-Group . . . . . . . . . . . . . . 5
3. Mechanisms Used in Multi-Path Systems . . . . . . . . . . . . 5
3.1. Destination Selection . . . . . . . . . . . . . . . . . . 6
3.2. Path Selection . . . . . . . . . . . . . . . . . . . . . 6
3.3. Chunking . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Scheduling . . . . . . . . . . . . . . . . . . . . . . . 7
4. Cost of Transport Option Selection . . . . . . . . . . . . . 8
5. Involvement of On-Path Elements . . . . . . . . . . . . . . . 8
6. Overview of Mechanisms provided by selected IETF Protocols . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. Informative References . . . . . . . . . . . . . . . . . . . 11
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Today's Internet architecture faces a communication endpoint with a
set of choices, including choosing a transport protocol and picking
an IP protocol version. In many cases, e.g., when fetching data from
a CDN, an endpoint has also the choice of which endpoint instance,
[I-D.brunstrom-taps-impl] calls these instances "Derived Endpoint",
to contact as DNS can return multiple alternative addresses.
If endpoints want to take advantage of multiple available paths,
there is another bunch of, partially interdependent, choices:
o Which path(s) between the endpoints could be used?
o Which path(s) between the endpoints should be used?
Tiesel & Enghardt Expires May 7, 2019 [Page 2]
Internet-Draft Communication Units Granularity November 2018
o Should the paths be used in an active/active way or only as
active/fallback?
o Which protocols or sets of protocols should be used?
o Which role will other on-path elements, e.g. middle-boxes, take in
servicing this flow?
Implementing an heuristic or strategy for choosing from this
overwhelming set of transport options by each application puts a huge
burden on the application developer. Thus, the decisions regarding
all transport options mentioned so far should be supported and, if
requested by the application, automated within a the transport layer.
In order to build such automatization, we need to be able to compare
the product of all transport options (destinations, paths, transport
protocols and protocol options) available to choose the most
appropriate.
As the protocols to be used are not known a priori and can differ
depending on other transport options, this reasoning has to be
independent of a specific protocol or implementation and allow to
compare them even if they operate on different communication unit
granularities.
1.1. Communication Units vs. Layering
When reasoning about network systems, layering traditionally has been
the main guidance on where functionality is placed. Looking at
modern systems, the classical concept of layers and their mapping to
protocols becomes blurry. Protocols can operate on different
granularities of communication units, i.e., the semantic units such
as messages that the protocols distinguish. These communication
units often do not match the PDUs used by the protocols, e.g., TCP
segments do not necessarily align with messages at the application
layer.
In this document, we do not want to take a protocol-centric
perspective, but we focus on mechanisms a multi-access system is
composed of and the communication units they operate on. This has
several advantages:
o We can much easier abstract from the protocols used and look at
the composition itself.
o By disseminate on which kind of communication unit these
mechanisms can operate, we can reason about the overall design
space.
Tiesel & Enghardt Expires May 7, 2019 [Page 3]
Internet-Draft Communication Units Granularity November 2018
o If seeing the same mechanism multiple times within the same system
composition, we can reason about possibly conflicting
optimizations.
Overall, this perspective allows us to compare mechanism like
distributing requests of an application among different paths, MPTCP
and using bandwidth aggregation proxies (as discussed within the IETF
in the BANANA working group) despite their different nature and layer
of implementation.
2. Abstract Hierarchy of Communication Units
These communication units definitions are primarily used for
reasoning about automatic stack composition. Therefore, depending on
the protocol stack instance, a communication unit can span multiple
protocol instances.
Some of these hierarchy levels correspond to objects in
[I-D.ietf-taps-minset], but in case of Association and Association
Set, we have to split categories as they may indeed be separate on
the transport. Note the naming confusion concerning the term "flow"
deriving from different perspective.
We also annotate the corresponding terminology used in
[I-D.ietf-taps-arch] if it differs from the one used in this
document.
2.1. Message
An Message is a piece of data that has a meaning for the application.
It is the smallest communication unit that we consider.
[I-D.ietf-taps-minset] correspondent: Message
Examples:
o A HTTP-Request/Response-Header/Body for HTTP/2
o An XML message in XMPP
2.2. Stream
A Stream is an ordered sequence of related Messages that should be
treated the same by the transport system.
[I-D.ietf-taps-minset] correspondent: Flow
Examples:
Tiesel & Enghardt Expires May 7, 2019 [Page 4]
Internet-Draft Communication Units Granularity November 2018
o A Stream in QUIC or SCTP
o A TCP connection used as transport for XMPP
2.3. Association / Connection Group
An Association multiplexes a set of Messages or Streams within the
same Flow with common source and destination. Therefore these
communication units become indistinguishable for the network.
Association and flow describe the same concept, the former from the
perspective of the application, the latter from the perspective of
the network.
[I-D.ietf-taps-minset] correspondent: Flow-Group
[I-D.ietf-taps-arch] correspondent: Connection Group
Examples:
o A TCP connection carrying HTTP/2 frames
o A set of IP packets that carry TCP or UDP segments and share the
same 5-tuple of src-address, dst-address, protocol, src-port,
dest-port.
2.4. Association Set / Flow-Group
An Association Set or Flow Set is a set of Associations or Flows that
belong together from an application point of view.
[I-D.ietf-taps-minset] correspondent: Flow-Group
Examples:
o Two flows, one carrying RTP payloads and one used for RTCP control
messages.
3. Mechanisms Used in Multi-Path Systems
Transport protocols on the Internet provide a large variety of
functionality. While the functionality of simple protocols like UDP
is easy to describe (multiplexing streams of messages), describing
the functionality of complex protocols such as QUIC, MPTCP or SCTP is
manyfold as these protocols provide a set of commonly used
functionality. Also, the same functionality can be provided at many
places throughout the whole stack. In the following, we explore the
set of functionality that can be provided by transport protocols.
Tiesel & Enghardt Expires May 7, 2019 [Page 5]
Internet-Draft Communication Units Granularity November 2018
3.1. Destination Selection
Destination Selection refers to selecting one of multiple different
destinations. This mechanism is applicable to any kind of
communication unit and can occur on all layers.
Typical cases for destination selection include:
o Choosing one address of a multi-homed server for an upcoming
communication.
o Choosing a server among a list of servers retuned by DNS, e.g for
servers that host the same content as part of a CDN.
o Choosing a backend server within a load balancer.
In practice, destination address selection is often tied to name
resolution. As name resolution relies on both local decisions on the
endpoint as well as decisions within the DNS infrastructure, this
mechanism spreads across different administrative domains which each
independently contribute to the overall selection result.
3.2. Path Selection
Path Selection refers to choosing which of the available paths to
use. and can occur on the network layer and any layer below.
o Within an end-host, path selection is usually realized by choosing
the source IP address and thus choosing one of the local network
interfaces for the communication to the remote endpoint.
o Within a path layer traffic system like an MPTCP-Proxy or a
BANANA-Box, path selection is usually realized by choosing the
outer source and destination address.
o In case of an ECMP router, path selection is usually done based on
a 3- or 5-tupel and just determines the interface to the next hop.
o Within MPTCP, each TCP segment has to be assigned to one or more
subflows for transmission to the receiver.
While path selection involves a choice of access network it does not
need knowledge of or changes to the routing choices within the core
network.
When doing path selection on small communication units like TCP
segments, it is not uncommon to split path selection into two
subproblems: _Candidate Path Selection_ determines feasible and
Tiesel & Enghardt Expires May 7, 2019 [Page 6]
Internet-Draft Communication Units Granularity November 2018
preferred choices, e.g., in case of MPTCP by establishing subflows.
Afterwards, _Per-Chunk Path Selection_ selects among these
alternatives for each chunk. Thus, the first can be more expensive
while the latter should be easy to execute.
TODO: Discuss difference between Multiple Provisioning Domains
[RFC7556] or multiple access networks within the same provisioning
domain - especially when it comes to integrating 3GPP mechanisms like
[RFC5555] or [RFC7864].
3.3. Chunking
Chunking refers to splitting an message, a stream or a set of
associations into one or more parts. Typically, chunking splits only
large messages or streams into multiple ones while keeping smaller
entities untouched. Associations or Flows are typically not split,
but sets of Associations or Flows might be partitioned. Once split
into chunks, each chunk can be transferred individually over
different transfer options.
Chunking can and does occur at different layers within a system:
o A Web site consists of multiple objects or files. Thus, the files
can be seen as the natural chunks of a Web site.
o TCP takes as input a byte stream and chunks it into segments. TCP
chunking (segmentation) occurs at arbitrary byte ranges, thus it
will most likely not align with boundaries of Messages that were
multiplexed within an application layer Association on top of a
TCP connection.
In practice, chunking is often constrained in order to maintain
certain properties that are desirable for the overall system.
Examples such restrictions include the following:
o Segmentation in TCP restrict the chunk size, i.e. TCP segment
size, to the IP MTU or IP Path MTU to avoid fragmentation at the
IP layer.
o Equal cost multipath routing does not distribute packets, but
Flows to avoid reordering.
3.4. Scheduling
Scheduling refers to distributing chunks or sets of chunks across
multiple pre-chosen path. Thus, depending on the objectives, it can
make sense to see scheduling as is nothing else than per-chunk path
selection as defined above. In other cases, e.g. when trying to
Tiesel & Enghardt Expires May 7, 2019 [Page 7]
Internet-Draft Communication Units Granularity November 2018
balance traffic, it makes sense to look at scheduling as a concept
itself that uses chunking and per-chunk path selection as sub-
mechanisms.
Examples of scheduling strategies include:
o Schedule all chunks on one path as long as this path is available,
otherwise fall pack to another.
o Distribute chunks based on path capacity.
4. Cost of Transport Option Selection
Transport option selection mechanisms are often intertwined. Which
mechanism is used by which layer or which network component depends
on the transfer objectives as well as the state of the network, e.g.,
availability, path throughput, path RTT, server load.
The cost and complexity of transport option selection depends on the
network state used and the number of transfer options. If the
transfer option selection only uses local state e.g., link
availability, and the mechanism is predetermined and/or uses simple
mechanisms, e.g., a simple hash function, the cost can even be
negligible. An example where transfer option selection is cheap is
ECMP within a router. In other cases, the cost can be non-trivial,
e.g. when the selection involves queries to remote entities or even
active network performance measurements. Such examples include DNS
or DHT lookups, as used by some file sharing protocols, or network
measurements like RTT and bandwidth estimations used by many video
streaming applications. Indeed, costs may be prohibitive, e.g when
requiring multiple DNS lookups for every 1 second chunk of a 20
minute video.
5. Involvement of On-Path Elements
It may become necessary to take path layer components (middle-boxes)
into account that interfere with the transport layer.
While the classical "End-To-End Arguments in System Design"
[End-To-End] advocates for a dumb network and placing functionality
as close to the edge and up in the stack as possible, there are
always tussles of moving functionality up or down the stack. This
document does not argue against pushing some multi-path functionality
down the stack, but advocates to maintain the control of the overall
system composition at the end host. Functionality provided by a path
can indeed be a reason to choose this path for a given communication
unit.
Tiesel & Enghardt Expires May 7, 2019 [Page 8]
Internet-Draft Communication Units Granularity November 2018
Some flow off-loading mechanisms that come in gestalt of of logical
interfaces, e.g., [RFC7847]. These interfaces treat some association
sets differently, which can be considered on-path functionality.
6. Overview of Mechanisms provided by selected IETF Protocols
+-----+-----+-----+------+------+--------+-------+-------+----------+
| Pro | Con | Ord | Reli | Inte | Confid | Authe | Chunk | Multiple |
| toc | ges | eri | abil | grit | ential | ntici | ing | xing |
| ol | tio | ng | ity | y P. | ity P. | ty P. | | |
| | n C | | | | | | | |
| | ont | | | | | | | |
| | rol | | | | | | | |
+-----+-----+-----+------+------+--------+-------+-------+----------+
| HTT | r | r | r | | | | bytes | requests |
| P | | | | | | | | |
| | | | | | | | | |
| HTT | r | r | r | r | r | r | bytes | requests |
| PS | | | | | | | | |
| | | | | | | | | |
| XMP | r | r | r | (r) | (r) | (r) | | messages |
| P | | | | | | | | |
| | | | | | | | | |
| SIP | | | + | (r) | (r) | (r) | | messages |
| | | | | | | | | |
| DTL | | | | + | + | + | | services |
| S | | | | | | | | ,name |
| | | | | | | | | |
| TLS | | r | r | + | + | + | | services |
| | | | | | | | | ,name |
| | | | | | | | | |
| RTP | +(p | +(p | | | | | messa | messages |
| | rf) | rf) | | | | | ges(p | |
| | | | | | | | rf) | |
| | | | | | | | | |
| SRT | +(p | +(p | | + | + | r(sig | messa | messages |
| P | rf) | rf) | | | | ) | ges(p | |
| | | | | | | | rf) | |
| | | | | | | | | |
| QUI | + | + | + | + | + | +(tls | bytes | connecti |
| C | | | | | | ) | | on-id,+( |
| | | | | | | | | tls) |
| | | | | | | | | |
| UDP | | | | | | | | ports |
| | | | | | | | | |
| DCC | + | | | | | | | ports |
| P | | | | | | | | |
| | | | | | | | | |
Tiesel & Enghardt Expires May 7, 2019 [Page 9]
Internet-Draft Communication Units Granularity November 2018
| TCP | + | + | + | | | | bytes | ports |
| | | | | | | | | |
| MPT | + | + | + | | | | bytes | ports |
| CP | | | | | | | | |
| | | | | | | | | |
| SCT | + | + | + | | | | bytes | ports,st |
| P | | | | | | | | reams |
| | | | | | | | | |
| IPs | | | | + | + | r(ike | | spi |
| ec | | | | | | ) | | ,next- |
| (ES | | | | | | | | header |
| P) | | | | | | | | |
| | | | | | | | | |
| IPs | | | | + | | r(ike | | spi |
| ec | | | | | | ) | | ,next- |
| (AH | | | | | | | | header |
| ) | | | | | | | | |
| | | | | | | | | |
| IP | | (+( | | | | | (frag | address |
| | | fr) | | | | | ments | ,next- |
| | | ) | | | | | ) | header |
| | | | | | | | | |
| NEM | | | | | r | r | assoc | |
| O/I | | | | | | | . | |
| FOM | | | | | | | | |
+-----+-----+-----+------+------+--------+-------+-------+----------+
Legend:
r: Protocol requires transport service.
+: Protocol provides transport service.
prf: Realized by content specific profiles.
tls: Uses TLSv1.3 as sub-protocol; imports authenticity protection
and multiplexing from TLS.
ike: Realized externally by external protocol IKE/IKEv2.
sig: Realized externally by external signaling protocol (e.g., SIP,
XMPP, WebRTC).
fr: :Only when fragmentation is used and only to re-assemble IP PUDs
Tiesel & Enghardt Expires May 7, 2019 [Page 10]
Internet-Draft Communication Units Granularity November 2018
7. Acknowledgements
This work has been supported by Leibniz Prize project funds of DFG -
German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
FE 570/4-1).
8. Informative References
[End-To-End]
Saltzer, J., Reed, D., and D. Clark, "End-to-end arguments
in system design", ACM Transactions on Computer
Systems Vol. 2, pp. 277-288, DOI 10.1145/357401.357402,
November 1984.
[I-D.brunstrom-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", draft-
brunstrom-taps-impl-00 (work in progress), March 2018.
[I-D.ietf-taps-arch]
Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
Perkins, C., Tiesel, P., and C. Wood, "An Architecture for
Transport Services", draft-ietf-taps-arch-02 (work in
progress), October 2018.
[I-D.ietf-taps-interface]
Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
Kuehlewind, M., Perkins, C., Tiesel, P., and C. Wood, "An
Abstract Application Layer Interface to Transport
Services", draft-ietf-taps-interface-02 (work in
progress), October 2018.
[I-D.ietf-taps-minset]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for End Systems", draft-ietf-taps-minset-11 (work
in progress), September 2018.
[RFC5555] Soliman, H., Ed., "Mobile IPv6 Support for Dual Stack
Hosts and Routers", RFC 5555, DOI 10.17487/RFC5555, June
2009, <https://www.rfc-editor.org/info/rfc5555>.
[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.
Tiesel & Enghardt Expires May 7, 2019 [Page 11]
Internet-Draft Communication Units Granularity November 2018
[RFC7847] Melia, T., Ed. and S. Gundavelli, Ed., "Logical-Interface
Support for IP Hosts with Multi-Access Support", RFC 7847,
DOI 10.17487/RFC7847, May 2016,
<https://www.rfc-editor.org/info/rfc7847>.
[RFC7864] Bernardos, CJ., Ed., "Proxy Mobile IPv6 Extensions to
Support Flow Mobility", RFC 7864, DOI 10.17487/RFC7864,
May 2016, <https://www.rfc-editor.org/info/rfc7864>.
Appendix A. Changes
A.1. Since -00
o Replaced granularity "Object" with "Message" to align with other
TAPS documents.
o Removed empty section on protocol instance selection - this topic
will go into a separate document later.
o Minor clarifications.
o Removed definition of normative terms not needed for this document
o Added acknowledgments and updated authors' affiliation
(compliance).
A.2. Since -01
o Updated drafts references
o Added Overview of Mechanisms provided by selected IETF Protocols
o Minor clarifications
o Removed superfluous IANA and Security Considerations section
A.3. Since -02
o Prevent expirary (minor formatting fixes)
Authors' Addresses
Tiesel & Enghardt Expires May 7, 2019 [Page 12]
Internet-Draft Communication Units Granularity November 2018
Philipp S. Tiesel
TU Berlin
Marchstr. 23
Berlin
Germany
Email: philipp@inet.tu-berlin.de
Theresa Enghardt
TU Berlin
Marchstr. 23
Berlin
Germany
Email: theresa@inet.tu-berlin.de
Tiesel & Enghardt Expires May 7, 2019 [Page 13]
