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Versions: 00 01 02 03

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
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   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



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   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?





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   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.




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   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:



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   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.




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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



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   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



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   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.




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   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   |     |     |      |      |        |       |       |          |
   |     |     |     |      |      |        |       |       |          |



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   | 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







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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>.






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   [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










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   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



































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