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TAPS Working Group                                             P. Tiesel
Internet-Draft                                               T. Enghardt
Intended status: Informational            Berlin Institute of Technology
Expires: December 29, 2017                                 June 27, 2017

  Communication Units Granularity Considerations for Multi-Path Aware
                          Transport Selection


   This document provides an abstract framework to reason about the
   composition of multi-path aware systems in a protocol-independent
   fashion.  It discusses basic mechanisms that are used in multi-path
   systems and their applicability to different granularities of
   communication units.  This document is targeted as consideration
   basis for automation of destination, path and transport protocol
   selection within the transport layer.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 29, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Communication Units vs. Layering  . . . . . . . . . . . .   3
   3.  Abstract Hierarchy of Communication Units . . . . . . . . . .   4
     3.1.  Object  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Stream  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Association, Flow . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Association Set, Flow Set (Flow-Group)  . . . . . . . . .   5
   4.  Mechanisms Used in Multi-Path Systems . . . . . . . . . . . .   5
     4.1.  Destination Selection . . . . . . . . . . . . . . . . . .   5
     4.2.  Path Selection  . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Chunking  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Scheduling  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Transport Protocol Stack Instance Selection . . . . . . .   8
   5.  Cost of Transport Option Selection  . . . . . . . . . . . . .   8
   6.  Involvement of On-Path Elements . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Conventions and Definitions

   The words "MUST", "MUST NOT", "SHALL", "SHALL NOT", "SHOULD", and
   "MAY" are used in this document.  It's not shouting; when these words
   are capitalized, they have a special meaning as defined in [RFC2119].

2.  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.pauly-taps-guidelines] 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?

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   o  Which path(s) between the endpoints should be used?

   o  Should the paths be used in an active/active way or only as

   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

   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

2.1.  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.

   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

   o  If seeing the same mechanism multiple times within the same system
      composition, we can reason about possibly conflicting

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   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.

3.  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.gjessing-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.trammell-taps-post-sockets] if applicable.

3.1.  Object

   An Object is a piece of data that has a meaning for the application.
   It is the smallest communication unit that we consider.

   [I-D.gjessing-taps-minset] correspondent: Message

   [I-D.trammell-taps-post-sockets] correspondent: Message


   o  A HTTP-Request/Response-Header/Body for HTTP/2

   o  An XML message in XMPP

3.2.  Stream

   A Stream is an ordered sequence of related Objects that should be
   treated the same by the transport system.

   [I-D.gjessing-taps-minset] correspondent: Flow

   [I-D.trammell-taps-post-sockets] correspondent: Stream


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   o  A Stream in QUIC or SCTP

   o  A TCP connection used as transport for XMPP

3.3.  Association, Flow

   An Association multiplexes a set of Objects 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.gjessing-taps-minset] correspondent: Flow-Group

   [I-D.trammell-taps-post-sockets] correspondent: Association


   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,

3.4.  Association Set, Flow 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.gjessing-taps-minset] correspondent: Flow-Group

   [I-D.trammell-taps-post-sockets] correspondent: Association


   o  Two flows, one carrying RTP payloads and one used for RTCP control

4.  Mechanisms Used in Multi-Path Systems

4.1.  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:

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   o  Choosing one address of a multi-homed server for an upcoming

   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.

4.2.  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

   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
   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

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   domain - especially when it comes to integrating 3GPP mechanisms like
   IFOM/ [RFC5555].

4.3.  Chunking

   Chunking refers to splitting an object, a stream or a set of
   associations into one or more parts.  Typically, chunking splits only
   large objects 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 Objects 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.

4.4.  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
   balance traffic, it makes sense to look at scheduling as a concept
   itself that uses chunking and per-chunk path selection as sub-

   Examples of scheduling strategies include:

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   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.

4.5.  Transport Protocol Stack Instance Selection

   TODO - There are many examples in TAPS - still unsure what will go
   here or will be cited here.

5.  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.

6.  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.

   Especially in the 3GPP context, a lot of off-loading mechanisms have
   been specified that are implemented as path level components, within
   virtual network adapters.

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7.  Security Considerations

   Security related transport service request must take priority over
   performance, therefore, transport options or stack compositions that
   don't provide the transport service requested should be ignored for
   transport option selection.

   Note:  This discussion is not exhaustive - more considerations will
      be added in later versions of this draft.

8.  IANA Considerations


9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

9.2.  Informative References

              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.

              Gjessing, S. and M. Welzl, "A Minimal Set of Transport
              Services for TAPS Systems", draft-gjessing-taps-minset-05
              (work in progress), June 2017.

              Pauly, T., "Software Guidelines for Protocol Evolution",
              draft-pauly-taps-guidelines-00 (work in progress),
              February 2017.

              Trammell, B., Perkins, C., Pauly, T., and M. Kuehlewind,
              "Post Sockets, An Abstract Programming Interface for the
              Transport Layer", draft-trammell-taps-post-sockets-00
              (work in progress), March 2017.

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   [RFC5555]  Soliman, H., Ed., "Mobile IPv6 Support for Dual Stack
              Hosts and Routers", RFC 5555, DOI 10.17487/RFC5555, June
              2009, <http://www.rfc-editor.org/info/rfc5555>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

Authors' Addresses

   Philipp S. Tiesel
   Berlin Institute of Technology
   Marchstr. 23

   Email: philipp@inet.tu-berlin.de

   Theresa Enghardt
   Berlin Institute of Technology
   Marchstr. 23

   Email: theresa@inet.tu-berlin.de

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