ICCRG Working Group                                       O. Bonaventure
Internet-Draft                                                 M. Piraux
Intended status: Experimental                              Q. De Coninck
Expires: March 13, 2021                                        UCLouvain
                                                               M. Baerts
                                                               C. Paasch
                                                                M. Amend
                                                        Deutsche Telekom
                                                      September 09, 2020

                          Multipath schedulers


   This document proposes a series of abstract packet schedulers for
   multipath transport protocols equipped with a congestion controller.

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 13, 2021.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  An abstract multipath transport protocol  . . . . . . . . . .   4
   3.  Packet scheduling challenges  . . . . . . . . . . . . . . . .   5
   4.  Packet schedulers . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Round-Robin . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Weighted Round-Robin  . . . . . . . . . . . . . . . . . .   7
     4.3.  Strict Priority . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Round-Trip-Time Threshold . . . . . . . . . . . . . . . .   9
     4.5.  Lowest Round-Trip-Time First  . . . . . . . . . . . . . .   9
     4.6.  Combination of schedulers type: Priority and Lowest
           round-trip-time first . . . . . . . . . . . . . . . . . .  11
   5.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Since draft-bonaventure-iccrg-schedulers-00 . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Internet was designed under the implicit assumption that hosts
   are equipped with a single network interface while routers are
   equipped with several ones.  Under this assumption, an Internet host
   is usually identified by the IP address of its network interface.

   This assumption does not hold anymore today for two reasons.  First,
   a growing fraction of the Internet hosts are equipped with several
   network interfaces, usually through different datalink networks.
   These multihomed hosts are reachable via different IP addresses.
   Second, a growing fraction of the hosts that are attached through a
   single network interface are dual-stack and are thus reachable over
   both IPv4 and IPv6.

   Several Internet transport protocols have been extended to leverage
   the different paths that are exposed on such hosts: Multipath TCP
   [RFC6824], the load sharing extensions to SCTP
   [I-D.tuexen-tsvwg-sctp-multipath], Multipath DCCP
   [I-D.amend-tsvwg-multipath-dccp] and Multipath QUIC
   [I-D.deconinck-quic-multipath].  These multipath transport protocols
   differ in the way they are organized and exchange control information
   and user data.  However, they all include algorithms to handle three
   problems that any multipath transport protocol needs to solve:

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   o  Congestion controller

   o  Path manager

   o  Packet scheduler

   o  Packet re-assembly

   From a congestion control viewpoint, the main concern for a multipath
   transport protocol is that a multipath connection should not be
   unfair to single-path transport connections that share a common
   bottleneck.  This problem can be solved by coupling the congestion
   windows of the different paths.  The solution proposed in [RFC6356]
   is applicable to any transport protocol.  Beside providing fairness,
   congestion control can also be a valuable input for different kind of
   traffic distribution algorithm within a packet scheduler.  Typically
   metrics like RTT and available capacity can be derived.

   A multipath transport protocol uses different flows during the
   lifetime of a connection.  The Path Manager contains the logic that
   regulates the creation/deletion of these flows.  This logic usually
   depends on the requirements of the application that uses the
   multipath transport.  Some applications use multipath in failover
   situations.  In this case, the connection can use one path and the
   path manager can create another path when the primary one fails.  An
   application that wishes to share its load among different paths can
   request the path manager to establish different paths in order to
   simultaneously use them during the connection.  Many path managers
   have been proposed in the literature [CONEXT15], but these are
   outside the scope of this document.

   The packet scheduler is the generic term for the algorithm that
   selects the path that will be used to transmit each packet on a
   multipath connection.  This logic is obviously only useful when there
   are at least two active paths for a given multipath transport
   connection.  A variety of packet schedulers have been proposed in the
   literature [ACMCS14] and implemented in multipath transport
   protocols.  Experience with multipath transport protocols shows that
   the packet scheduler can have a huge impact on the performance
   achieved by such protocols.

   Packet re-assembly or re-ordering in multipath transport has the
   functionality to equalize the effect of packet scheduling across
   paths with different characteristics and restore the original packet
   order to a certain extent.  Obviously, packet re-assembly is the
   counterpart of packet scheduling and located at the far end of the
   multipath transport.  However, packet scheduling schemes exists which
   render the re-assembly superfluous or lowering at least its effort.

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   In this document, we document a series of multipath packet schedulers
   that are known to provide performance that matches well the
   requirements of specific applications.  To describe these packet
   schedulers, we assume an abstract transport that is briefly presented
   in Section 2.  In Section 3 we describe the challenges and
   constraints around a multipath scheduler.  Finally, we describe the
   different schedulers in Section 4.  To keep the description as simple
   and intuitive as possible, we assume here multipath connections that
   are composed of two paths, a frequent deployment scenario for
   multipath transport.  This does not restrict the proposed schedulers
   to using only two paths.  Implementations are encouraged to support
   more than 2 paths.  We leave the discussion on how to adapt these
   abstract schedulers to concrete multipath transport protocols in
   future drafts.

2.  An abstract multipath transport protocol

   For simplicity, we assume a multipath transport protocol which can
   send packets over different paths.  Some protocols such as Multipath
   TCP [RFC6824] support active and backup paths.  We do not assume this
   in this document and leave the impact of these active/backup paths in
   specific documents.

   Furthermore, we assume that there are exactly two active paths for
   the presentation of the packet schedulers.  We consider that a path
   is active as long as it supports the transmission of packets.
   Meaning, A Multipath TCP subflow TCP segment with the FIN or RST
   flags set is not considered as an active path.  Other constraints are
   possible on whether or not a path is active.  These are specific to
   the scheduler and vary depending on the goal of the scheduler.  An
   example of these is that when a path has experienced a certain number
   N of retransmission timeouts, the path can be considered inactive.

   We assume that the transport protocol maintains one congestion
   controller per path as in [RFC6356].  We do not assume a specific
   congestion controller, but assume that it can be queried by the
   packet scheduler to verify whether a packet of length l would be
   blocked or not by the congestion control scheme.  A window-based
   congestion controller such as [RFC6356] can block a packet from being
   transmitted for some time when its congestion window is full.  The
   same applies to a rate-based congestion controller although the
   latter could indicate when the packet could be accepted while the
   former cannot.

   We assume that the multipath transport protocol maintains some state
   at the connection level and at the path level.  On both level, the
   multipath transport protocol will maintain send and receive windows,

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   and a Maximum Segment Size that is negotiated at connection

   It may also contain some information that is specific to the
   application (e.g. total amount of data sent or received) and
   information about non-active flows.  At the path level, we expect
   that the multipath transport protocol will maintain an accurate
   estimation of the round-trip-time over that path, possibly a send/
   receive window, per path MTU information, the state of the congestion
   controller, and optionally information that is specific to the
   application or the packet scheduler (e.g. priority for one path over
   another one).

3.  Packet scheduling challenges

   Packet scheduling tries to balance different quality of service goals
   with different constraints of the paths.  The balance depends on
   which of the goals or constraints is the primary factor for the
   experience the application is aiming for.  In the following we list
   these goals and constraints and conclude by how they can influence
   each other.

   Each path can be subject to a different cost when transmitting data.
   For example, a path can introduce a per-byte monetary cost for the
   transmission (e.g., metered cellular link).  Another cost can be the
   power consumption when transmitting or receiving data.  These costs
   are imposing restrictions on when a path can be used compared to the
   lower-cost path.

   A goal for many applications is to reduce the latency of their
   transaction.  With multiple paths, each path can have a significantly
   different latency compared to the other paths.  It is thus crucial to
   schedule the traffic on a path such that the latency requirements of
   the application are satisfied.

   Achieving high throughput is another goal of many applications.
   Streaming applications often require a minimum bit rate to sustain
   playback.  The scheduler should try to achieve this bit rate to allow
   for a flawless streaming experience.  Beyond that, adaptive streaming
   requires also a more stable throughput experience to ensure that the
   bit rate of the video stream is consistent.  When sending traffic
   over multiple paths the bit rate can experience more variance and
   thus the scheduler for such a streaming application needs to take
   precautions to ensure a smooth experience.

   Finally, transport protocols impose a receive-window that signals to
   the sender how much data the application is willing to receive.  When
   the paths have a large latency difference, a multipath transport can

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   quickly become receive-window limited.  This limitation comes from
   the fact that a packet might have been sent on a high-latency path.
   If the transport imposes in-order delivery of the data, the receiver
   needs to wait to receive this packet over the high-latency path
   before providing it to the application.  The sender will thus become
   receive-window limited and may end up under-utilizing the low-latency
   path.  This can become a major challenge when trying to achieve high

   All of these quality of service goals and constraints need to be
   balanced against each other.  A scheduler might decide to trade
   latency for higher throughput.  Or reduce the throughput with the
   goal of reducing the cost.

4.  Packet schedulers

   The packet scheduler is executed every time a packet needs to be
   transmitted by the multipath transport protocol.  A packet scheduler
   can consider three different types of packets:

   o  packets that carry new user data

   o  packets that carry previously transmitted user data

   o  packets that only carry control information (e.g.,
      acknowledgements, address advertisements)

   In Multipath TCP, the packet scheduler is only used for packets that
   carry data.  Multipath TCP will typically return acknowledgements on
   the same path as the one over which data packets were received.  For
   Multipath QUIC, the situation is different since Multipath QUIC can
   acknowledge over one path data that was previously received over
   another path.  In Multipath TCP, this is only partially possible.
   The subflow level acknowledgements must be sent on the subflow where
   the data was received while the data-level acknowledgements can be
   sent over any subflow.

   This document uses the Python language to represent multipath
   schedulers.  A multipath scheduler is represented as a Python
   function.  This function takes the length of the next packet to
   schedule as argument and returns the path on which it will be send.
   A path is represented as a Python class with the following

   o  srtt: The smoothed RTT of the path [RFC6298].

   o  cc_state: The state of the congestion controller, i.e. either
      slow_start, congestion_avoidance or recovery.

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   o  blocked(l): A function indicating whether a packet of length l
      would be rejected by the congestion controller.

   The schedulers presented can be executed in a simulator
   [MultipathSim] implementing the abstract multipath protocol presented
   in Section 2.  It can be used to simulate a file transfer between a
   client and a server over multiple paths.

4.1.  Round-Robin

   We use the Round-Robin scheduler as a simple example to illustrate
   how a packet scheduler can be specified, but we do not recommend its
   usage.  Experiments with Multipath TCP [ACMCS14] indicate that it
   does not provide good performance.

   This packet scheduler uses one additional state at the connection
   level: last_path.  This stores the identifier of the last path that
   was used to send a packet.  The scheduler is defined by the code
   shown in Figure 1.

   class RoundRobin(Scheduler):
       """ Chooses an available path in a round-robin manner. """
       last_path: Optional[Path] = None

       def schedule(self, packet_len: int):
           if self.last_path in self.paths:
               next_idx = self.paths.index(self.last_path) + 1
               next_idx = 0
           sorted_paths = self.paths[next_idx:] + self.paths[:next_idx]
           for p in sorted_paths:
               if not p.blocked(packet_len):
                   self.last_path = p
                   return p

                 Figure 1: A simple Round Robin scheduler

   This scheduler does not distinguish between the different types of
   packets.  It iterates over the available paths and sends over the
   ones whose congestion window is open.

4.2.  Weighted Round-Robin

   The Weighted Round-Robin scheduler is a more advanced version of the
   Round-Robin scheduler.  It allows specifying a particular
   distribution of paths.  This can be used to non-uniformly spread
   packets over paths.

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   This packet scheduler adds two states:

   o  distribution: A list containing the distribution of paths to
      consider.  Paths to which more importance is given will be present
      several times in the list.  The ordering of the list allows to
      choose whether interleaved or burst sending is preferred.

   o  last_idx: It stores the index in the distribution of the last path
      used to send a packet.

class WeightedRoundRobin(Scheduler):
    """ Chooses an available path in a following a fixed distribution. """
    distribution: List[Path]
    last_idx: int = -1

    def schedule(self, packet_len: int) -> Optional[Path]:
        next_idx = (self.last_idx + 1) % len(self.distribution)
        sorted_paths = self.distribution[next_idx:] + self.distribution[:next_idx]
        for i, p in enumerate(sorted_paths):
            if not p.blocked(packet_len):
                self.last_idx = (self.last.idx + i) % len(self.distribution)
                return p

                Figure 2: A Weighted Round Robin scheduler

   This scheduler does not distinguish between the different types of
   packets.  It iterates over the available paths following the given
   distribution and sends over the ones whose congestion window is open.
   A variant of this algorithm could maintain a deficit per path and
   consider the length of packets when distributing them.

4.3.  Strict Priority

   The Strict Priority scheduler's aim is to select paths based on a
   priority list.  Some paths might go through networks that are more
   expensive to use than others.  Then the idea is to select the path
   with the highest priority if it is available before looking at others
   by priority.  This scheduler is described by the code shown in
   Figure 3.

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class StrictPriority(Scheduler):
    """ Chooses the first available path in a priority list of paths. """

    def schedule(self, packet_len: int):
        for p in sorted(self.paths, key=lambda p: p.priority, reverse=True):
            if not p.blocked(packet_len):
                return p

               Figure 3: A simple Strict Priority scheduler

   This scheduler can face performance issues if, compared to others,
   paths with high priority accept a lot of data but delivered packets
   with a high latency.  When the path is experiencing bufferbloat, the
   receiver has to store packets for a long time in its buffers to
   ensure an in-order delivery.  It is then recommended to cover these
   cases in the scheduler implementation with the help of the congestion
   control algorithm.

4.4.  Round-Trip-Time Threshold

   The Round-Trip-Time Threshold scheduler selects the first available
   path with a smoothed round-trip-time below a certain threshold.  The
   goal is to keep the RTT of the multipath connection to a small value
   and avoid having the whole connection impacted by "bad" paths.  A
   prototype is shown in Figure 4.

class RTTThreshold(Scheduler):
    """ Chooses the first available path below a certain RTT threshold. """
    threshold: float

    def schedule(self, packet_len: int):
        for p in self.paths:
            if p.srtt < self.threshold and not p.blocked(packet_len):
                return p

          Figure 4: A simple Round-Trip-Time Threshold scheduler

   This kind of protection can of course be added to other existing

4.5.  Lowest Round-Trip-Time First

   The Lowest round-trip-time first scheduler's goal is to minimize
   latency for short flows while at the same time achieving high
   throughput for long flows [ACMCS14].  To handle the latency
   differences across the paths when being limited by the receive-

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   window, this scheduler deploys a fast reinjection mechanism to
   quickly recover from the head-of-line blocking.

   At each round, the scheduler iterates over the list of paths that are
   eligible for transmission.  To decide whether or not a path is
   eligible, a few conditions need to be satisfied:

   o  The congestion window needs to provide enough space for the

   o  The path is not in fast-recovery or experiencing retransmission

   Among all the eligible paths, the scheduler will choose the path with
   the lowest RTT and transmit the segment with the new data on that
   path.  Figure 5 illustrates a simple lowest RTT scheduler which does
   not include fast reinjections.

   class LowestRTTFirst(Scheduler):
       """ Chooses the first available path with the lowest RTT. """

       def schedule(self, packet_len: int):
           # Sort paths by ascending SRTT
           for p in sorted(self.paths, key=lambda path: path.srtt):
               if not p.blocked(packet_len) \
                  and p.cc_state != 'recovery':
                   return p

               Figure 5: A simple Lowest RTT First scheduler

   To handle head-of-line blocking situations when the paths have a
   large delay difference the scheduler uses a strategy of opportunistic
   retransmission and path penalization as described in [NSDI12].

   Opportunistic retransmission kicks in whenever a path is eligible for
   transmission but the receive-window advertised by the receiver
   prevents the sender from transmitting new data.  In that case the
   sender can transmit previously transmitted data over the eligible
   path.  To overcome the head-of-line blocking the sender will thus
   transmit the packet at the head of the transmission queue over this
   faster path (if it hasn't been transmitted on this particular path
   yet).  This packet has thus a chance to quickly reach the receiver
   and fill the hole created by the head-of-line blocking.

   Whenever the previously mentioned mechanism kicks in, it is and
   indication that the path's round-trip-time is too high to allow the
   path with the lower RTT to fully use its capacity.  We thus should

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   reduce the transmission rate on this path.  This mechanism is called
   penalization and is achieved by dividing the congestion window by 2.

   [comment:] ## Out-of-order transmission for in-order arrival

4.6.  Combination of schedulers type: Priority and Lowest round-trip-
      time first

   Combining some types of schedulers can be a way to address some use
   cases.  For example, a scheduler using the priority and the round-
   trip-time attributes can be used to give more priorities to some
   links having a lower cost (e.g. fixed vs. mobile accesses) while
   still being able to benefit from the advantages of the "Lowest RTT
   First" scheduler described in Section 4.5.  A prototype of this
   "hybrid" scheduler is shown in Figure 6.

class PriorityAndLowestRTTFirst(Scheduler):
    """ Chooses the first available path with the highest priority and then the lowest RTT. """

    def schedule(self, packet_len: int):
        # Sort paths by ascending priority (2nd sort) and then ascending SRTT (1st sort)
        paths = sorted(self.paths, key=lambda path: path.srtt)
        paths = sorted(paths, key=lambda path: path.priority, reverse=True)
        for p in paths:
            if not p.blocked(packet_len) and p.cc.state is not CCState.recovery:
                return p

        Figure 6: A scheduler combining priority and RTT attributes

   Combining some properties can have new undesired effects.  In the
   case presented here, paths with a higher priority but also a higher
   RTT can affect performances compared to a setup having a scheduler
   not looking at the priority but only the round-trip-time.  If paths
   with a higher priority are used first whatever the network conditions
   are on these paths, it is normal to sacrifice the total bandwidth
   capacity but fully use the capacity of these links with a higher
   priority.  If the paths with a lower priority are seen as extra
   capacity that can be used only when the other links are congested, it
   is fine if they are not fully used when the sender is limited by the
   global sending window of the multipath connection.

   For this kind of scheduler, it could be interesting to also associate
   the benefits associated to a "Round-Trip-Time Threshold" scheduler
   described in Section 4.4.  This scheduler prevents being too impacted
   by links having a higher priority but a very high RTT while other
   paths, with a lower priority and a lower RTT, can be used.  It is a
   matter of qualifying what is important: maximizing the use of paths

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   over reducing the latency and probably the total bandwidth as well if
   the sender and/or the receiver are limited by congestion windows.

   It is also important to note that the penalization mechanism
   described in the "Lowest round-trip-time first" scheduler in
   Section 4.5 also needs to take into account the priority.  If the
   goal is to maximize the use of some links over others, links with a
   higher priority cannot be penalized over the ones with a lower
   priority.  The consequence of this would be that links with higher
   priority are under used due to the penalization.

   ASCII figure

                         Figure 7: A simple figure

5.  Informative References

   [ACMCS14]  Paasch, C., Ferlin, S., Alay, O., and O. Bonaventure,
              "Experimental Evaluation of Multipath TCP Schedulers",
              Proceedings of the 2014 ACM SIGCOMM workshop on Capacity
              sharing workshop , n.d..

              Hesmans, B., Detal, G., Barre, S., Bauduin, R., and O.
              Bonaventure, "SMAPP &#58; Towards Smart Multipath TCP-
              enabled APPlications", CoNEXT '15: Proceedings of the 11th
              ACM Conference on Emerging Networking Experiments and
              Technologies , n.d..

              Amend, M., Bogenfeld, E., Brunstrom, A., Kassler, A., and
              V. Rakocevic, "DCCP Extensions for Multipath Operation
              with Multiple Addresses", draft-amend-tsvwg-multipath-
              dccp-03 (work in progress), November 2019.

              Coninck, Q. and O. Bonaventure, "Multipath Extensions for
              QUIC (MP-QUIC)", draft-deconinck-quic-multipath-05 (work
              in progress), August 2020.

              Amer, P., Becke, M., Dreibholz, T., Ekiz, N., Iyengar, J.,
              Natarajan, P., Stewart, R., and M. Tuexen, "Load Sharing
              for the Stream Control Transmission Protocol (SCTP)",
              draft-tuexen-tsvwg-sctp-multipath-20 (work in progress),
              July 2020.

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              Piraux, M., "Multipath simulator for the IETF draft
              Multipath schedulers", n.d.,

   [NSDI12]   Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
              Duchene, F., Bonaventure, O., and M. Handley, "How Hard
              Can It Be? Designing and Implementing a Deployable
              Multipath TCP", 9th USENIX Symposium on Networked Systems
              Design and Implementation (NSDI 12) , n.d..

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols",
              RFC 6356, DOI 10.17487/RFC6356, October 2011,

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

Appendix A.  Change log

A.1.  Since draft-bonaventure-iccrg-schedulers-00

   o  Renamed Delay Threshold to RTT Threshold

   o  Added the Priority And Lowest RTT First scheduler

Authors' Addresses

   Olivier Bonaventure

   Email: Olivier.Bonaventure@uclouvain.be

   Maxime Piraux

   Email: Maxime.Piraux@uclouvain.be

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   Quentin De Coninck

   Email: quentin.deconinck@uclouvain.be

   Matthieu Baerts

   Email: Matthieu.Baerts@tessares.net

   Christoph Paasch

   Email: cpaasch@apple.com

   Markus Amend
   Deutsche Telekom

   Email: markus.amend@telekom.de

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