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TSVWG                                                              Y. Li
Internet-Draft                                                   X. Zhou
Intended status: Informational                                    Huawei
Expires: January 8, 2020                                    M. Boucadair
                                                                  Orange
                                                                 J. Wang
                                                           China Telecom
                                                           July 07, 2019


 LOOPS (Localized Optimizations on Path Segments) Problem Statement and
       Opportunities for Network-Assisted Performance Enhancement
             draft-li-tsvwg-loops-problem-opportunities-03

Abstract

   In various network deployments, end to end forwarding paths are
   partitioned into multiple segments.  For example, in some cloud-based
   WAN communications, stitching multiple overlay tunnels are used for
   traffic policy enforcement matters such as to optimize traffic
   distribution or to select paths exposing a lower latency.  Likewise,
   in satellite communications, the communication path is decomposed
   into two terrestrial segments and a satellite segment.  Such long-
   haul paths are naturally composed of multiple network segments with
   various encapsulation schemes.  Packet loss may show different
   characteristics on different segments.

   Traditional transport protocols (e.g., TCP) respond to packet loss
   slowly especially in long-haul networks: they either wait for some
   signal from the receiver to indicate a loss and then retransmit from
   the sender or rely on sender's timeout which is often quite long.
   Non-congestive loss may make the TCP sender over-reduce the sending
   rate unnecessarily.  With the increase of end-to-end transport
   encryption (e.g., QUIC), traditional PEP (performance enhancing
   proxy) techniques such as TCP splitting are no longer applicable.

   LOOPS (Local Optimizations on Path Segments) is a network-assisted
   performance enhancement over path segment and it aims to provide
   local in-network recovery to achieve better data delivery by making
   packet loss recovery faster and by avoiding the senders over-reducing
   their sending rate.  In an overlay network scenario, LOOPS can be
   performed over a variety of the existing, or purposely created,
   tunnel-based path segments.








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Status of This Memo

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   This Internet-Draft will expire on January 8, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  The Problem . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Sketching a Work Direction: Rationale & Goals . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Cloud-Internet Overlay Network  . . . . . . . . . . . . . . .   7
     3.1.  Tail Loss or Loss in Short Flows  . . . . . . . . . . . .   9
     3.2.  Packet Loss in Real Time Media Streams  . . . . . . . . .   9
     3.3.  Packet Loss and Congestion Control in Bulk Data Transfer   10
     3.4.  Multipathing  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Satellite Communication . . . . . . . . . . . . . . . . . . .  11
   5.  Branch Office WAN Connection  . . . . . . . . . . . . . . . .  13
   6.  Features and Impacts to be Considered for LOOPS . . . . . . .  14
     6.1.  Local Recovery and End-to-end Retransmission  . . . . . .  15
       6.1.1.  OE to OE Measurement, Recovery, and Multipathing  . .  17



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     6.2.  Congestion Control Interaction  . . . . . . . . . . . . .  18
     6.3.  Overlay Protocol Extensions . . . . . . . . . . . . . . .  19
     6.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   10. Informative References  . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

1.1.  The Problem

   Tunnels are widely deployed within many networks to achieve various
   engineering goals, including long-haul WAN interconnection or
   enterprise wireless access networks.  A connection between two
   endpoints can be decomposed into many connection legs.  As such, the
   corresponding forwarding path can be partitioned into multiple path
   segments that some of them are using network overlays by means of
   tunnels.  This design serves a number of purposes such as steering
   the traffic, optimize egress/ingress link utilization, optimize
   traffic performance metrics (such as delay, delay variation, or
   loss), optimize resource utilization by invoking resource bonding,
   provide high-availability, etc.

   A reliable transport layer normally employs some end-to-end
   retransmission mechanisms which also address congestion control
   [RFC0793] [RFC5681].  The sender either waits for the receiver to
   send some signals on a packet loss or sets some form of timeout for
   retransmission.  For unreliable transport protocols such as RTP
   [RFC3550], optional and limited usage of end-to-end retransmission is
   employed to recover from packet loss [RFC4585] [RFC4588].

   End-to-end retransmission to recover lost packets is slow especially
   when the network is long-haul.  When a path is partitioned into
   multiple path segments that are realized typically as overlay
   tunnels, LOOPS (Local Optimizations on Path Segments) aims to provide
   local segment based in-network recovery to achieve better data
   delivery by making packet loss recovery faster and by avoiding the
   senders over-reducing their sending rate.  In an overlay network
   scenario, LOOPS can be performed over the existing, or purposely
   created, overlay tunnel based path segments.  Figure 1 show a basic
   usage scenario of LOOPS.

   Some link types (satellite, microwave, drone-based networking, etc.)
   may exhibit unusually high loss rate in special conditions (e.g.,
   fades due to heavy rain).  The traditional TCP sender interprets loss
   as congestion and over-reduces the sending rate, degrading the



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   throughput.  LOOPS is also applicable to such scenarios to improve
   the throughput.

   Also, multiple paths may be available in the network that may be used
   for better performance.  These paths are not visible to endpoints.
   Means to make use of these paths while ensuring the overall
   performance is enhanced would contribute to customer satisfaction.
   Blindly implementing link aggregation may lead to undesired effects
   (e.g., underperform compared to single path).

1.2.  Sketching a Work Direction: Rationale & Goals

   This document sketches a proposal that is meant to experimentally
   investigate to what extent a network-assisted approach can contribute
   to increase the overall perceived quality of experience in specific
   situations (e.g., Sections 3.5 and 3.6 of [RFC8517]) without
   requiring access to internal transport primitives.  The rationale
   beneath this approach is that some information (loss detection,
   better visibility on available paths and their characteristics, etc.)
   can be used to trigger local actions while avoiding as much as
   possible undesired side effects (e.g., expose a behavior that would
   be interpreted by an endpoint as an anomaly (corrupt data) and which
   would lead to exacerbate end-to-end recovery.  Such local actions
   would have a faster effect (e.g., faster recovery, used multiple
   paths simultaneously).

   To that aim, the work is structured into two (2) phased stages:

   o  Stage 1: Network-assisted optimization.  This one assumes that
      optimizations (e.g., support latency-sensitive applications) can
      be implemented at the network without requiring defining new
      interaction with the endpoint.  Existing tools such as ECN will be
      used.  Some of these optimizations may be valuable in deployments
      where communications are established over paths that are not
      exposing the same performance characteristics.

   o  Stage 2: Collaborative networking optimization.  This one requires
      more interaction between the network and an endpoint to implement
      coordinated and more surgical network-assisted optimizations based
      on information/instructions shared by an endpoint or sharing
      locally-visible information with endpoint for better and faster
      recovery.

   The document focuses on the first stage.  Effort related to the
   second stage is out of scope of the initial planned work.
   Nevertheless, future work will be planned once progress is
   (hopefully) made on the first stage.




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   The proposed mechanism is not meant to be applied to all traffic, but
   only to a subset which is eligible to the network-assisted
   optimization service.

   Which traffic is eligible is deployment-specific and policy-based.
   For example, techniques for dynamic information of optimization
   function (e.g., SFC) may be leveraged to unambiguously identify the
   aggregate of traffic that is eligible to the service.  Such
   identification may be triggered by subscription actions made by
   customers or be provided by a network provider (e.g., specific-
   applications, during specific events such as during severe DDoS
   attack or flash crowds events).

   Likewise, whether the optimization function is permanently
   instantiated or on-demand is deployment-specific.

   This document does not intend to provide a comprehensive list of
   target deployment cases.  Sample scenarios are described to
   illustrate some LOOPS potentials.  Similar issues and optimizations
   may be helpful in other deployments such as enhancing the reliability
   of data transfer when a fleet of drones are used for specific
   missions (e.g., site inspection, live streaming, and emergency
   service).  Captured data should be reliably transmitted via paths
   involving radio connections.

   It is not required that all segments are LOOPS-aware to benefit from
   LOOPS advantages.

   Section 3 presents some of the issues and opportunities found in
   Cloud-Internet overlay networks that require higher performance and
   more reliable packet transmission over best effort networks.
   Section 4 discusses applications of LOOPS in satellite communication.
   Section 6 describes the corresponding solution features and their
   impact on existing network technologies.

















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                                                      ON=overlay node
                                                      UN=underlay node

   +---------+                                               +---------+
   |   App   | <---------------- end-to-end ---------------> |   App   |
   +---------+                                               +---------+
   |Transport| <---------------- end-to-end ---------------> |Transport|
   +---------+                                               +---------+
   |         |                                               |         |
   |         |        +--+  path  +--+  path segment2  +--+  |         |
   |         |        |  |<-seg1->|  |<--------------> |  |  |         |
   | Network |  +--+  |ON|  +--+  |ON|  +--+   +----+  |ON|  | Network |
   |         |--|UN|--|  |--|UN|--|  |--|UN|---| UN |--|  |--|         |
   +---------+  +--+  +--+  +--+  +--+  +--+   +----+  +--+  +---------+
     End Host                                                  End Host
                       <--------------------------------->
                        LOOPS domain: path segment enables
                        local optimizations for better experience

             Figure 1: LOOPS in Overlay Network Usage Scenario

2.  Terminology

   This document makes use of the following terms:

   LOOPS:  Local Optimizations on Path Segments.  LOOPS includes to the
      local in-network (i.e., non end-to-end) recovery functions and
      other supporting features such as local measurement, loss
      detection, and congestion feedback.

   LOOPS Node:  A node supporting LOOPS functions.

   Overlay Node (ON):  A node having overlay functions (e.g., overlay
      protocol encapsulation/decapsulation, header modification, TLV
      inspection) and LOOPS functions in LOOPS overlay network usage
      scenario.

   Overlay Tunnel:  A tunnel with designated ingress and egress nodes
      using some network overlay protocol as encapsulation, optionally
      with a specific traffic type.

   Overlay Edge (OE):  Edge node of an overlay tunnel.  It can behave as
      ingress or egress as a function of the traffic direction.

   Path segment:  A LOOPS enabled tunnel-based network subpath.  It is
      used interchangeably with overlay segment in this document when
      the context wants to emphasize on its overlay encapsulated nature.
      It is also called segment for simplicity in this document.



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   Overlay segment:  Refers to path segment.

   Underlay Node (UN):  A node not participating in the overlay network.

3.  Cloud-Internet Overlay Network

   CSPs (Cloud Service Providers) are connecting their data centers
   using the Internet or via self-constructed networks/links.  This
   expands the traditional Internet's infrastructure and, together with
   the original ISP's infrastructure, forms the Internet underlay.

   Automation techniques and NFV (Network Function Virtualization)
   further ambitions to make it easier to dynamically provision a new
   virtual node/function as a workload in a cloud for CPU/storage
   intensive functions.  With the aid of various mechanisms such as
   kernel bypassing and Virtual IO, forwarding based on virtual nodes is
   becoming more and more effective.  The interconnection among the
   purposely positioned virtual nodes and/or the existing nodes with
   virtualization functions potentially form an overlay infrastructure.
   It is called the Cloud-Internet Overlay Network (CION) in this
   document for short.

   This architecture scenario makes use of overlay technologies to
   direct the traffic going through the specific overlay path regardless
   of the underlying physical topology, in order to achieve better
   service delivery.  It purposely creates or selects overlay nodes (ON)
   from providers.  By continuously measuring the delay of path segments
   and use them as metrics for path selection, when the number of
   overlay nodes is sufficiently large, there is a high chance that a
   better path could be found [DOI_10.1109_ICDCS.2016.49]
   [DOI_10.1145_3038912.3052560].  [DOI_10.1145_3038912.3052560] further
   shows all cloud providers experience random loss episodes and random
   loss accounts for more than 35% of total loss.

   Some of the considerations that are discussed below may also apply
   for interconnecting DCs owned by a network provider.

   Figure 2 shows an example of an overlay path over large geographic
   distances.  Three path segments, i.e., ON1-ON2, ON2-ON3, ON3-ON4 are
   shown.  ON is usually a virtual node, though it does not have to be.
   Each segment transmits packets using some form of network overlay
   protocol encapsulation.  ON has the computing and memory resources
   that can be used for some functions like packet loss detection,
   network measurement and feedback, and packet recovery.  ONs are
   managed by a single administrator though they can be workloads
   created from different CSPs.





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                  _____________
                 /  domain 1   \
                /               \
            ___/                 -------------\
           /                                   \
    PoP1 ->--ON1                                \
          |   |                            ON4------>-- PoP2
          |   |   ON2                     ___|__/
           \__|_ |->|         _____      /   |
              | \|__|__      /     \    /    |
              |  |  |  \____/       \__/     |
             \|/ |  |        _____           |
              |  |  |    ___/     \          |
              |  | \|/  /          \_____    |
              |  |  |  /         domain 2 \ /|\
              |  |  | |       ON3         |  |
              |  |  |  \      |->|        |  |
              |  |  |   \_____|__|_______/   |
              | /|\ |         | \|/          |
              |  |  |         |  |           |
              |  |  |        /|\ |           |
       +--------------------------------------------------+
       |      |  |  |         |  |           |   Internet |
       |      o--o  o---o->---o  o---o->--o--o   underlay |
       +--------------------------------------------------+

              Figure 2: Cloud-Internet Overlay Network (CION)

   We tested based on 37 overlay nodes from multiple cloud providers
   globally.  Each pair of the overlay nodes are used as sender and
   receiver.  When the traffic is not intentionally directed to go
   through any intermediate virtual nodes, we call the path followed by
   the traffic in the test as the default path.  When any of the virtual
   nodes is intentionally used as an intermediate node to forward the
   traffic, the path that the traffic takes is called an overlay path.
   The preliminary experiments showed that the delay of an overlay path
   is shorter than the one of the default path in 69% of cases at 99%
   percentile and improvement is 17.5% at 99% percentile when we probe
   Ping packets every second for a week.  More experimental information
   can be found in [OCN].

   Lower delay does not necessarily mean higher throughput.  Different
   path segments may have different packet loss rates.  Loss rate is
   another major factor impacting the overall TCP throughput.  From some
   customer requirements, the target loss rate is set in the test to be
   less than 1% at 99% percentile and 99.9% percentile, respectively.
   The loss was measured between any two overlay nodes, i.e., any
   potential path segment.  Two thousand Ping packets were sent every 20



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   seconds between two overlay nodes for 55 hours.  This preliminary
   experiment showed that the packet loss rate satisfaction are 44.27%
   and 29.51% at the 99% and 99.9% percentiles, respectively.

   Hence packet loss in an overlay segment is a key issue to be solved
   in such architecture.  In long-haul networks, the end-to-end
   retransmission of lost packet can result in an extra round trip time
   (RTT).  Such extra time is not acceptable in some latency-sensitive
   applications.  As CION naturally consists of multiple overlay
   segments, LOOPS leverages this to perform local optimizations on a
   single hop between two overlay nodes.  ("Local" here is a concept
   relative to end-to-end, it does not mean such optimization is limited
   to LAN networks.)

   The following subsections present different scenarios using multiple
   segment-based overlay paths with a common need of local in-network
   loss recovery in best effort networks.

3.1.  Tail Loss or Loss in Short Flows

   When the lost segments are at the end of a transaction, TCP's fast
   retransmit algorithm does not work as there are no ACKs to trigger
   it.  When a sender does not receive an ACK for a given segment within
   a certain amount of time called retransmission timeout (RTO), it re-
   sends the segment [RFC6298].  RTO can be as long as several seconds.
   Hence the recovery of lost segments triggered by RTO is lengthy.
   [I-D.dukkipati-tcpm-tcp-loss-probe] indicates that large RTOs make a
   significant contribution to the long tail on the latency statistics
   of short flows such as loading web pages.

   The short flow often completes in one or two RTTs.  Even when the
   loss is not a tail loss, it can possibly add another RTT because of
   end-to-end retransmission (not enough packets are in flight to
   trigger fast retransmit).  In long-haul networks, it can result in
   extra time of tens or even hundreds of milliseconds.

   An overlay segment transmits the aggregated flows from ON to ON.  As
   short-lived flows are aggregated, the probability of tail loss over
   this specific overlay segment decreases compared to an individual
   flow.  The overlay segment is much shorter than the end-to-end path
   in a Cloud- Internet overlay network, hence loss recovery over an
   overlay segment is faster.

3.2.  Packet Loss in Real Time Media Streams

   The Real-time transport protocol (RTP) is widely used in interactive
   audio and video.  Packet loss degrades the quality of the received
   media.  When the latency tolerance of the application is sufficiently



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   large, the RTP sender may use RTCP NACK feedback from the receiver
   [RFC4585] to trigger the retransmission of the lost packets before
   the playout time is reached at the receiver.

   In a Cloud-Internet overlay network, the end-to-end path can be
   hundreds of milliseconds.  End-to-end feedback based retransmission
   may be not be very useful when applications can not tolerate one more
   RTT of this length.  Loss recovery over an overlay segment can then
   be used for the scenarios where RTCP NACK triggered retransmission is
   not appropriate.

3.3.  Packet Loss and Congestion Control in Bulk Data Transfer

   TCP congestion control algorithms such as Reno and CUBIC basically
   interpret packet loss as congestion experienced somewhere in the
   path.  When a loss is detected, the congestion window will be
   decreased at the sender to make the sending slower.  It has been
   observed that packet loss is not an accurate way to detect congestion
   in the current Internet [I-D.cardwell-iccrg-bbr-congestion-control].
   In long-haul links, when the loss is caused by non-persistent burst
   which is extremely short and pretty random, the sender's reaction of
   reducing sending rate is not able to respond in time to the
   instantaneous path situation or to mitigate such bursts.  On the
   contrary, reducing window size at the sender unnecessarily or too
   aggressively harms the throughput for application's long lasting
   traffic like bulk data transfer.

   The overlay nodes are distributed over the path with computing
   capability, they are in a better position than the end hosts to
   quickly deduce the underlying links' instantaneous situation from
   measuring the delay, loss or other metrics over the segment.  Shorter
   round trip time over a path segment will benefit more accurate and
   immediate measurements for the maximum recent bandwidth available,
   the minimum recent latency, or trend of change.  ONs can further
   decide if the sending rate reduction at the sender is necessary when
   a loss happened.  Section 6.2 talks more details on this.

3.4.  Multipathing

   As an overlay path may suffer from an impairment of the underlying
   network, two or more overlay paths between the same set of ingress
   and egress overlay nodes can be combined for reliability purpose.
   During a transient time when a network impairment is detected,
   sending replicating traffic over two paths can improve reliability.

   When two or more disjoint overlay paths are available as shown in
   Figure 3 from ON1 to ON2, different sets of traffic may use different
   overlay paths.  For instance, one path is for low latency and the



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   other is for higher bandwidth, or they can be simply used as load
   balancing for better bandwidth utilization.

   Two disjoint paths can be, for example, found by measurement to
   figure out the segments with very low "mathematical correlation" in
   latency change.  When the number of overlay nodes is large, it is
   easy to find disjoint or partially disjoint segments.  This
   information may be available if the ONs are managed by the network
   provider managing the underlying forwarding paths.

   Different overlay paths may have varying characteristics, obviously.
   The overlay tunnel should allow the overlay path to handle the packet
   loss depending on its own path measurements.

                       ON-A
             +----------o------------------+
             |                             |
             |                             |
      A -----o ON1                      ON2o----- B
             |                             |
             +-----------------------o-----+
                                   ON-B

                Figure 3: Example of Multiple Overlay Paths

   In reference to Figure 3, both A and B are not aware of the existence
   of these multiple paths.  A network-assistance would be valuable for
   the sake of better resilience and performance.  Note that in a
   collaborative context (a.k.a., stage 2 mentioned in Section 1.2)
   LOOPS may target means to advertise the available path
   characteristics to an endpoint A/B, to allow an endpoint A/B to
   control the traffic distribution policy to be enforced by ON1/ON2, or
   to let endpoint A/B notify ON1/ON2 with their multipathing
   preference.

4.  Satellite Communication

   Traditionally, satellite communications deploy PEP (performance
   enhancing proxy [RFC3135]) nodes around the satellite link to enhance
   end-to-end performance.  TCP splitting is a common approach employed
   by such PEPs, where the TCP connection is split into three: the
   segment before the satellite hop, the satellite section (uplink,
   downlink), and the segment behind the satellite hop.  This requires
   heavy interactions with the end-to-end transport protocols, usually
   without the explicit consent of the end hosts.  Unfortunately, this
   is indistinguishable from a man-in-the-middle attack on TCP.  With
   end-to-end encryption moving under the transport (QUIC), this
   approach is no longer useful.



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   Geosynchronous Earth Orbit (GEO) satellites have a one-way delay (up
   to the satellite and back) on the order of 250 milliseconds.  This
   does not include queueing, coding and other delays in the satellite
   ground equipment.  The Round Trip Time for a TCP or QUIC connection
   going over a satellite hop in both directions, in the best case, will
   be on the order of 600 milliseconds.  And, it may be considerably
   longer.  RTTs on this order of magnitude have significant performance
   implications.

   Packet loss recovery is an area where splitting the TCP connection
   into different parts helps.  Packets lost on the terrestrial links
   can be recovered at terrestrial latencies.  Packet loss on the
   satellite link can be recovered more quickly by an optimized
   satellite protocol between the PEPs and/or link layer FEC than they
   could be end to end.  Again, encryption makes TCP splitting no longer
   applicable.  Enhanced error recovery at the satellite link layer
   helps for the loss on the satellite link but doesn't help for the
   terrestrial links.  Even when the terrestrial segments are short, any
   loss must be recovered across the satellite link delay.  And, there
   are cases when a satellite ground station connects to the general
   Internet with a potentially larger terrestrial segment (e.g., to a
   correspondent host in another country).  Faster recovery over such
   long terrestrial segments is desirable.

   Another aspect of recovery is that terrestrial loss is highly likely
   to be congestion related but satellite loss is more likely to be
   transmission errors due to link conditions.  A transport endpoint
   slowing down because of mis-interpreting these errors as congestion
   losses unnecessarily reduces performance.  But, at the end points,
   the difference between the two is not easily distinguished.  To
   elaborate more on the loss recovery for satellite communications,
   while the error rate on the satellite paths is generally very low
   most of the time, it might get higher during special link conditions
   (e.g.  fades due to heavy rain).  The satellite hop itself does know
   which losses are due to link conditions as opposed to congestion, but
   it has no mechanism to signal this difference to the end hosts.

   We will need the protocol under QUIC to try to minimize non-
   congestion packet drop.  Specific link layers may have techniques
   such as satellite FEC to recover.  Where the capabilities of that may
   be exceeded (e.g., rain fade), we can look at LOOPS-like approaches.

   There are two high level classes of solutions for making encrypted
   transport traffic like QUIC work well over satellite:

   o  Hooks in the transport protocol which can adapt to large BDPs
      where both the bandwidth and the latency are large.  This would
      require end to end enhancement.



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   o  Capabilities (such as LOOPS) under the transport protocol to
      improve performance over specific segments of the path.  In
      particular, separating the terrestrial from the satellite losses.
      Fixing the terrestrial loss quickly and keeping throughput high
      over satellite segment by not causing the end-hosts to over-reduce
      their sending window in case of non-congestion loss.

   This document focuses on the latter.

5.  Branch Office WAN Connection

   Enterprises usually require network connections between the branch
   offices or between branch offices and cloud data center over
   geographic distances.  With the increasing deployment of vCPE
   (virtual CPE), some services usually hosted on the CPE are moved to
   the provider network from the customer site.  Such vCPE approach
   enables some value added service to be provided such as WAN
   optimization and traffic steering.

   Figure 4 shows an example of two branch offices WAN connection via
   Internet.  Figure 5 shows a branch office access to public cloud via
   a selected PoP (point of presence). vCPE connects to that PoP which
   can be hundreds of kilometers away via Internet.  In both cases, the
   path segments over Internet is subject to loss.  Similar problems
   presented in subsections of Section 3 should be solved.  The GW1 may
   be reachable via multiple paths.

   Requirements to steer traffic through different sub-paths for latency
   optimization, resource optimization, balancing, or other purposes are
   increasing.  For example, directing the traffic from vCPE to a
   lightly loaded PoP rather than to the closest one.  Mere best effort
   transport is not sufficient.  New technologies like SFC (Service
   Function Chaining), SRv6 (segment routing over IPv6), and NFV/SDN
   used together with vCPE to enable the potentials to embed more
   complicated loss recovery functions at intermediate nodes in end-to-
   end path.

     +------+       +-----+   Internet    +------+       +-----+
     | GW1  |-------|vCPE1|---------------| vCPE2|-------+ GW2 |
     +------+       +-----+               +------+       +-----+

      Site A                                              Site B


            Figure 4: Branch Office WAN Connection via Internet






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                                            +-------------+
                                            |   +------+  |
                                            |   | PoP1 |  |
     +------+      +-----+   Internet       |   +------+  |
     | GW1  |------|vCPE1|------------------|       |     |
     +------+      +-----+                  |       |     |
                                            |   +------+  |
      Site A                                |   | vPC1 |  |
                                            |   +------+  |
                                            |public cloud |
                                            +-------------+
                                                   |
                                                   |
                                                   | DC
                                                   | Interconnection
                                                   |
                                            +-------------+
                                            |   +------+  |
                                            |   | vPC2 |  |
                                            |   +------+  |
                                            |       |     |
                                            |       |     |
                                            |   +------+  |
                                            |   | PoP2 |  |
                                            |   +------+  |
                                            |public cloud |
                                            +-------------+



                     Figure 5: Enterprise Cloud Access

6.  Features and Impacts to be Considered for LOOPS

   This section provides an overview of the proposed LOOPS solution.
   This section is not meant to document a detailed specification, but
   it is meant to highlight some design choices that may be followed
   during the solution design phase.

   LOOPS aims to improve the transport performance "locally" in addition
   to native end-to-end mechanism supported by a given transport
   protocol.  This is possible because LOOPS nodes will be instantiated
   to partition the path into multiple segments.  With the advent of
   automation and technologies like NFV and virtual IO, it is possible
   to dynamically instantiate functions to nodes.  Some overlay
   protocols such as VXLAN [RFC7348], GENEVE [I-D.ietf-nvo3-geneve],
   LISP [RFC6830] or CAPWAP [RFC5415] may be used in the network.  In
   overlay network usage scenario, LOOPS can extend a specific overlay



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   protocol header to perform local measurement and local recovery
   functions, like the example shown in Figure 6.

    +------------+------------+-----------------+---------+---------+
    |Outer IP hdr|Overlay hdr |LOOPS information|Inner hdr|payload  |
    +------------+------------+-----------------+---------+---------+


                 Figure 6: LOOPS Extension Header Example

   LOOPS should be designed to minimize its overhead while increasing
   the benefit (e.g., reduces the completion time of a video
   application, reduces the loss).  Also, LOOPS should be designed to
   auto-tune itself in case its overhead is exceeding a threshold.

   For example, LOOPS uses packet number space independent from that of
   the transport layer.  Acknowledgment should be generated from ON
   receiver to ON sender for packet loss detection and local
   measurement.  To reduce overhead, negative ACK over each path segment
   is a good choice here.  A Timestamp echo mechanism, analogous to
   TCP's Timestamp option, should be employed in-band in LOOPS extension
   to measure the local RTT and variation for an overlay segment.  Local
   in-network recovery is performed.  The measurement over segment is
   expected to give a hint on whether the lost packet of locally
   recovered one was caused by congestion.  Such a hint could be further
   feedback, using like by ECN Congestion Experienced (CE) markings, to
   the end host sender.  It directs the end host sender if congestion
   window adjustment is necessary.  LOOPS normally works on the overlay
   segment which aggregates the same type of traffic, for instance TCP
   traffic or finer granularity like TCP throughput sensitive traffic.
   LOOPS does not look into the inner packet (when an encapsulation
   scheme is used).  Elements to be considered in LOOPS are discussed
   briefly here.

6.1.  Local Recovery and End-to-end Retransmission

   There are basically two ways to perform local recovery,
   retransmission and FEC (Forward Error Correction).  They are possibly
   used together in some cases.  Such approaches between two overlay
   nodes recover the lost packet in relatively shorter distance and thus
   shorter latency.  Therefore the local recovery is always faster
   compared to end-to- end.

   At the same time, most transport layer protocols have their own end-
   to-end retransmission to recover the lost packet.  It would be ideal
   if end-to-end retransmission at the sender was not triggered when the
   local recovery is successful.




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   End-to-end retransmission is normally triggered by a NACK as in RTCP
   or multiple duplicate ACKs as in TCP.

   When FEC is used for local recovery, it may come with a buffer to
   make sure the recovered packets delivered are in order subsequently.
   Therefore the receiver side is unlikely to see the out-of-order
   packets and then send a NACK or multiple duplicate ACKs.  The side
   effect to unnecessarily trigger end-to-end retransmit is minimum.
   When FEC is used, if redundancy and block size are determined, extra
   latency required to recover lost packets is also bounded.  Then RTT
   variation caused by it is predictable.  In some extreme case like a
   large number of packet loss caused by persistent burst, FEC may not
   be able to recover it.  Then end-to-end retransmit will work as a
   last resort.  In summary, when FEC is used as local recovery, the
   impact on end-to-end retransmission is limited.

   When local retransmission is used, more care is required.

   For packet loss in RTP streaming, local retransmission can recover
   those packets which would not be retransmitted end-to-end otherwise
   due to long RTT.  It would be ideal if the retransmitted packet
   reaches the receiver before it sends back information that the sender
   would interpret as a NACK for the lost packet.  Therefore when the
   segment(s) being retransmitted is a small portion of the whole end to
   end path, the retransmission will have a significant effect of
   improving the quality at receiver.  When the sender also re-transmits
   the packet based on a NACK received, the receiver will receive the
   duplicated retransmitted packets and should ignore the duplication.

   For packet loss in TCP flows, TCP RENO and CUBIC use duplicate ACKs
   as a loss signal to trigger the fast retransmit.  There are different
   ways to avoid the sender's end-to-end retransmission being triggered
   prematurely:

   o  The egress overlay node can buffer the out-of-order packets for a
      while, giving a limited time for a packet being retransmitted
      somewhere in the overlay path to reach it.  The retransmitted
      packet and the buffered packets caused by it may increase the RTT
      variation at the sender.  When the retransmitted latency is a
      small portion of RTT or the loss is rare, such RTT variation will
      be smoothed without much impact.  Another possible way is to make
      the sender exclude such packets from the RTT measurement.  The
      locally recovered packets can be specially marked and this marking
      is spin back to end host sender.  Then RTT measurement should not
      use that packet.

      The buffer management is nontrivial in this case.  It has to be
      determined how many out-of-order packets can be buffered at the



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      egress overlay node before it gives up waiting for a successful
      local retransmission.  In some extreme case the lost packet is not
      recovered successfully locally, the sender may invoke end-to-end
      fast retransmit slower than it would be in classic TCP.

   o  If LOOPS network does not buffer the out-of-order packets caused
      by packet loss, TCP sender can use a time based loss detection
      like RACK [I-D.ietf-tcpm-rack] to prevent the TCP sender from
      invoking fast retransmit too early.  RACK uses the notion of time
      to replace the conventional DUPACK threshold approach to detect
      losses.  RACK is required to be tuned to fit the local
      retransmission better.  If there are n similar segments over the
      path, segment retransmission will at least add RTT/n to the
      reordering window by average when the packet is lost only once
      over the whole overlay path.  This approach is more preferred than
      one described in previous bullet.  On the other hand, if time
      based loss detection is not supported at the sender, end to end
      retransmission will be invoked as usual.  It wastes some
      bandwidth.

6.1.1.  OE to OE Measurement, Recovery, and Multipathing

   When multiple segments are stitched, another type of local recovery
   can be is performed between OE (Overlay Edge) to OE.  When the
   segments of an overlay path have similar characteristics and/or only
   OE has the expected processing capability, OE to OE based local
   recovery can be used instead of per-segment based recovery.

   If there is more than one overlay path between two OEs, multipathing
   can split and recombine the traffic.  Measurements such as RTT and
   loss rate between OEs have to be specific to each path.  The ingress
   OE can use the feedback measurement to determine the FEC parameter
   settings for different path.  FEC can also be configured to work over
   the combined path.  FEC should not increase redundancy over the path
   where a congestion is found.  The egress OE should be able to remove
   the duplicated packets when multipathing is available.

   OE to OE measurement can help each segment determine its proportion
   in edge to edge delay.  It is useful for ON to decide if it is
   necessary to turn on the per segment recovery or how to fine tune the
   parameter settings.  When the segment delay ratio is small, the
   segment retransmission is more effective.  Such approach requires
   nested LOOPS function.  This draft does not focus on the nest LOOPS
   now.  More details will be discussed later if comments showing
   interests in it are received.






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6.2.  Congestion Control Interaction

   When a TCP-like transport layer protocol is used, local recovery in
   LOOPS has to interact with the upper layer transport congestion
   control.  Classic TCP adjusts the congestion window when a loss is
   detected and fast retransmit is invoked.

   The local recovery mechanism breaks the assumption of the necessary
   and sufficient conditional relationship between detected packet loss
   and congestion control trigger at the sender in classic TCP.  The
   loss that is locally recovered can be caused by a non-persistent
   congestion such as a random loss or a microburst, both of which
   ideally would not let the sender invoke the congestion control
   mechanism.  But then, loss can also possibly caused by a real
   persistent congestion which should let the sender aware of it and
   reduces its sending rate.

   When a local recovery takes effect, we consider the following two
   cases.  Firstly, the classic TCP sender does not see enough number of
   duplicate ACKs to trigger fast retransmit.  This may be due to the
   local recovery procedures, which hides the out-of-order packet from
   receiver using mechanisms like reordering buffer at egress node.
   Classic TCP sender in this case will not reduce congestion window as
   no loss is detected.  Secondly, if a time based loss detection such
   as RACK is used, as long as the locally recovered packet's ACK
   reaches the sender before the reordering window expires, the
   congestion window will not be reduced.

   Such behavior brings the desirable throughput improvement when the
   recovered packet is lost due to non-persistent congestion.  It solves
   the throughput problem mentioned in Section 3.3 and Section 4.
   However, it also brings the risk that the sender is not able to
   detect a real persistent congestion in time, and then overshooting
   may occur.  Eventually a severe congestion that is not recoverable by
   a local recovery mechanism will be detected by sender.  In addition,
   it may be unfriendly to other flows (possibly pushing them out) if
   those flows are running over the same underlying bottleneck links.

   There is a spectrum of approaches.  On one end, each locally
   recovered packet can be treated exactly as a loss in order to invoke
   the congestion control at the sender to guarantee the fair sharing as
   classic TCP by setting its CE (Congestion Experienced) bit.  Explicit
   Congestion Notification (ECN) can be used here as ECN marking was
   required to be equivalent to a packet drop [RFC3168].  Congestion
   control at the sender works as usual and no throughput improvement
   could be achieved (although the benefit of faster recovery is still
   there).  On the other hand, ON can perform its congestion measurement
   over the segment, for instance local RTT and its variation trend.



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   Such measurement can help to determine if a lost packet by
   congestion.  It will further decide if it is necessary to set CE
   marking or even what ratio is set to make the sender adjust the
   sending rate.

   There are possible cases that the sender detects the loss even with
   local recovery in function.  For example, when the re-ordering window
   in RACK is not optimally adapted, the sender may trigger the
   congestion control at the same time of end-to-end retransmission.  If
   spurious retransmission detection based on DSACK [RFC3708] is used,
   such end-to-end retransmission will be found out unnecessary when
   locally recovered packets reaches the receiver successfully.  Then
   congestion control changes will be undone at the sender.  This
   results in similar pros and cons as described earlier.  Pros are
   preventing the unnecessary window reduction and improving the
   throughput when the loss is caused by non-congestive loss.  Cons are
   some mechanisms like ECN or its variants should be used wisely to
   make sure the congestion control is invoked in case of persistent
   congestion.

   An approach where the losses on a path segment are not immediately
   made known to the end-to-end congestion control can be combined with
   a "circuit breaker" style congestion control on the path segment.
   When the usage of path segment by the overlay flow starts to become
   unfair, the path segment sends congestion signals up to the end-to-
   end congestion control.  This must be carefully tuned to avoid
   unwanted oscillation.

   In summary, local recovery can improve Flow Completion Time (FCT) by
   eliminating tail loss in small flows.  As it may change loss event to
   out-of-order event in most cases to TCP sender, if TCP sender uses
   loss based congestion control, there is no much throughput
   improvement.  We suggest ECN and spurious retransmission to be
   enabled when local recovery is in use, it would give the desirable
   throughput performance, i.e. when loss is caused by congestion,
   reduce congestion window; otherwise keep sender's sending rate.  We
   do not suggest to use spurious retransmission alone together with
   local recovery as it may cause the TCP sender falsely undo window
   reduction when congestion occurs.  If only ECN is enabled or neither
   ECN nor spurious retransmission is enabled, the throughput with local
   recovery in use is no much difference from that of the tradition TCP.

6.3.  Overlay Protocol Extensions

   The overlay usually has no control over how packets are routed in the
   underlying network between two overlay nodes, but it can control, for
   example, the sequence of overlay nodes a message traverses before
   reaching its destination.  LOOPS assumes the overlay protocol can



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   deliver the packets in such designated sequence.  Most forms of
   overlay networking use some sort of "encapsulation".  The whole path
   taken can be performed by stitching multiple overlay paths, like
   VXLAN [RFC7348], GENEVE [I-D.ietf-nvo3-geneve], or it can be a single
   overlay path with a sequence of intermediate overlay nodes specified,
   as in SRv6 [I-D.ietf-6man-segment-routing-header].  In either way,
   LOOPS information is required to be embedded in some form to support
   the data plane measurement and feedback.  Retransmission or FEC based
   loss recovery can be either per ON-hop or OE to OE based.

   LOOPS alone has no setup requirement on control plane.  Some overlay
   protocols, e.g., CAPWAP [RFC5415], has session setup phase, it can be
   used to exchange the information such as dynamic FEC parameters.

6.4.  Summary

   LOOPS is expected to extend the existing overlay protocols in data
   plane.  Path selection is assumed a feature provided by the overlay
   protocols via SDN techniques [RFC7149] or other approaches and is not
   a part of LOOPS.  LOOPS is a set of functions to be implemented on
   Overlay Nodes, that will be involved in forwarding packets in a long
   haul overlay network.  LOOPS targets the following features.

   1.  Local recovery: Retransmission, FEC, or combination thereof can
       be used as local recovery method.  Such recovery mechanism is in-
       network.  It is performed by two network nodes with computing and
       memory resources.

   2.  Local congestion measurement: Ingress/Egress overlay nodes
       measure the local segment RTT, loss and/or throughput to
       immediately get the overlay segment status.

   3.  Signal to end-to-end congestion control: Strategy to set ECN CE
       marking or simply not to recover the packet to signal the end
       host sender about if and/or how to adjust the sending rate is
       required.

7.  Security Considerations

   LOOPS does not require access to the traffic payload in clear, so
   encrypted payload does not affect functionality of LOOPS.

   The use of LOOPS introduces some issues which impact security.  ON
   with LOOPS function represents a point in the network where the
   traffic can be potentially manipulated and intercepted by malicious
   nodes.  Means to ensure that only legitimate nodes are involved
   should be considered.




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   Denial of service attack can be launched from an ON.  A rogue ON
   might be able to spoof packets as if it come from a legitimate ON.
   It may also modify the ECN CE marking in packets to influence the
   sender's rate.  In order to protected from such attacks, the overlay
   protocol itself should have some build-in security protection which
   inherently be used by LOOPS.  The operator should use some
   authentication mechanism to make sure ONs are valid and non-
   compromised.

8.  IANA Considerations

   No IANA action is required.

9.  Acknowledgements

   Thanks to etosat mailing list about the discussion about the SatCom
   and LOOPS use case.

10.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,
              <https://www.rfc-editor.org/info/rfc3135>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              DOI 10.17487/RFC3708, February 2004,
              <https://www.rfc-editor.org/info/rfc3708>.





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   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <https://www.rfc-editor.org/info/rfc4588>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7149]  Boucadair, M. and C. Jacquenet, "Software-Defined
              Networking: A Perspective from within a Service Provider
              Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
              <https://www.rfc-editor.org/info/rfc7149>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.

   [RFC8517]  Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
              Jacquenet, "An Inventory of Transport-Centric Functions
              Provided by Middleboxes: An Operator Perspective",
              RFC 8517, DOI 10.17487/RFC8517, February 2019,
              <https://www.rfc-editor.org/info/rfc8517>.



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   [I-D.dukkipati-tcpm-tcp-loss-probe]
              Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
              Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
              in progress), February 2013.

   [I-D.ietf-nvo3-geneve]
              Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
              Network Virtualization Encapsulation", draft-ietf-
              nvo3-geneve-13 (work in progress), March 2019.

   [I-D.ietf-tcpm-rack]
              Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-05 (work in progress), April 2019.

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d. daniel.voyer@bell.ca, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-21 (work in progress), June 2019.

   [I-D.cardwell-iccrg-bbr-congestion-control]
              Cardwell, N., Cheng, Y., Yeganeh, S., and V. Jacobson,
              "BBR Congestion Control", draft-cardwell-iccrg-bbr-
              congestion-control-00 (work in progress), July 2017.

   [DOI_10.1109_ICDCS.2016.49]
              Cai, C., Le, F., Sun, X., Xie, G., Jamjoom, H., and R.
              Campbell, "CRONets: Cloud-Routed Overlay Networks", 2016
              IEEE 36th International Conference on Distributed
              Computing Systems (ICDCS), DOI 10.1109/icdcs.2016.49, June
              2016.

   [DOI_10.1145_3038912.3052560]
              Haq, O., Raja, M., and F. Dogar, "Measuring and Improving
              the Reliability of Wide-Area Cloud Paths", Proceedings of
              the 26th International Conference on World Wide Web -
              WWW '17, DOI 10.1145/3038912.3052560, 2017.

   [OCN]      Xu, Z., Ju, R., Gu, L., Wang, W., Li, J., Li, F., and L.
              Han, "Using Overlay Cloud Network to Accelerate Global
              Communications", INFOCOM ICCN 2019, April 2019,
              <https://github.com/zhaoguixu/INFOCOM19_ICCN/blob/master/
              ocn.pdf>.






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Authors' Addresses

   Yizhou Li
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012
   China

   Phone: +86-25-56624584
   Email: liyizhou@huawei.com


   Xingwang Zhou
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012
   China

   Email: zhouxingwang@huawei.com


   Mohamed Boucadair
   Orange

   Email: mohamed.boucadair@orange.com


   Jianglong Wang
   China Telecom

   Email: wangjl1.bri@chinatelecom.cn




















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