Multipath sequence maintenance

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Authors Markus Amend  , Dirk Hugo 
Last updated 2021-03-08 (latest revision 2021-02-22)
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ICCRG Working Group                                             M. Amend
Internet-Draft                                               D. von Hugo
Intended status: Experimental                                         DT
Expires: 26 August 2021                                 22 February 2021

                     Multipath sequence maintenance


   This document discusses the issue of packet re-ordering which occurs
   as a specific problem in multi-path connections without reliable
   transport protocols such as TCP.  The topic is relevant for devices
   connected via multiple accesses technologies towards the network as
   is foreseen e.g. within Access Traffic Selection, Switching, and
   Splitting (ATSSS) service of 3rd Generation Partnership Project
   (3GPP) enabling fixed mobile converged (FMC) scenario.

Status of This Memo

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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  State of the Art  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Scheduling mechanisms . . . . . . . . . . . . . . . . . . . .   6
   5.  Resequencing mechanisms . . . . . . . . . . . . . . . . . . .   7
     5.1.  Passive . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Exact . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Static Expiration . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Adaptive Expiration . . . . . . . . . . . . . . . . . . .   8
     5.5.  Delay Equalization  . . . . . . . . . . . . . . . . . . .   8
     5.6.  Fast Packet Loss Detection  . . . . . . . . . . . . . . .   9
       5.6.1.  Connection sequencing . . . . . . . . . . . . . . . .   9
       5.6.2.  Per-path sequencing . . . . . . . . . . . . . . . . .   9
       5.6.3.  Combination connection and per-path sequencing  . . .   9
   6.  Recovery mechanisms . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  FEC (Forward Error Correction)  . . . . . . . . . . . . .  10
     6.2.  Network Coding  . . . . . . . . . . . . . . . . . . . . .  10
   7.  Retransmission mechanisms . . . . . . . . . . . . . . . . . .  11
     7.1.  Signalling  . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  Anticipated . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  Flow-selection  . . . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Mobile end user devices nowadays are mostly equipped with multiple
   network interfaces allowing to connect to more than one network at a
   time and thus increase data throughput, reliability, coverage, and so
   on.  Ideally the user data stream originating from the application at
   the device is split between the available (here: N) paths at the
   sender side and re-assembled at an intermediate aggregation node
   before transmitted to the corresponding host in the network as
   depicted in Figure 1.

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                     /            \
          +---------| Access Net 1 |--+
          |         \             /   |
          |          -------------    |
          |          ------------     |
          |         /            \    |
          | +------| Access Net 2 |-+ |
          | |       \            /  | |
          | |        ------------   | |
          | |                       | |
          | |                       | |
          | |               +-------+-+---+
       +--+-+-+             |             |           +------+
       |End   |             | Aggregation +----/.../--| Host |
       |User  |             |    Node     |           +------+
       |Device|             |             |
       +--+-+-+             +-------+-+---+
          | |                       | |
          | |     --------------    | |
          | |    /              \   | |
          | +---| Access Net N-1 |--+ |
          |      \              /     |
          |       --------------      |
          |                           |
          |          ------------     |
          |         /            \    |
          +--------| Access Net N |---+
                    \            /

        Figure 1: Reference Architecture for multi-path re-ordering

   However, when several paths are utilized concurrently to transmit
   user data between the sender and the receiver, different
   characteristics of the paths in terms of bandwidth, delay, or error
   proneness can impact the overall performance due to delayed packet
   arrival and need for re-transmit in case of lost packets.  Without
   further arrangements the original order of packets at the sending UE
   side is no longer maintained at the receiving host and a re-ordering
   or re-arrangement has to occur before delivery to the application at
   the far end site.  This can be performed at earliest at the
   aggregation node with a minimum additional delay due to re-
   transmission requests or at latest either by the application on the
   host itself or the transmission protocol.

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   It is a goal of the present document to collect and describe
   mechanisms to maintain the sequence of split traffic over multiple
   paths.  These mechanisms are generic and not dedicated to a specific
   multipath network protocol, but give clear guidance on requirements
   and benefits to maintainers of multipath network protocols.

2.  State of the Art

   Regular TCP protocol [RFC0793] offers such mechanism with queues for
   in-order and out-of order (including damaged, lost, duplicated)
   arrival of packets.

   This is also provided by MPTCP [RFC8684] as the first and successful
   Multipath protocol which however also requires new methods as
   sequence numbers both on (whole) data (stream) and subflow level to
   ensure in-order delivery to the application layer on the receiver
   side [RFC8684].  Moreover, careful design of buffer sizes and
   interpretation of sequence numbers to distinguish between (delayed)
   out-of-order packets and completely lost ones has to be considered.

   [I-D.bonaventure-iccrg-schedulers] already reflects on proper packet
   scheduling schemes (at the sender side) to reduce the effort for re-
   assembly or even make such (time consuming) treatment unnecessary.

   MP-QUIC [I-D.deconinck-quic-multipath] introduces the concept of
   uniflows with own IDs claiming to get rid of additional sequence
   numbers for re-ordering as required in Multipath TCP [RFC8684].
   Although [I-D.liu-multipath-quic] admits that statistical performance
   information should help a host in deciding on optimum packet
   scheduling and flow control a dedicated packet scheduling policy is
   out of scope of that document.  A further improvement versus MPTCP
   can be achieved by decoupling paths used for data transmission from
   those for sending acknowledgments (ACKs) or claiming for re-
   transmission by NACKs to not introduce further latency.

   [I-D.ietf-quic-recovery] specifies algorithms for QUIC Loss Detection
   and Congestion Control by using measurement of Round Trip Time (RTT)
   to determine when packets should be retransmitted.  Draft
   [I-D.huitema-quic-ts] proposes to enable one way delay (1WD)
   measurements in QUIC by defining a TIME_STAMP frame to carry the time
   at which a packet is sent and combine the ACKs sent with a timestamp
   field and thus allow for more precise estimation of the (one-way)
   delay of each uniflow, assisting proper scheduling decisions.

   Also other protocols as Multi-Access Management Services (MAMS)
   [RFC8743] consider the need for re-ordering on User Plane level which
   may be done at network and client level by introducing a new Multi-
   Access (MX) Convergence Layer.  [I-D.zhu-intarea-mams-user-protocol]

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   introduces accordingly Traffic Splitting Update (TSU) messages and
   Packet Loss Report (PLR) messages including beside others Traffic
   Splitting Parameters and an expected next (in-order) sequence number,

   [I-D.zhu-intarea-gma] on Generic Multi-Access (GMA) Convergence
   Encapsulation Protocols introduces a trailer-based encapsulation
   which carries one or multiple IP packets or fragments thereof in a
   Protocol Data Unit (PDU).  At receiver side PDUs with identical
   Sequence Numbers (in the trailer) are to be placed in the relative
   order indicated by a so-called Fragment Offset.

3.  Problem Statement

   Assuming for simplicity the minimum multipath scenario with two
   separate paths for transmission of a flow of packets with sequence
   numbers (SN) SN1 ... SM.  In case the scheduling of packets is done
   equally to both paths and path 2 exhibits a delay of the duration of
   transmission time required for e.g. two packets (assuming fixed
   packet size and same constant data for both paths) for an exemplary
   App-originated sequence of packets as SN1 SN2 SN3 SN4 SN5 SN6 SN7 SN8
   ... the resulting sequence of packets could look as depicted in
   Figure 2 which of course depends on the queue processing and
   buffering at the Aggregation Proxy.

   APP              UE             Aggregation Node                 Host
    |  SN1 ... SN8  |                         |                       |
    |-------------->|path 1 SN1 SN3 SN5 SN7...|                       |
    |               |------------------------>|                       |
    |               |path 2 SN2 SN4 SN6 SN8...|                       |
    |               |------------------------>|                       |
    |               |                         |SN1 SN3 SN2 SN5 SN4 SN7|
    |               |                         |======================>|
    |               |                         |                       |

       Figure 2: Exemplary data transmission for a dual-path scenario

   In such a case re-ordering at the Aggregation Node would be simple
   and straight forward.  It even could be avoided if the scheduling
   would already take the expected different delays into account (e.g.
   by pre-delaying the traffic on path 1 thus of course not leveraging
   the lower delay).  Different from this simplistic scenario in general
   the data rate on both paths will vary in time and be not equal, also
   different and variable latency (jitter) per path will be introduced
   and in addition loss of packets as well as potential duplication may
   occur making the situation much more complicated.  In case of loss
   detection after a threshold waiting time a retransmission could be
   initiated by the Host or if possible already by the Aggregation Node.

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   Alternatively the UE could send redundant packets in advance coded in
   such a way that it allows for derivation of e.g. one lost packet per
   M correctly received ones or by a (real-time) application able to
   survive singular lost packets.

   Holding multiple queues and a large enough buffer both at UE and at
   the Aggregation Node would be required to apply proper scheduling at
   UE and reordering during re-assembly at Aggregation Node to mitigate
   the sketched impact of multiple paths' variable characteristics in
   terms of transmission performance.


4.  Scheduling mechanisms

   Scheduling mechanisms decide on sender side how traffic is
   distributed over the paths of a multipath-setup.
   [I-D.bonaventure-iccrg-schedulers] gives an overview of possible
   distribution schemes.  For this document it is assumed, that
   schedulers are used, which simultaneously distribute traffic over
   more than one path, whereas path characteristics differ between those
   multiple paths (e.g. a latency difference exists).  While on the one
   hand, the traffic scheduling causes out-of-order multipath delivery
   when simultaneously utilize heterogeneous paths, it can also be used
   to mitigate this problem.  Pre-delaying data on a fast path,
   according to the latency difference of the slowest path is aimed e.g.
   by OTIAS [OTIAS], DAPS [DAPS], and BLEST [BLEST].  However, the
   success is much dependent on the accuracy of path information like
   path latency, throughput, and packet loss rate.  In heterogeneous and
   volatile environments most often such information have to be
   estimated, e.g. using congestion control.  That means, it takes at
   least one RTT to gain first indications and probably several RTTs to
   converge to a worthwhile accuracy.  Changes of path characteristics
   in sub-RTT time frames put such a system to test.  Dependent on the
   demand on in-order delivery and/or the accuracy of the relevant path
   information, scheduling might be an exclusive alternative or can be
   applied in conjunction with other discussed mechanisms in this

   Scheduling will not help to overcome any degree of out-of-order
   delivery, when the scheduling goal is different to this.  For example
   a strict cost prioritization of Wi-Fi over cellular access in a
   mobile phone might be assumed counterproductive.

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5.  Resequencing mechanisms

   Resequencing mechanisms are responsible to modify the sequence of
   received data split over multiple paths according to a sequencing
   scheme.  The degree of resequencing can reach from no measure up to
   re-generating the exact order.

   Typically at least one sequencing scheme, describing the order of how
   data was generated on sender side is prerequisite.  This is referred
   to as "connection sequencing".  Under certain circumstances an
   additional sequencing scheme per path of the multi-path setup can be
   leveraged, to optimize packet loss detection and is further
   elaborated in Section 5.6.  For most multipath protocols both
   sequencing schemes are already available.  Packet loss detection
   becomes important when multipath protocols are applied which do not
   guarantee successful transmission as TCP achieves by acknowledgement
   of successful reception.  For example,
   [I-D.amend-tsvwg-multipath-dccp] or the combination of
   [I-D.deconinck-quic-multipath] and [I-D.ietf-quic-datagram] are
   unreliable protocols in that sense.

   For simplicity all the mechanism described in the following are
   explained based on two paths but in principle would work with any
   other amount though.

5.1.  Passive

   This approach includes no active change or reordering at the
   transport level and purely re-combines the packet flows incoming from
   both paths as is.  All modification of the resulting sequence of
   packets is left to the application at the end node.  Here no
   processing delay is added due to the resequencing but since no early
   packet loss detection with subsequent re-transmission request on
   transport level is possible the risk of a larger delay due to late
   loss detection at the application will arise in case of lossy

5.2.  Exact

   This approach covers all mechanisms which attempt to re-generate the
   original order of packets in the flow exactly, independent of the
   expected or resulting delay due to waiting time for all packets on
   all paths to arrive.  In case of unreliable transport protocols this
   may result in a large delay due to Head-of-Line blocking and for
   actual packet loss in a remaining packet gap which causes a stand
   still without an option to recover.  For applications demanding near
   real-time delivery of packets it should not be applied.

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5.3.  Static Expiration

   This method to detect and decide on packet loss assumes a certain
   fixed time threshold for the gap between packets within a sequence
   after re-combination of both paths.  A possible re-transmission -
   either in the multipath system internally or based on the piggybacked
   protocol/service - will possibly not be requested before this
   threshold is exceeded.  Thus an additional delay in the overall
   latency budget will occur so that this simple approach is only
   recommended for non-time critical applications.  Every packet loss or
   simultaneous transmission of data over the short and long latency
   path will cause spikes in the service perceived latency.

5.4.  Adaptive Expiration

   Here the packet gap is assumed as packet loss after exceeding a
   flexibly decided on time threshold which may be derived dynamically
   from the differences between latencies both paths exhibit.  As the
   latency may vary due to propagation conditions or routing paths this
   latency difference has to be monitored and statistically evaluated
   (smoothed) which introduces additional effort.  A possible solution
   for this is the determination of the the one way latency as described
   in [] or sending available RTT information from the
   sender from which the receiver can calculate the latency difference.

5.5.  Delay Equalization

   This is an ordering mechanism which delays data forwarding on the
   faster path by the latency difference to the slower path.  Ideally
   the resequencing effort on the aggregated packet flow can be greatly
   reduced up to no resequencing at all.  Due to time variation in path
   delays (jitter) and delay differences and the required time for
   decision and feedback on the delay, some re-sequencing still remains
   to be executed.  Similar to Section 5.4, explicit knowledge of the
   latency difference is required.  Strictly speaking this method allows
   to avoid resequencing based on sequencing information.  However, the
   overall delay may be larger since the advantage of the short-delay
   path is not exploited.  In combination with Section 5.3 or
   Section 5.4 resequencing can be added with a presumably lower
   resequencing effort to scenarios without delay equalization.  The
   essence is a in-order stream with a unified latency across the
   multiple paths.

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5.6.  Fast Packet Loss Detection

   The following sections describe methods to achieve unambiguous
   detection of packet loss independent from thresholds in Section 5.3
   or Section 5.4.  Furthermore, packet loss can be differentiated from
   delayed delivery.  The benefit is a much faster decision plane based
   on monitoring the sequence space of consecutive packets.  For that,
   the sequencing coming along with the receiver based re-sequencing is
   further leveraged.  Two sequencing schemes are considered here, the
   connection and the per-path sequencing.

5.6.1.  Connection sequencing

   Connection sequencing marks the outgoing packets in the order they
   enter the multipath system and is independent from a particular
   selected path for transmission.  After arrival at the aggregation
   node the lowest packet sequence number at each of the multiple paths
   is compared the that of the last correctly received packet.  When the
   numbers are not consecutive (i.e., when on all paths a higher number
   is received than the next expected in-order packet), an overall
   packet loss is detected.  While only a single comparison of packet
   numbers has to be performed and the out-of-order arrival on a single
   path can be partly compensated this scheme does not allow for
   immediate detection of where reordering happens.

5.6.2.  Per-path sequencing

   Per-path sequencing is a path inherent sequencing mechanism valid in
   the particular path domain only.  In this case the packets are marked
   by path-specific sequence numbers at the sender side and at each
   interface of the aggregation node the sequence numbers of arriving
   packets are compared on per-path level.  When a higher sequence
   number is received than the one which is waited for (next expected
   in-order packet), a packet loss for this specific path is declared.
   This may prevent partly misinterpretation of out-of-order arrival as
   packet loss and allow for path specific countermeasures towards
   overall performance improvement, as e.g., chosing a moire robust
   transmission technique for this path.

5.6.3.  Combination connection and per-path sequencing

   While the benefits from the individual sequencing schemes above can
   be combined, a further benefit crystallizes. [to be continued...]

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6.  Recovery mechanisms

   Recovering packets, in particular lost packets or assumed lost
   packets on receiver side avoids re-transmission and potentially
   mitigates the resequencing process in respect to detecting packet
   loss.  Shorter latencies will be an expected outcome.  Discussing the
   complexity, computation overhead and reachable benefit is subject of
   this section.

6.1.  FEC (Forward Error Correction)

   This approach is based on introduction of redundancy to user data to
   detect errors and subsequently reconstruct data in case of a limited
   number of bit or Byte errors.  As such packet with corrupted data can
   be recovered up to a certain degree but in case of a too high bit
   error rate (BER) a packet is completely lost.  However, in
   combination with scrambling, i.e. the sequence of original data
   stream is distributed over multiple packets and re-compiled
   afterwards also data from lost packets could be recovered.  As such
   methods introduce additional delay and overhead it is mainly applied
   in case of long re-transmission delays as e.g. is typical for
   satellite transmission.  FEC can be applied to each path separately
   (e.g., if they exhibit deviating performance characteristics to not
   degrade the 'better one') or in an overall FEC fashion before split
   and recombination which would support scrambling and facilitate
   recovery of completely lost packets on the 'worse path'.
   Unsuccessful application of FEC may enable quick detection of
   unrecoverable errors in a packet and thus trigger re-transmission
   from the sender side before time-out.

6.2.  Network Coding

   In linear network coding (LNC) network nodes (or interfaces of a
   device) do not simply relay the packets of information they receive,
   but combine several packets together for transmission.  After
   reception of combined and separate packets the maximum possible
   information flow in a network can be detected and throughput,
   efficiency and scalability, as well as resilience to attacks and
   eavesdropping can be improved.  The method in general improves with
   the number of paths in excess of two.  According to [COPE] drawbacks
   of LNC are high decoding computational complexity, high transmission
   overhead, and linear dependency among coefficients vectors for en-
   and decoding.  Triangular network coding (TNC) addresses the high
   encoding and decoding computational complexity without degrading the
   throughput performance, with code rate comparable to that of LNC.
   TNC is therefore advantageous for implementation on small devices
   mobile phones and wireless sensors with limited processing capability
   and power supply [TNC].

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7.  Retransmission mechanisms

   Re-transmission becomes interesting when it can help to reduce the
   time spent on waiting for outstanding packets for re-sequencing.  In
   particular scenarios when for example a known path RTT (Round Trip
   Time) lets expect a shorter time to re-transmit than wait for packet
   loss detection, a likely scenario in e.g.  Figure 1.  It could also
   avoid a potential late triggering of re-transmission by the end-to-
   end service.  On the other hand for sake of resource efficiency the
   amount of unnecessary retransmissions should be limited to not
   degrade the overall throughput of the connection.

7.1.  Signalling

   Signal to the sender to re-transmit a missing packet.  This requires
   a send buffer which keeps information for a reasonable time, which
   allows the beneficial use of this mechanism. [to be continued]

7.2.  Anticipated

   Re-transmit data without being notified from the receiver when packet
   loss can be assumed, e.g. from lower layer information. [to be

7.3.  Flow-selection

   Repeating data on the same path is not always useful.  In some
   scenarios it make sense to re-transmit data on another path, e.g.
   when the original path is broken or another path is known to be

   *  Requires path independent identification of data, e.g. the
      connection sequencing

   *  Has to consider MTU discrepancies between paths

   Flow selection for re-transmitting data can be combined with
   Section 7.1 or Section 7.2.  In certain scenarios data to be re-
   transmitted can be duplicated across paths to increase reliability.
   [to be continued]

8.  Informative References

   [BLEST]    Ferlin, S., Alay, Oe., Mehani, O., and R. Boreli, "BLEST:
              Blocking estimation-based MPTCP scheduler for
              heterogeneous networks", IFIP Networking Conference , May

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   [COPE]     Katti, S., Rahul, H., Katabi, W.H.D., Medard, M., and J.
              Crowcroft, "XORs in The Air: Practical Wireless Network
              Coding", September 2006,

   [DAPS]     Kuhn, N., Lochin, E., Mifdaoui, A., Sarwar, G., Mehani,
              O., and R. Boreli, "DAPS: Intelligent delay-aware packet
              scheduling for multipath transport", ICC IEEE
              International Conference on Communications, June 2014,

              Amend, M., Bogenfeld, E., Brunstrom, A., Kassler, A., and
              V. Rakocevic, "DCCP Extensions for Multipath Operation
              with Multiple Addresses", Work in Progress, Internet-
              Draft, draft-amend-tsvwg-multipath-dccp-03, 4 November
              2019, <

              Bonaventure, O., Piraux, M., Coninck, Q., Baerts, M.,
              Paasch, C., and M. Amend, "Multipath schedulers", Work in
              Progress, Internet-Draft, draft-bonaventure-iccrg-
              schedulers-01, 9 September 2020, <

              Coninck, Q. and O. Bonaventure, "Multipath Extensions for
              QUIC (MP-QUIC)", Work in Progress, Internet-Draft, draft-
              deconinck-quic-multipath-06, 2 November 2020,

              Huitema, C., "Quic Timestamps For Measuring One-Way
              Delays", Work in Progress, Internet-Draft, draft-huitema-
              quic-ts-03, 10 August 2020, <

              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-datagram-01, 24 August 2020,

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              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", Work in Progress, Internet-Draft,
              draft-ietf-quic-recovery-34, 14 January 2021,

              Liu, Y., Ma, Y., Huitema, C., An, Q., and Z. Li,
              "Multipath Extension for QUIC", Work in Progress,
              Internet-Draft, draft-liu-multipath-quic-02, 15 December
              2020, <

              Song, F., Zhang, H., Chan, A., and A. Wei, "One Way
              Latency Considerations for MPTCP", Work in Progress,
              Internet-Draft, draft-song-mptcp-owl-06, 18 June 2019,

              Zhu, J. and S. Kanugovi, "Generic Multi-Access (GMA)
              Encapsulation Protocol", Work in Progress, Internet-Draft,
              draft-zhu-intarea-gma-07, 14 May 2020,

              Zhu, J., Seo, S., Kanugovi, S., and S. Peng, "User-Plane
              Protocols for Multiple Access Management Service", Work in
              Progress, Internet-Draft, draft-zhu-intarea-mams-user-
              protocol-09, 4 March 2020, <

   [OTIAS]    Yang, F., Wang, Q., and P.D. Amer, "Out-of-Order
              Transmission for In-Order Arrival Scheduling for Multipath
              TCP", AINAW 28th International Conference on Advanced
              Information Networking and Applications Workshops, May
              2014, <>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

Amend & von Hugo         Expires 26 August 2021                [Page 13]
Internet-Draft       Multipath sequence maintenance        February 2021

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <>.

   [RFC8743]  Kanugovi, S., Baboescu, F., Zhu, J., and S. Seo, "Multiple
              Access Management Services Multi-Access Management
              Services (MAMS)", RFC 8743, DOI 10.17487/RFC8743, March
              2020, <>.

   [TNC]      Qureshi, J., Foh, C.H., and J. Cai, "Optimal solution for
              the index coding problem using network coding over GF(2)",
              June 2012, <>.

Authors' Addresses

   Markus Amend
   Deutsche Telekom
   Deutsche-Telekom-Allee 9
   64295 Darmstadt


   Dirk von Hugo
   Deutsche Telekom
   Deutsche-Telekom-Allee 9
   64295 Darmstadt


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