Internet-Draft Multipath sequence maintenance October 2021
Amend & von Hugo Expires 28 April 2022 [Page]
ICCRG Working Group
Intended Status:
M. Amend
D. von Hugo

Multipath sequence maintenance


This document discusses the issue of packet reordering 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

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 28 April 2022.

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.

                  /            \
       +---------| 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 reordering

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 reordering 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.

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 reordering 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 reordering 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] 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, respectively.

[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 reordering 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. 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 document.

[AOPS] proposes to use a predictive Adaptive Order Prediction Scheduling (AOPS) mechanism considering both the anticipated time of packet delivery and the reliability of each path to optimize the traffic scheduling for MP-DCCP, thus coping with reordering and achieving in-order delivery.

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.

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 connections.

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.

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.

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 more 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. Since the out-of-order arrival is detected on per-path basis, the path specific out-of-order delivery rate can be used as a criterion to choose repair parameters
on a per-path basis (which thus may work more efficiently). In addition the decision on the path selection and weighting can be made based on this criterion. Thus an improved overall performance can be achieved in this case. [to be checked/continued...]

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].

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. Signaling

In case of detected packet loss the receiver has to send a corresponding signalling message to the sender to re-transmit a missing packet. This is the traditional way of negative acknowledgement in case of missing the correct reception of packets
within a time window and sending a repeat-request. This approach requires a send buffer which keeps information for a reasonable time, thus allowing the beneficial use of this mechanism. On the other
hand the additional delay in terms of at least once the RTT until the
lost packet is correctly received results in performance degradation
for time-critical applications. ... [to be continued?]

7.2. Anticipated

To speed up the induced re-transmission delay a pro-active or anticipated approach would allow to trigger the sender to re-transmit data without needing to wait for notification from the receiver. This method can be applied when the assumed packet loss can be estimated based on other data, e.g., from lower layer, such as information on path or
link quality degradation derived from, e.g., an increased raw BER
detected by FEC mechanism (see Section 6.1).
[to be continued/extended?]

7.3. Flow-selection

Repeating data on the same path is not always useful. In some scenarios it makes sense to re-transmit data on another path, e.g., when the original path is broken or another path is known to provide higher throughput or lower packet loss. To apply such a selection of the flow for re-transmission.

  • 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 detection mechanisms as described in Section 7.1 or Section 7.2.

7.4. Other re-transmission issues

In certain scenarios data to be re-transmitted can be duplicated across paths (either in advance or after loss detection) to increase reliability and reduce potential overall transmission delay.
However, such approaches decrease the resource efficiency and reduce
the overall user throughput. A more pro-active measure would
be to encode multiple packets either on per-path or on per-connection
basis in a single 'repair packet' in 'XOR style' to be injected after
a set of packets (similarly as described in [COPE]. This would allow
to recreate exactly one lost packet out of the set in case the others
have been correctly received. Depending on the anticipated loss rate
the amount of packets within a set is chosen to more efficiently use
the transmission resources. [to be continued]

8. Security Considerations

This document does not add any additional security considerations in
addition to the ones introduced by multipath extensions to other
transmission protocols as, e.g., described for MPTCP in [RFC8684].
Also the described issues for GMA [I-D.zhu-intarea-gma], MP-DCCP
[I-D.amend-tsvwg-multipath-dccp], and MP-QUIC
[I-D.liu-multipath-quic] may apply here.

9. Informative References

Huang, C., Chen, Y., and S. Linn, "Packet scheduling and congestion control schemes for Multipath Datagram Congestion Control Protocol", The Computer Journal 58, no. 2, pp. 188--203 ] , .
Ferlin, S., Alay, Oe., Mehani, O., and R. Boreli, "BLEST: Blocking estimation-based MPTCP scheduler for heterogeneous networks", IFIP Networking Conference , , <>.
Katti, S., Rahul, H., Katabi, W.H.D., Medard, M., and J. Crowcroft, "XORs in The Air: Practical Wireless Network Coding", , <>.
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, , <>.
Amend, M., Hugo, D. V., Brunstrom, A., Kassler, A., Rakocevic, V., and S. Johnson, "DCCP Extensions for Multipath Operation with Multiple Addresses", Work in Progress, Internet-Draft, draft-amend-tsvwg-multipath-dccp-05, , <>.
Bonaventure, O., Piraux, M., Coninck, Q. D., Baerts, M., Paasch, C., and M. Amend, "Multipath schedulers", Work in Progress, Internet-Draft, draft-bonaventure-iccrg-schedulers-02, , <>.
Coninck, Q. D. and O. Bonaventure, "Multipath Extensions for QUIC (MP-QUIC)", Work in Progress, Internet-Draft, draft-deconinck-quic-multipath-07, , <>.
Huitema, C., "Quic Timestamps For Measuring One-Way Delays", Work in Progress, Internet-Draft, draft-huitema-quic-ts-06, , <>.
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable Datagram Extension to QUIC", Work in Progress, Internet-Draft, draft-ietf-quic-datagram-06, , <>.
Iyengar, J. and I. Swett, "QUIC Loss Detection and Congestion Control", Work in Progress, Internet-Draft, draft-ietf-quic-recovery-34, , <>.
Liu, Y., Ma, Y., Huitema, C., An, Q., and Z. Li, "Multipath Extension for QUIC", Work in Progress, Internet-Draft, draft-liu-multipath-quic-04, , <>.
Song, F., Zhang, H., Chan, H. A., and A. Wei, "One Way Latency Considerations for MPTCP", Work in Progress, Internet-Draft, draft-song-mptcp-owl-06, , <>.
Zhu, J. and S. Kanugovi, "Generic Multi-Access (GMA) Encapsulation Protocol", Work in Progress, Internet-Draft, draft-zhu-intarea-gma-12, , <>.
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, , <>.
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, , <>.
Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, , <>.
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, , <>.
Kanugovi, S., Baboescu, F., Zhu, J., and S. Seo, "Multiple Access Management Services Multi-Access Management Services (MAMS)", RFC 8743, DOI 10.17487/RFC8743, , <>.
Qureshi, J., Foh, C.H., and J. Cai, "Optimal solution for the index coding problem using network coding over GF(2)", , <>.

Authors' Addresses

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