GMA Traffic Splitting Control
draft-zhu-gma-tsc-00

Network Working Group                                      J. Zhu, Ed.
Internet Draft                                               M. Zhang
Intended status: Informational                                  Intel
Expires: September 25,2024                               March 25, 2024

                      GMA Traffic Splitting Control

                           draft-zhu-gma-tsc-00

Abstract

   This document specifies the GMA (Generic Multi-Access) traffic
   splitting control algorithm. The receiving endpoint measures one-
   way-delay, round-trip time, and delivery rate for multiple
   connections and determines how a data flow is split across them.
   When update is needed, it will send out a control message, aka
   Traffic Splitting Update (TSU), to notify the transmitting
   endpoint of the new traffic splitting configuration. Relative to
   other sender-based multi-path transport protocols, e.g. MPTCP,
   MPQUIC, the GMA traffic splitting algorithm is receiver-based and
   does not require per-packet feedback, e.g. Ack. It is designed
   specifically to support the Generic Multi-Access (GMA) convergence
   protocol as introduced in [MAMS] [GMA]. The solution has been
   developed by the authors based on their experiences in multiple
   standards bodies including IETF and 3GPP, is not an Internet
   Standard and does not represent the consensus opinion of the IETF.

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   This Internet-Draft will expire on September 25, 2024.

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Table of Contents

   1 Introduction .................................................2
   2  Conventions used in this document ...........................4
   3  GMA Traffic Splitting Control Algorithm .....................4
      3.1   Minimum OWD Measurement ...............................4
      3.2   Congestion Measurement ................................5
      3.3   Connection Failure Detection ..........................7
      3.4   Multi-path Traffic Splitting Control ..................8
   4  Enhancements ................................................9
      4.1   Adaptive Control in Medium Congestion .................9
      4.2   Minimizing TSU/TSA Overhead ...........................9
      4.3   Adaptive Splitting Burst Size ........................10
      4.4   Traffic Splitting Ratio Quantization .................10
   5  Security Considerations ....................................11
   6  IANA Considerations ........................................11
   7  References .................................................11
      7.1   Informative References ...............................11

1 Introduction

   A device can simultaneously connect to multiple networks, e.g.,
   Wi-Fi, LTE, 5G, DSL, and SATCOM (Satellite Communications). It is
   desirable to seamlessly combine multiple connections over these
   networks below the transport layer (L4) to improve quality of
   experience for applications that do not have built-in multi-path
   capabilities.

   The Multi-Access Management Service (MAMS) framework has been
   recently specified in [MAMS] to support various multi-access
   solutions [ATSSS] [LWIPEP] [GRE1] [GRE2]. As shown in Figure 1,
   its user-plane protocol stack consists of two layers: convergence
   and adaptation. The convergence layer is responsible for multi-

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   access operations, including multi-link (path) aggregation,
   splitting/reordering, lossless switching/retransmission, etc. It
   operates on top of the adaptation layer. From the perspective of a
   transmitter, a user payload (e.g., IP packet) is processed by the
   convergence layer first, and then by the adaptation layer before
   being transported over a delivery connection; from the receiver's
   perspective, an IP packet received over a delivery connection is
   processed by the adaptation layer first, and then by the
   convergence layer.

          +-----------------------------------------------------+
          |   User Payload, e.g., IP Protocol Data Unit (PDU)   |
          +-----------------------------------------------------+
       +-----------------------------------------------------------+
       |  +-----------------------------------------------------+  |
       |  | Multi-Access (MX) Convergence Layer                 |  |
       |  +-----------------------------------------------------+  |
       |  +-----------------------------------------------------+  |
       |  | MX Adaptation   | MX Adaptation   | MX Adaptation   |  |
       |  | Layer           | Layer           | Layer           |  |
       |  +-----------------+-----------------+-----------------+  |
       |  | Access #1 IP    | Access #2 IP    | Access #3 IP    |  |
       |  +-----------------------------------------------------+  |
       |                            MAMS User-Plane Protocol Stack |
       +-----------------------------------------------------------+

             Figure 1: MAMS User-Plane Protocol Stack [MAMS]

   A UDP-based GMA control protocol [GMA] has been proposed for the
   MX convergence layer in the MAMS framework. From the perspective
   of applications, the GMA protocol is a multi-path tunneling
   protocol operating below the network layer (L3), and therefore can
   support any legacy single-path transport protocol, e.g. TCP, UDP,
   QUIC, etc. From the perspective of an underlay access network, it
   is a light-weight transport protocol designed specifically for
   multi-path operation, removing unnecessary complexity and overhead
   (e.g., end-to-end encryption, congestion control, reliable
   transmission, etc.) as seen in a modern transport protocol [QUIC].
   Moreover, it can be easily extended to support advanced multi-path
   operations, e.g., network coding, network-based traffic steering,
   in-band QoS monitoring, etc.

   This document presents a receiver-based multi-path traffic
   splitting algorithm for the MX convergence layer. Unlike other
   sender-based multi-path solutions, e.g. MPTCP, it does not require

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   per-packet feedback, e.g. ACK, and leverages One-Way-Delay (OWD)
   measurements that are only available at the receiver.

   The solution described in this document has been developed by the
   authors based on their experiences in multiple standard bodies
   including the IETF and 3GPP. However, it is not an Internet
   Standard and does not represent the consensus opinion of the IETF.

2  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

3 GMA Traffic Splitting Control (GMA-TSC) Algorithm

   There are four components involved in the GMA-TSC algorithm:

    o minimum OWD measurement
    o congestion measurement
    o connection failure detection
    o multi-path traffic splitting control

3.1   Minimum OWD Measurement

   The GMA receiver performs minimum OWD measurement periodically
   based on received data and control packets. The time unit is
   milliseconds (ms). Define the following notations:

    o d(k, i): the OWD of the k-th received packet over the i-th
       connection
    o Y(i): the minimum OWD of the i-th connection
    o d'(i): the OWD of the last received packet over the i-th
       connection

   The GMA receiver SHOULD update Y(i) at the end of each minimum OWD
   measurement interval, and obtain the minimum OWD of the i-th
   connection as following:

                      Y(i) = min(d(k, i), for all k)

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   In addition, the receiver SHOULD set Y(i) to d'(i) immediately
   when receiving a packet with d'(i) < Y(i) - B1, where B1 is a
   configurable parameter, say 5ms.

3.2   Congestion Measurement

   The GMA receiver performs congestion measurement periodically
   based on received data & control packets. The congestion
   measurement interval SHOULD be set much shorter, e.g. < 1 s, than
   the minimum OWD measurement interval, e.g. 12 seconds. It will
   start right after a successful TSU/TSA exchange [GMA] or the
   previous interval if the TSU/TSA exchange is not triggered.

   Define the following control parameters:

    o T1: the minimum congestion measurement duration
    o T2: the minimum number of data packets for congestion
       measurement
    o Dmin: the lower bound of congestion measurement interval
    o Dmax: the upper bound of congestion measurement interval

   T1 is configured as follows:

                               T1 = a1 * V

   Herein, a1 is a configurable coefficient, e.g., 1.5. V indicates
   the maximum of average RTT of all connections and SHOULD be
   updated at the end of each interval.

   Define the following notations:

    o t(i): the RTT of the last control message exchange over the i-th
       connection
    o d0(i): the OWD of the received control message for the last RTT
       measurement t(i)
    o v(i): the average RTT of the i-th connection

   The average round-trip time of the i-th connection can be measured
   as

              v(i)=t(i) - d0(i) + average(d(k, i), for all k)

   Then, we can get V as

                         V = max(v(i), for all i)

   T2 is configured as follows:

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                               T2 = a2 * L

   Herein, a2 is another configurable coefficient, e.g. 2, and L is
   the splitting burst size, defined as the total number of packets
   per traffic splitting cycle.

   A congestion measurement interval will end only if both T1 and T2
   are reached. It is further bounded by a configurable range of
   [Dmin, Dmax], e.g. [10ms, 1s].

   The GMA receiver will measure the following metrics during each
   congestion measurement interval:

    o k(i): the number of received data packets experiencing
       congestions for the i-th connection
    o n(i): the total number of received data packets for the i-th
       connection
    o b(i): the estimated bandwidth for the i-th connection in unit of
       packets/second

   Let's use u to indicate one of the last U (e.g. 10) congestion
   measurement intervals, including the current one. Figure 2 shows
   an example of four (U=4) consecutive congestion measurement
   intervals.

               |----u=3---|---u=2----|--u=1---|--u=0---|
      -------------------------------------------------------> Time
          Figure 2: Congestion Measurement Intervals (U=4)

   Moreover, define the following notations:

      o D(u): the interval length of the u-th interval
      o n(i, u): the number of received packets for the u-th interval
      o k(i, u): the number of received data packets experiencing
        congestions on the i-th connection during the u-th interval.

   We can calculate b(i) as follows:

   if (max(k(i, u), for all u) > 0

                    b(i) = max(n(i, u) / D(u), for all u)

   else

                                 b(i) = 0

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   When u = 0, it refers to the current interval, i.e., n(i, u=0) =
   n(i), k(i, u=0)= k(i), and D(u=0) = D. In short, b(i) measures the
   maximum throughput of the i-th connection in the last U intervals
   when at least one received data packet is experiencing congestion.

   Next, we define that a packet is experiencing congestion if its
   OWD exceeds the minimum OWD with a specified threshold, i.e.,

                           d(k, i) > Y(i) + B2

   Here, B2 is a configurable threshold, e.g. 10ms.

   Notice that no congestion measurement SHOULD be performed during
   the TSU/TSA exchange.

3.3   Connection Failure Detection

   At the end of a congestion measurement interval, if the GMA
   receiver detects any potential connection failure, it will send a
   control message, e.g. probe, to check the connection status
   explicitly. Connection failure can then be confirmed through
   successive retransmission failures of the control message.
   Otherwise, it is a false alarm.

   Define s(i) as the traffic splitting ratio in the range of [0, 1]
   to indicate how much data traffic of a flow is delivered through
   the i-th connection. The connection failure detection method works
   as follows:

   o When a flow is being split over two or more connections, a
      connection will be flagged with "failure" if the connection has
      s(i)>0 but n(i)==0, and the total number of received packets
      exceeds T3, i.e. sum(n(i))>T3.
   o When a flow is being steered to a single connection, the
      connection will be flagged with "failure" if no packets are
      received in the current interval (u=0), e.g., sum(n(i)) == 0 and
      Q == 1. Here, Q is a bit flag to indicate if any data packet of
      the flow is received ("1") or not ("0") in the previous interval
      (u=1).

   T3 is configured as follows:

                            T3 = max(P, a2 * L)

   and P is a configurable lower-bound, e.g. 128 to ensure that when
   L is too small, e.g. 8, there are still enough measurement samples
   for connection failure detection to minimize false alarms.

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3.4   Multi-path Traffic Splitting Control

   The connections are ordered according to their preference, and its
   index is used to indicate its preference. A connection is
   preferred over another if its index is smaller. For example, the
   1-st connection with i=1 is the most preferred one, and the 2nd
   connection with i=2 is the next preferred one.

   At the end of a congestion measurement interval, the GMA receiver
   SHOULD recalculate traffic splitting ratio s(i) using the
   following principals:

   o For any connection flagged as a failure (see 3.3), its traffic
      splitting ratio s(i) is set to 0.
   o For other connections, consider the following four cases:
        + Case #1 (No Traffic): If no packet is received at all,
           i.e., sum(n(i)) == 0, stop splitting and steer the flow to
           the most preferred available connection.
        + Case #2 (No Congestion): If no packet experiences
           congestion, i.e. sum(k(i)) == 0, stop sending over the
           least preferred connection with none-zero traffic load,
           i.e., max(i | n(i)>0), and split the flow over the more
           preferred available connections.
        + Case #3 (Medium Congestion): When only a subset of
           connections experience congestion, i.e., min(k(i))==0 and
           max(k(i)) > 0, reallocate a portion of the flow from a
           congested connection to non-congested ones.
        + Case #4 (Heavy Congestion): If all connections are
           congested, i.e., min(k(i))>0, split the flow in proportion
           to n(i).

   The GMA traffic splitting algorithm can be described as follows:

   If (sum(n(i)) == 0) //no traffic

      o x: the most preferred available connection
      o j: other available connections
      o s(x) = 1.0 and s(j) = 0

   else if (sum(k(i)) == 0) //no congestion

      o x: the least preferred connection with none-zero load, i.e.,
        x == max(i | n(i)>0)
      o j: another available connection more preferred than "x"
      o S: the total number of available connections more preferred
        than "x"
      o R = min(n(x), p*sum(n(i))), where p = S/L
      o s(x) = (n(x) - R)/sum(n(i))

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      o if (min(b(j)) == 0)
        + s(j) = (n(j) + R/S)/sum(n(i))
      o else
        + s(j) = (sum(n(j)) + R)*b(j)/sum(b(j))/(sum(n(i))

   else if (min(k(i))==0 && max(k(i)) > 0) //medium congestion

      o x: the congested connection, i.e., k(x)>0
      o j: the uncongested connection, i.e., k(j)==0
      o s(x) = (n(x) - e*k(x))/sum(n(i)), where e = 0.3
      o if (min(b(j)) == 0)
        + s(j)=(n(j) + e*sum(k(x))/M)/sum(n(i)), where M is the
           number of uncongested connections
      o Else
        + s(j)=(sum(n(j)) + e *sum(k(x)))*b(j)/sum(b(j)/sum(n(i))

   else if (min(k(i)) > 0) //heavy congestion

      o s(i) = n(i)/sum(n(i))

4 Enhancements

4.1   Adaptive Control in Medium Congestion

   In the case of medium congestion (case #3 in 3.4), some data
   packets are reallocated from a congested connection to uncongested
   ones. However, the total amount of reallocated traffic, given by
   e*sum(k(x)) should not exceed the total available bandwidth of
   uncongested connections, given by sum(b(j))*D - sum(n(j)), where D
   is the current interval length. Hence, we can adaptively configure
   "e" with the following two steps:

   o step 1: e = (sum(b(j))*D - sum(n(j)))/sum(k(x))
   o step 2: e = max(Emin, min(a, Emax)), where Emin = 0.1 and Emax =
      0.5.

   Notice that step 2 limits the range of "e" to [Emin, Emax].

4.2   Minimizing TSU/TSA Overhead

   If the splitting ratio update is too small, the GMA receiver
   SHOULD NOT initiate a TSU/TSA exchange to minimize signaling
   overhead. Using s'(i) and s(i) to indicate the current and new
   splitting ratio respectively, the TSU/TSA exchange will be
   triggered only if

                         max(|s'(i) - s(i)|) > B3

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   Here, B3 is a configurable threshold, e.g. 3%.

4.3   Adaptive Splitting Burst Size

   The traffic splitting control granularity is given by 1/L. Larger
   the splitting burst size L, finer the control granularity. L also
   controls the congestion measurement interval. Smaller the
   splitting burst size, shorter the measurement interval so that the
   algorithm will converge faster. Therefore, L should be set as
   large as possible but not increasing the congestion measurement
   interval, i.e. D . T1. Let's denote L as

                            L = 2^(max(p, a3))

   Wherein, a3 is a configurable constant to determine the minimum
   splitting burst size. For example, with a3=3, we have L > or = 8.
   The GMA receiver SHOULD dynamically adjust p at the end of each
   measurement interval as

                 p = floor(log2(sum(n(i))* T1/(D * a2)))

   If D = T1, p can be simplified as

                      p = floor(log2(sum(n(i))/a2))

   For example, if sum(n(i)) = 50 and a2 = 2, we will have L = 16.

4.4   Traffic Splitting Ratio Quantization

   Let's use S(i) to indicate the number of packets sent to the i-th
   connection for every traffic splitting burst, i.e., sum(S(i))==L.
   S(i) is an integer in the range of [0, L], and the following
   method MAY be used to obtain S(i) from the traffic splitting ratio
   s(i), which is a decimal in the range of [0, 1].

   o step 1: determine S(j) for the connections with reduced traffic
      splitting ratio, i.e. s(j)<s'(j), where s'(j) is the current
      splitting ratio.

                         S(j)=round(L*s(j) - a4)

   o Step 2: determine S(i) for other connections:

 S(i)=round((L - sum(S(j)|s(j)<s'(j)))*s(i)/sum(s(i)|s(i)>or=s'(i)))

   o Step 3: adjust S(m) for the connection with highest traffic
      splitting ratio, i.e., S(m)==max(S(i)), to ensure sum(S(i))==L

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                        S(m)=S(m) + L - sum(S(i))

   Herein, a4 is a configurable constant, e.g. 0.3.

5 Security Considerations

   This proposal makes no changes to the underlying security of GMA
   protocol [GMA].

6 IANA Considerations

This document makes no requests of IANA.

7 References

7.1   Informative References

[MAMS] RFC 8743, "Multi-Access Management Services (MAMS)"
          <https://tools.ietf.org/rfc/rfc8743.txt>

[GMA] J. Zhu, M. Zhang, A UDP-based GMA (Generic Multi-Access)
          Protocol <https://www.ietf.org/archive/id/draft-zhu-
          intarea-gma-control-05.txt>

Authors' Addresses

   Jing Zhu

   Intel

   Email: jing.z.zhu@intel.com

   Menglei Zhang

   Intel

   Email: menglei.zhang@intel.com

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