Internet DRAFT - draft-widjaja-mpls-mate


Internet Engineering Task Force                            Indra Widjaja
                                          Fujitsu Network Communications
INTERNET DRAFT                                             Anwar Elwalid
Expired in six months                     Bell Labs, Lucent Technologies
                                                             August 1998

                MATE: MPLS Adaptive Traffic Engineering

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   This document describes an MPLS Adaptive Traffic Engineering scheme,
   called MATE. The main goal of MATE is to avoid network congestion by
   balancing the loads among the LSPs.  MATE makes minimal assumptions
   in that the intermediate LSRs are not required to perform traffic
   engineering activities or measurements beside forwarding packets.
   Moreover, MATE does not impose any particular scheduling, buffer
   management, architecture, or a priori traffic characterization, on
   the LSRs.

1.0 Introduction

   One of the main advantages of MPLS is its efficient support of expli-
   cit routing through the use of Label Switched Paths (LSPs) [1][2].
   With destination-based forwarding as in the conventional datagram
   networks, explicit routing is usually provided by attaching to each

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   packet the network-layer address of each node along the explicit
   path.  This approach makes the overhead in the packet prohibitively
   expensive.  Switched forwarding using label swapping in MPLS enables
   a label to identify an LSP regardless of whether the LSP is esta-
   blished through hop-by-hop or explicit routing.  In other words, MPLS
   imposes the same amount of packet header overhead irrespective of the
   use of explicit routing.

   Explicit routing has useful applications such as Virtual Private Net-
   works (VPNs) and Traffic Engineering.  This memo focuses on engineer-
   ing the traffic across multiple explicit routes.  The purpose of
   traffic engineering is to manage network resources as efficiently as
   possible so that congestion is minimized.  This may be done by avoid-
   ing links that are already heavily stressed. Traffic engineering typ-
   ically becomes more effective as the network provides more alternate
   paths. Traffic engineering also becomes more critical in large Auto-
   nomous Systems where maximal operational efficiency should be
   emphasized [3].

   It is envisioned that traffic engineering is performed only for
   traffic that does not require resource reservation, but may need pro-
   visioning on an aggregated basis.  Examples include best-effort and
   differentiated services.  This memo proposes that traffic engineering
   be done by establishing multiple LSPs between a given ingress LSR and
   a given egress LSR in an MPLS domain. The main objective is to have
   the ingress LSR distribute traffic across the multiple LSPs effec-
   tively so that network resources among the LSPs are equitably util-
   ized. This memo describes a scheme called MPLS Adaptive Traffic
   Engineering (MATE).  It is to be noted that MATE is intended for
   quasi-stationary situations where traffic statistics changes rela-
   tively slowly (much longer than the round-trip delay between the
   ingress and egress LSRs). Recent measurements in the Internet indi-
   cate traffic stationarity in at least 5-min intervals [4].

   Some of the features of MATE include:

     o Traffic engineering on a per-class basis.
     o Distributed load-balancing algorithm.
     o End-to-end protocol between ingress and egress LSRs.
     o No new hardware or protocol requirement on intermediate LSRs,
       nor a priori traffic distributions.
     o No assumption on the scheduling or buffer management
       schemes at the LSR.
     o Optimization decision based on LSP congestion measure.
     o Minimal packet reordering due to traffic engineering.
     o No clock synchronization between two LSRs.

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2.0 Traffic Engineering Using MATE

   This section provides the description of MATE.  While the specifics
   of the scheme are tailored for best-effort service, the basic princi-
   ples are intended to be extensible to other services (e.g., differen-
   tiated service) as well.

2.1 Overview

   It is assumed that M explicit LSPs have been established between an
   ingress LSR and an egress LSR when traffic engineering is to be
   facilitated between these two LSRs.  The value of M is typically
   between 2 and 5 (the case with M=1 is not interesting).  The M LSPs
   may be chosen to be the "best" M paths calculated by a link-state
   protocol or configured manually. Explicit LSPs may be established by
   RSVP, LDP, or other means.  The mechanism to select and establish the
   LSPs is beyond the scope of this document.

   Once the LSPs are setup, the ingress LSR tries to distribute the
   traffic across the LSPs so that the traffic loads are balanced and
   congestion is thus minimized. The traffic to be engineered at the
   ingress LSR is the aggregated flow (called traffic trunk in [5]) that
   shares the same Forwarding Equivalence Class.  This document assumes
   that the traffic to be engineered consists of the best-effort ser-
   vice. Future work will consider differentiated service.

   In order to perform effective traffic distribution, the characteris-
   tics of the LSPs and the QoS requirements of the traffic must be
   known.  In general, the LSP characteristics may include the average
   packet delay, packet delay variance, loading factor/utilization,
   packet loss rate, bottleneck bandwidth, available bandwidth, etc.
   For best-effort traffic, there is no explicit QoS requirement, except
   that it is desirable to have minimum packet loss rate.  Since MATE is
   intended to be as flexible as possible, the pertinent LSP charac-
   teristics are not assumed to be given quantities, but must be gath-
   ered through some measurement.  In MATE, the ingress LSR transmits
   probe packets periodically to the egress LSR which will return the
   probe packets back to the ingress LSR.  Based on the information in
   the returning probe packets, the ingress LSR is able to compute the
   LSP characteristics.  Intermediate LSRs are not required to modify
   the contents of the probe packets, but such optional capabilities may
   be used to refine the measurement process.

   MATE employs a four-phase algorithm for load balancing.  The first
   phase initializes the congestion measure for each LSP.  The conges-
   tion measure may be a function of delay derivative and packet loss.

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   In the second phase, the algorithm tries to equalize the congestion
   measure for each LSP. Once the measures are equalized, the algorithm
   moves to the third phase.  The algorithm monitors each LSP in the
   third phase. If an appreciable change in the network state is
   detected, the algorithm moves to the fourth phase where the conges-
   tion measures are appropriate adjusted. Then, the algorithm goes to
   the second phase and the whole process repeats.

2.2 Traffic Filtering and Distribution

   MATE performs a two-stage traffic distribution.  First, MATE distri-
   butes the engineered traffic for a given ingress-egress pair equally
   among N bins at the ingress LSR. If the total incoming traffic to be
   engineered is of rate R bps, each bin receives an amount of r = R/N
   bps.  Then, the traffic from the N bins is mapped into M LSPs accord-
   ing to the rule defined below.

   The engineered traffic can be filtered and distributed into the N
   bins in a number of ways.  A simple method is to distribute the
   traffic on a per-packet basis without filtering.  For example, one
   may distribute incoming packets at the ingress LSR to the bins in a
   round-robin fashion.  Although it does not have to maintain any per-
   flow state, the method suffers from potentially having to reorder an
   excessive amount of packets for a given flow which is undesirable for
   TCP applications.

   On the other extreme, one may filter the traffic on a per-flow basis
   (e.g., based on <source IP address, source port, destination IP
   address, destination port, IP protocol> tuple), and distribute the
   flows to the bins such that the loads are similar.  Although per-flow
   traffic filtering and distribution preserves packet sequencing, it
   has to maintain a large number of states to keep track of each active

   Another method, which is described in [6], is to filter the incoming
   packets by using a hash function on the IP field(s). The fields can
   be based on the source and destination address pair, or other combi-
   nations.  A typical hash function is based on CRC.  The purpose of
   the hash function is to randomize the address space to prevent hot
   spots.  Traffic can be distributed into the N bins by applying a
   modulo-N operation on the hash space.  Note that packet sequence for
   each flow is maintained with this method.

   After the engineered traffic is distributed into the N bins, a second
   function maps each bin to the corresponding LSP.  The rule for the
   second function is very simple.  If LSP(i) is to receive twice as
   much traffic as LSP(j), then LSP(i) should receive traffic from twice

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   as many bins as LSP(j).  The value N should be chosen so that the
   smallest traffic that can be shifted, which is equal to 1/N of the
   total incoming traffic, has reasonable granularity.

2.3 Traffic Measurement

   The efficacy of any traffic engineering scheme depends crucially on
   the traffic measurement process. MATE does not require each LSR to
   perform traffic measurement. Only the ingress and egress LSRs are
   required to participate in the measurement process.

   For the purpose of balancing the loads on each LSP, the available
   bandwidth appears to be a desirable metric to measure. The methods
   for measuring the available bandwidth of a given path have been
   described in the past (e.g., see [7] and [8]).  Based on our experi-
   ence, this metric turns out to be difficult to measure accurately
   using minimal requirements assumed in MATE.

   To this end, we found that packet delays across the LSPs are the most
   reliable known metric. The delay of a packet on an LSP can be
   obtained by transmitting a probe packet from the ingress LSR to the
   egress LSR. The probe packet is time-stamped at the ingress LSR at
   time T1 and recorded at the egress LSR at time T2.  If the ingress's
   clock is faster than the egress's clock by Td, then the total packet
   delay (i.e, queueing time, propagation time, and processing time) is
   T2-T1+Td.  A bunch of probe packets can easily yield an estimate on
   the mean packet delay Tm+Td, where Tm is the long-term average of
   T2-T1.  The reliability of the estimator can be evaluated by
   bootstrapping or jacknife to give the confidence interval for the
   mean delay.  One important point to note is that the value of Td is
   not required when only the marginal delay is needed.  MATE exploits
   this property by relying only on marginal delays rather than absolute
   delays.  Therefore, clock synchronization is not necessary.

   Packet loss probability is another metric that can be estimated by a
   bunch of probe packets. In general, only reasonably high packet loss
   rates can be reliably observed.  Packet loss probability can be
   estimated by encoding a sequence number in the probe packet to notify
   the egress LSR how many probe packets have been transmitted by the
   ingress LSR, and another field in the probe packet to indicate how
   many probe packets have been received by the egress LSR.  When a
   probe packet returns, the ingress LSR is able to estimate the one-way
   packet loss probability based on the number of probe packets that has
   been transmitted and the number that has been received.  The advan-
   tage of this approach is that it is resilient to losses in the
   reverse direction.

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2.4 Objective Function

   For best-effort traffic, our objective is to avoid congested LSPs.
   The congestion measure can be characterized by the sensitivity of the
   LSP.  An LSP is qualitatively said to be sensitive if a perturbation
   in the load changes the mean delay significantly.  MATE computes the
   derivative of the mean packet delay with respect to the offered load
   to quantify the sensitivity of an LSP.  For a given LSP, the deriva-
   tive increases as the load increases.  This is evident since the mean
   delay is an increasing and convex function with respect to the load.
   The derivative can be derived by observing the mean delays at two
   different loads.  One advantage for using the sensitivity measure is
   that the fixed propagation delay has no effect on the derivative.

   An LSP is said to be lossless if each LSR along the LSP never dis-
   cards packets. Otherwise, the LSP is said to be lossy.  Although the
   lossless case may not be realistic, it is much simpler to understand.
   Here, MATE performs load balancing by simply equalizing the deriva-
   tives across the M LSPs.  The implementation is much simplified since
   only the relative derivatives should be known.  MATE essentially
   applies a Min-Max operation; that is minimizing the load on an LSP
   that has maximum congestion.  The objective is also equivalent to
   minimizing the sum of all LSP delays.  MATE can be easily made to
   respond to changes in network state adaptively as long as the traffic
   is quasi-stationary.  Network changes can be tracked by periodically
   measuring packet delays.  If the changes in delays exceed a certain
   threshold, the algorithm minimized the most congested LSP by re-
   equalizing the derivatives.

   For the lossy case, the packet loss information must be incorporated
   into the load balancing process.  The lossy case will be treated in
   the later section.

2.5 State Table

   MATE keeps a state table for each traffic class that will be
   engineered between an ingress LSR and an egress LSR. The state table
   needs to maintain the mean delay and derivative of each LSP. One con-
   venient realization is depicted below:

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     |      LSP        |     D_old    |    D_new    |   Inc   |
     |       1         |              |             |         |
     |       2         |              |             |         |
     |       3         |              |             |         |
     |      ...        |              |             |         |
     |       M         |              |             |         |

   D_old (D_new) denotes the mean packet delay before (after) a traffic
   shift is made to the LSP.  Inc (for Increments) denotes the number of
   bins for which traffic was recently shifted to/from the LSP.

   The derivative for LSP(i) can be simply computed by |D_new(i) -

2.6 Algorithm: Lossless Case

   We now present the algorithm in an ideal case with quasi-stationary
   traffic, zero packet loss, and perfect measurements.

   Initially, the traffic to be engineered at the ingress LSR is sent
   through the shortest path, say through LSP(1).  The algorithm can be
   broken into four phases:

   Initially the state table is empty, and phase 1 is run.

   Phase 1: Initialize delays and derivatives

          Inc(1) <- N
          for i from 2 to M
                    Send probe packets on LSP(1) and LSP(i)
                    Compute mean packet delays D_old(1) and D_old(i)
                    Shift [N/M] bins of traffic from LSP(1) to LSP(i)
                    Inc(1) <- N - [N/M]
                    Inc(i) <- [N/M]
                    Send probe packets on LSP(1) and LSP(i)
                    Compute mean packet delays D_new(1) and D_new(i)
          Go to Phase 2

   Here, [N/M] indicates the largest integer less than or equal to N/M.
   At the end of Phase 1, all columns of the state table are filled up.
   The traffic at the ingress LSR is distributed almost equally across all M LSPs.

   Phase 2: Equalize derivatives

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                    Let LSP(i) be the LSP that has the highest derivative
                    Let LSP(j) be the LSP that has the lowest derivative
                    D_old(i) <- D_new(i)
                    D_old(j) <- D_new(j)
                    Shift n bins of traffic from LSP(i) to LSP(j)
                    Send probe packets on LSP(i) and LSP(j)
                    Compute mean packet delays D_new(i) and D_new(j)
                    Inc(i) <- n
                    Inc(j) <- n
          while (|D_old(i) + D_old(j)| > |D_new(i) + D_new(j)|)
          Go to Phase 3

   The main stopping rule for the optimization process is
   when the total LSP delay can no longer be decreased.
   This is achieved when all derivatives are approximately equal.
   Note that phase 2 actually stops when the total LSP delay overshoots.
   A compensation could be made by shifting back
   the traffic from LSP(j) to LSP(i) after the overshoot is detected.

   The parameter n (n = 1, 2, ..., N)
   determines the amount of traffic to be shifted.
   A small value of n may cause the "noise" to mask the traffic
   that is shifted. On the other hand, a large value of n may
   overshoot and overload the LSP. Thus, this value should
   be properly engineered, and can be made adaptive.
   It is to be understood that MATE cannot shift more bins
   than it currently has for a given LSP.
   The algorithm terminates if no bin is available from
   the LSP with the lowest derivative.

   By the mean value theorem, the computed derivative is
   equal to the actual derivative between the points before and after
   the traffic is shifted.

   Phase 3: Track network dynamics

          Send probe packets intermittently on each LSP(i)
          Compute running mean packet delay D(i)
          If (|D(i) - D_new(i)| > alpha * |D_new(i) - D_old(i)|)
                    Go to Phase 4

   The condition in Phase 3 basically detects a network change by moni-
   toring an appreciable change in an LSP load.

   The amount of probe packets sent at phase 3 should be negligible com-
   pared to the engineered traffic that is sent to the same LSP (say,
   less than 1 percent). The parameter alpha determines the threshold on

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   the change in load.

   The mean packet delay on LSP(i) is updated by a low-pass filter:
           D(i) <- beta * D(i) + (1 - beta) * Measured delay
   where beta is weighting constant between 0 and 1, and D(i) is ini-
   tialized to D_new(i).

   In order to minimize synchronization due to multiple ingress LSRs
   detecting the network change simultaneously, it may be useful to ran-
   domize the detection time with a random backoff time. There are other
   schemes to eliminate synchronization.

   Phase 4: Refresh state table

          LSP(i) detects a significant change in its marginal delay
          set Flag(i)
          if (D(i) > D_new(i))
                    D_old(i) <- D(i) - 2 * (D_new(i) - D_old(i))
                    D_new(i) <- D(i)
                    Inc(i) <- 1
                    D_old(i) <- D(i) - (D_new(i) - D_old(i)) / 2
                    D_new(i) <- D(i)
                    Inc(i) <- 1
          For each LSP k such that (k != i)
                    D_old(k) <- D(k) - (D_new(k) - D_old(k))
                    D_new(k) <- D(k)

          Go to Phase 2

   When LSP(i) detects a significant change in its marginal delay, it
   changes  doubles or halfs its derivative depending on whether the
   delay has increased or decreased, respectively.  The associated flag
   is set to ensure that LSP(i) will be considered in Phase 2.  D_new's
   and D_old's of the rest of the LSPs are updated with the latest
   information. All computations should check for illegal values.

2.7 Time Scales

   Since MATE relies on measurements to estimate LSP characteristics, it
   is important to understand the time scale each phase typically
   operates.  After the initialization in Phase 1, MATE alternates in
   Phase 2, Phase 3 and Phase 4. MATE is designed to execute in Phase 2
   on the order of 1 minute. The effectiveness in this phase depends
   critically on the stationarity of the traffic.  Recent measurements
   indicate that traffic in the wide area network is stationary in at
   least a 5-minute interval [4].  Phase 3 is intended to detect a

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   persistent and appreciable change in the network after the algorithm
   has stabilized in Phase 2.  In general, Phase 3 operates at a much
   longer time scale (on the order of 30 minutes or longer).  Phase 4 is
   executed to update the State Table immediately after Phase 3. Phase 4
   typically operates on the order of a second.

2.8 Monotonous Property of Delay versus Load

   To cope with "noise" in the cross/background traffic and measurement
   errors, we describe some improvements to the above algorithm by using
   the property of delay-load curve.

   It is possible for the computed mean delay to violate the monotonous
   property of delay versus load.  A violation may occur in two cases.
   In the first case, the mean delay decreases when traffic is shifted
   into an LSP.  In another case, the mean delay increases when traffic
   is shifted out of an LSP.  The violation can be noted by associating
   an attribute flag to each delay variable in the state table.

   In order to correct an anomaly due to violation, MATE always repeats
   the measurement on the pair that is involved in violation until there
   is no more violation detected.  In the repetition, traffic is shifted
   back and forth between the pair until no more violation is detected
   in each LSP.

2.9 Inflection

   Another observation is that the mean delay is typically a convex
   function of the offered load, except beyond a certain point (called
   the inflection point) where the LSP becomes heavily congested.

   When MATE encounters an inflection point during the iteration pro-
   cess, it should adjust D_old so that the higher derivative between
   the previous and the current iterations is used.  This decision makes
   the derivative estimate more conservative when the inflection point
   is due to heavy congestion.

2.10 Dynamic Shift

   The number of bins to be shifted (Inc) can be dynamically adjusted as
   follows. If one of the LSPs to be shifted consists of an LSP whose
   new mean delay was invalidated previously due to a violation, then
   the number of bins to be shifted will be doubled from the previous
   value, subject to the maximum number of bins that can be shifted.
   Otherwise, the number of bins to be shifted will be halfed from the

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   previous value, subject to the minimum number of bins that is

2.11 Lossy Case

   The base algorithm described in the previous section assumes an ideal
   case where there is no packet loss.  This section incorporates the
   effect of packet loss on the algorithm by adjusting the derivative
   with a loss factor.

   MPLS traffic is likely to be dominated by TCP applications, in which
   case the packet loss inside an MPLS domain should be tightly con-
   trolled since TCP reacts to packet losses. It is well-known that TCP
   achieves bandwidth that is inversely proportional to the square root
   of the packet loss probability (p), under idealized condition. Note
   that this relationship is typically reasonable only in a certain
   regime of p (roughly, p is not too close to zero or one).

   We use a typical delay-load function which is convex and increasing
   in load.  The delay-load function blows up when the arrival rate
   approaches the service rate, which can be approximated by the simple

           D = A / (mu - lambda),                          (1)

   where mu is the offered traffic, lambda is the average service rate,
   and A is some constant. Most queueing systems would have the same
   factor in the denominator as the dominant one.  The derivative of the
   delay with respect to the offered traffic can then be computed by

           D' = A / (mu - lambda)^2.                       (2)

   Since the available bandwidth for the lossless case is

           BW_lossless = mu - lambda,                      (3)

   we can relate the lossless available bandwidth to the derivative by

           BW_lossless^2 = A / D'.                         (4)

   Because the bandwidth achieved by TCP is reduced by a factor of
   square root of p when there are packet losses, the available
   bandwidth for the lossy case can be correspondingly reduced by the
   same factor. In other words, we can write

           BW_lossy^2 = BW_lossless^2 * C / p,             (5)

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   where C is set to the lowest observable loss probability through the
   probe packets. If MATE cannot determine the packet loss probability
   because of its rare occurrences, it sets p to C.  Comparing (4) and
   (5), the lossy available bandwidth can be related to the derivative

           BW_lossy^2 = C / (p * D').                      (6)

   Therefore, equalizing (p * D') is equivalent to equalizing the avail-
   able bandwidth for the lossy case.

   It is interesting to see that one may define another metric called
   Power which is defined as the ratio of throughput to delay.  If the
   objective is to equalize power, one will find that this is equivalent
   to equalizing (p * D').

   For the lossy case, Phase 2 of the algorithm stops when the weighted
   delay increases.  In Phase 3, it is not possible to measure the
   packet loss rate since the probe traffic is low.  Instead, a lost
   probe packet is accounted for by assigning D(i) to the maximum
   measurable delay.

3.0 Protocol Specification

   To be added.

4.0 Security Considerations

   Security considerations are not addressed in the present document.

5.0 Acknowledgement

   The authors thank Cheng Jin for implementing MATE and suggesting
   improvements, and Dimitri Stiliadis and Eric Gray for providing
   insightful comments.

6.0 References

  [1] R. Callon, P. Doolan, N. Feldman, A. Fredette,
  G. Swallow and A. Viswanathan,

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  "A Framework for Multiprotocol Label Switching,"
  Internet Draft <draft-ietf-mpls-framework-02.txt>,
  Nov 1997.

  [2] E.C. Rosen, A. Viswanathan, and R. Callon,
  "Multiprotocol Label Switching Architecture,"
  Internet Draft <draft-ietf-mpls-arch-01.txt>, Mar 1998.

  [3] D.O. Awduche et. al., "Requirements for Traffic
  Engineering over MPLS,"
  Internet Draft <draft-awduche-mpls-traffic-eng-00.txt>,
  April 1998.

  [4] K. Thompson, G.J. Miller, and R. Wilder, "Wide-Area
  Internet Traffic Patterns and Characteristics, "
  IEEE Networks, vol. 6, no. 6, Dec. 1997.

  [5] T. Li and Y. Rekhter, "Provider Architecture for
  Differentiated Services and Traffic Engineering (PASTE),"
  Internet Draft <draft-li-paste-00.txt>, Jan 1998.

  [6] C. Villamizar, "OSPF Optimized Multipath (OSPF-OMP),"
  Internet Draft <draft-ietf-ospf-omp-00.txt>, Mar 1998.

  [7] S. Keshav, "A Control-Theoretic Approach to Flow
  Proc. SIGCOMM'91, pp.3-15, Sep. 1991.

  [8] R.L. Carter and M.E. Crovella, "Measuring Bottleneck
  Link Speed in Packet-Switched Networks," Technical Report,
  BU-CS-96-006, Boston University, Mar. 1996.

Author Information:

   Indra Widjaja
   Fujitsu Network Communications
   4403 Bland Road
   Raleigh, NC 27609, USA
   Phone: 919 790 2037

   Anwar Elwalid
   Bell Labs Lucent Technologies
   Murray Hill, NJ 07974, USA
   Phone: 908 582-7589

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