Abstract:
A new algorithm for clock offset estimation for resources distributed across a network (such as the Internet). By exchanging a sequence of time-stamped messages between pairs of network nodes and separately estimating variable delays for each message direction, present inventive embodiments provide more accurate estimates for clock offset between node pairs. Present inventive algorithms operate in a variety of peer and server network configurations while providing significant improvement in convergence speed and accuracy.

Description:
RELATED INVENTION  
       [0001]    This application is related to subject matter disclosed in co-pending, commonly owned application Ser. No. 09/740,252 by present applicants and others, which co-pending application is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to synchronization of clocks. More specifically, the present invention relates to estimation and correction of clock offset in distributed resources interconnected by a network, such as the Internet.  
         BACKGROUND OF THE INVENTION  
         [0003]    Accurate and reliable time information is necessary for many systems and applications involving distributed resources, including networked systems and processes typified by the Internet. In such systems, different functional elements are required to have their clocks synchronized. Clock synchronization involves two aspects: frequency synchronization and time synchronization. The former means that element clocks run at the same frequency, and the latter means that elements agree at a particular epoch with respect to the coordinated universal time (UTC), i.e., there is no offset between element clocks. For many purposes, it is appropriate to focus on estimating clock offset and to assume higher order effects, such as the frequency offset, can be ignored or provided for separately.  
           [0004]    One application in which clock synchronization becomes essential is in the transport of speech packets over the Internet. Thus, for example, if a network element knows the scheduled playout time of a voice packet at a destination and shares a common notion of time with the destination, it can schedule the transmission of the voice packet to the destination accordingly. Alternatively, if the network element knows that the playout time for the voice packet has passed, it can discard the packet and avoid unnecessarily transmitting it to the destination where it would be discarded anyway. Another example application in which synchronization proves necessary is in the transport of packets from a core network to base stations in a wireless network. That is, transmission of packets on a radio link (e.g., from the base station to the mobile device) is usually synchronous, and it is important to ensure that the core network delivers the packet to the base station in time.  
           [0005]    In certain wireless cellular systems, it is possible to transmit multiple copies of the same frame from different base stations to a mobile device whenever the mobile device can receive a sufficiently strong average signal from both base stations, although there will still be instantaneous variations on the received signal due to fast fading. The mobile device then combines the multiple copies prior to decoding. In an unreliable radio environment this greatly improves the probability that the frame is received correctly due to signal diversity. This technique, called soft handoff or sometimes macrodiversity, relies on the fact that the transmissions from the multiple base stations are synchronized in time, so that the mobile can combine the copies without ambiguity. In this context as well it is important for a common notion of time across multiple base stations when needed and an efficacious way to achieve this is through the mechanism and processing technique described in this invention. Therefore, it is often important that the base stations be synchronous with the core network.  
           [0006]    Synchronizing the clocks is a two step process: first, the magnitude of the offset between the clocks is estimated, and then one of the clocks is adjusted, as needed. Clocks inherently tend to drift and therefore offset estimation and correction must be done on a continuing basis.  
           [0007]    Clock synchronization issues have been extensively addressed in the literature. See, for example, D. Mills, Internet time synchronization: the Network Time Protocol,  IEEE Trans. Communications,  Vol. 39, No. 10, Oct. 1991; D. Mitra, Network synchronization: analysis of a hybrid of master-slave and mutual synchronization,  IEEE Trans. Communications,  COM-28, 8 (Aug. 1980), pp. 1245-1259; and N. W. Rickert, Non Byzantine clock synchronization—a programming experiment,  ACM Operating Systems Review  22,1 (Jan. 1988), pp. 73-78.  
           [0008]    A well-known clock synchronization protocol that has been successfully deployed in the Internet is the Network Time Protocol (NTP), described, for example, in D. Mills,  Network Time Protocol  ( version  3)  Specification, Implementation and Analysis RFC  1305, Mar. 1992. One of the most important network clock synchronization issues addressed by NTP is how to use the collected data to estimate the clock offset between a pair of network elements.  
           [0009]    In V. Paxson, On Calibrating Measurements of Packet Transit Times, LBNL-41535, ftp://ftp.ee.lbl.gov/papers/vp-clocks-sigmetrics98.ps.gz, Mar., 1998 (and in a shortened paper with the same title published in  Proc. ACM Sigmetrics 98, Jun. 22-26, 1998), the author proposed a new algorithm for clock offset estimation. For easy reference, this algorithm will be referred to as the Separate Direction Estimation Algorithm (SDEA) and the cited papers as Paxson. While SDEA can provide improved performance relative to the NTP algorithm, SDEA nevertheless suffers from significant limitations, especially in applying SDEA to contexts in which loading is different for each of the directional links between pairs of network elements. The incorporated application describes techniques by which limitations of the NTP algorithm and SDEA are overcome and SDEA techniques are extended and improved. Increasingly stringent demands on network performance have caused workers in the field to seek other broadly applicable improved clock offset estimation techniques.  
         SUMMARY OF THE INVENTION  
         [0010]    Limitations of the prior art are overcome and a technical advance is made in accordance with the present invention, illustrative embodiments of which are presented in the following detailed description. In particular, the present inventive provides methods for overcoming limitations of the NTP algorithm and SDEA in deriving estimates of clock offset with increased lower error.  
           [0011]    In addition to estimating the clock offset, present inventive techniques also develop an estimator for the bias of the offset estimator and provide a bias-corrected offset estimator. As a result, for typical transmission delay distributions, the present inventive algorithms show significant reductions in their mean squared error.  
           [0012]    In accordance with one aspect of present inventive methods, messages are exchanged (bi-directionally) between pairs of network elements, such messages including timestamps indicative of sending and receiving times noted at each stop. Because variable components of delay for each message direction need not be characterized by identical probability distribution functions, e.g., when links in each direction are differently loaded, undesired bias of estimates for clock offset can emerge. The present inventive methods propose a new estimator based on the exchange of timing messages, subsequently referred to as the sufficient statistic (SS) estimator. The SS estimator is shown, under one typical set of assumptions, to be the minimum variance estimator within the class of linear unbiased estimators based on the ordered one-way delay measurements. Illustrative embodiments of the present invention further employ a bootstrap methodology to obtain a closed form estimator of clock offset bias from which a bias-corrected estimator of the clock offset can be derived. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    [0013]FIG. 1 illustrates time relations in sending messages between network elements.  
         [0014]    [0014]FIG. 2 shows an illustrative network embodiment of the present invention having a single server node for interacting with a plurality of other nodes to effect clock correction at such other nodes.  
         [0015]    [0015]FIG. 3 shows an illustrative network embodiment of the present invention having a hierarchy of server nodes with lowest order server nodes for interacting with other network nodes to effect clock correction at such other nodes. 
     
    
     DETAILED DESCRIPTION  
       [0016]    Network clock synchronization of a pair of network elements typically involves exchanges of data packets (timing messages) between each element of the pair. One of such a pair of elements is referred to as a sender, and the other as a receiver with respect to a particular packet. Based on time stamps contained in these timing messages, the clock offset between the sender and the receiver is estimated.  
         [0017]    More specifically, timing messages are sent between network elements as shown in FIG. 1, where activities at a sender A ( 100 ) and a receiver B ( 110 ) are shown for a particular round of message exchanges. FIG. 1 illustrates the i-th exchange between A and B, that begins with A sending a message to B which includes a time stamp T i   0  indicating the time as known at A when the message is sent. Immediately upon reception of this message, B puts a time stamp T i   1  in the received message. Just before B sends the message back to A (which is not necessarily immediately), it places another time stamp T i   2  on the message. When A receives the returned message from B, it records another time stamp T i   3  on the message. After n such exchanges, A has the sequence of observations  
         {       T   i   0     ,     T   i   1     ,     T   i   2     ,     T   i   3       }       i   =   1     n                         
 
         [0018]    from which to estimate the clock offset between itself and B.  
         [0019]    Define θ to be the (unknown) offset of clock B, relative to clock A. It follows that if at a given instant clock B shows time t B  and clock A shows time t A  then t B =t A +θ. The sojourn times for the A→B and B→A timing messages are X i =T i   1 −T i   0  and Y i =T i   3 −T i   2 , respectively. Apart from the offset of clock B from clock A, two other components of the message sojourn times are propagation delay and network delay. Propagation delay corresponds to the amount of time required for a timing message to travel across the transmission medium (e.g., optical fiber) between A and B if there were no other intervening delays. In general, the propagation delay is fixed and quite small, on the order of nanoseconds, for example. It is customarily assumed that the (unknown) propagation delays in the two transmission directions A→B and B→A are approximately the same, and we denote the common value by d. Network delay arises due to congestion in the network that requires messages to sit in one or more queues before being granted the resources needed to continue its journey. Network delay can be on the order of milliseconds, or even seconds, and is the variable component of sojourn times. We let e i   AB  and e i   BA  denote the network delays associated with the A→B and B→A timing messages, respectively.  
         [0020]    Define X i =d+θ+e i   AB  and Y i =d−θ+e i   BA . In general the two directions would have different traffic characteristics and thus the network delays in each path are potentially different. Given a certain number of samples, N, Paxson, supra, proposed estimating θ by  
           θ   ^     =       (         min     1   ≤   i   ≤   N                       X   t       -       min     1   ≤   i   ≤   N                       Y   i         )     /   2       ,                         
 
         [0021]    based on the notion that the minimum sample value in each direction yields the sample that has the least amount of network delay associated with it. Note that the unknown network delay in each direction serves as “noise” in the offset estimation process and therefore, the minimum sample in each direction may be viewed as the clock offset sample least corrupted by “noise”. Consequently, it represents the samples of X and Y that are taken by Paxson as closest to θ.  
         [0022]    Derivation of Sufficient Statistic Estimator  
         [0023]    The present invention includes a method for processing the samples {X i : i=1, 2, . . . , N} and {Y i : i=1, 2, . . . , N} that yields a considerably more accurate estimate of θ than Paxson&#39;s method cited above. The development of the present inventive estimating method, hereinafter referred to as the Sufficient Statistic (SS) estimator will now be described in detail.  
         [0024]    The SS estimator is derived under the assumption that the unknown network delays e i   AB  and e i   BA  are exponentially distributed random variables. However, as will be illustrated later, the improved accuracy it exhibits is robust to the exponential assumption. The mean of the exponential distribution for all samples in a given direction is assumed to be the same, but the mean value in the two directions may be different. The derivation of the SS estimator and its properties are now described.  
         [0025]    Suppose FAB (·) and F BA (·) are exponential distributions with mean values λ AB  and λ BA , respectively. It follows that F(x)=1−e −λ     AB     (x−θ−d) ,x≧θ+d, and G(y)=1−e −λ     AB     (y−θ−d) , y≧θ+d. The likelihood function, L≡L(θ,d,λ AB ,λ BA ), based on the observations {(X i ,Y i )} i=1   n  is  
           L={λ   AB   n   e   nλ     AB     (θ+d)   e   −λ     AB     T     2   ×1( T   1   ≧θ+d )}×{λ BA   n   e   nλ     BA     (−θ+d)   e   −λ     BA     S     2   ×1( S   1   ≧θ+d )},  
         [0026]    where (T 1 , T 2 , S 1 , S 2 )  
         (       T   1     ,     T   2     ,     S   1     ,     S   2       )     =     (           min     1   ≤   i   ≤   n                       X   t       -       ∑     i   =   1     n          X   i         ,       min     1   ≤   i   ≤   n                       Y   i       ,       ∑     i   =   1     n          Y   i         )                           
 
         [0027]    is a sufficient statistic (see, for example, Mood, Graybill and Boes,  Introduction to the Theory of Statistics,  Prentice-Hall, 1997, for a discussion of a sufficient statistic). The marginal distributions of the sufficient statistic are easy to obtain. Indeed, T 1  and T 2  have translated (by θ+d) exponential and gamma distributions with parameters nλ AB  and (n,λ AB ), respectively. Likewise, S 1  and S 2  have translated (by −θ+d) exponential and gamma distributions with parameters nλ BA  and (n,λ BA ), respectively. It follows that the sufficient statistic estimator,  
         θ   ~     =       1   2          [           nT   1     -       T   2     /   n         n   -   1       -         nS   1     -       S   2     /   n         n   -   1         ]                             
 
         [0028]    is an unbiased estimator of θ.  
         [0029]    We now provide an argument that shows {overscore (θ)} is the best linear unbiased estimator (BLUE) of θ based on the order sequences {X (i) } i=1   n  and {Y (i) } i=1   n  (i.e., {overscore (θ)} is the ordered-BLUE or o-BLUE of θ) 1 . For arbitrary sequences of constants, {a i } i=1   n  and {b i } i=1   n , consider the estimator  
       T   =         ∑     i   =   1     n            a   i          X     (   i   )           +       ∑     i   =   1     n            b   i            Y     (   i   )       .                                 
 
         [0030]    For 1≦i≦n, define u,  
         μ   i     =       ∑     j   =     n   -   i   +   1       n          1   /   j                             
 
         [0031]    and  
         σ   2     =       ∑     j   =     n   -   i   +   1       n          1   /       j   2     .                               
 
         [0032]    It is well know (see, for example, H. A. David,  Order Statistics,  John Wiley, 1981, chapter 3) that E(X (i)) )=μ i / AB , Var(X (i) )=σ i   2 /λ 2   AB , and for 1 ≦r≦s≦n, Cov (X   (r) ,X (s) )=Var(X (r) ). Replacing λ AB  with λ BA  gives the corresponding expressions for the means, variances and covariances of the {Y (i)}   i=1   n  In sequence. It follows that  
               Var        (   T   )       =         ∑     i   =   1     n            a   i   2            σ   i   2       λ   AB   2           +     2          ∑     i   =   1     n            ∑     j   =     i   +   1       n            a   i          a   j            σ   i   2       λ   AB   2               +       ∑     i   =   1     n            b   i   2            σ   i   2       λ   BA   2           +     2          ∑     i   =   1     n            ∑     j   =     i   +   1       n            b   i          b   j            σ   i   2       λ   BA   2                         (   8   )                               
 
         [0033]    It can also be shown that there are four necessary and sufficient conditions for T to be an unbiased estimator of θ and they are  
                   ∑     i   =   1     n          a   i       =     1   /   2       ,         ∑     i   =   1     n          b   i       =       -   1     /   2       ,         ∑     i   =   1     n            a   i          μ   i         =   0     ,         ∑     i   =   1     n            b   i          μ   i         =   0.             (   9   )                               
 
         [0034]    Using Lagrange multipliers it can be shown that the minimum of (8), subject to (9), is achieved when b i =−a i  with a i =1/2+1/(2n), and a i =−1/[2n(n−1)], for 2≦i≦n. It follows that T≡θ and thus θ is the o-BLUE of θ.  
         [0035]    Two results that prove useful in considering the details of the Lagrange minimization are:  
         [0036]    1) the inverses of the variance-covariance matrices of {X (i) } i=1   n  and {Y (i) } i=1   n  are tridiagonal (useful when solving for the {a i } i=1   n  and {b i } i=1   n  in terms of the Lagrange multipliers), and  
         [0037]    2) for 2≦i≦n, (μ i −μ i−1 )/(σ i   2 σ i−1   2 )=n−i+1 (useful when determining the values of the Lagrange multipliers). Finally we note that in the special case where λ AB =λ BA , it can similarly be shown that θ is the o-BLUE of θ which provides new justification for Paxson&#39;s estimator.  
         [0038]    Mean Squared Error Comparison of {circumflex over (θ)} and {overscore (θ)} Under Exponential Delays  
         [0039]    Since {circumflex over (θ)}=(T i −S i )/2, it is easy to see that E({circumflex over (θ)})=θ+(1/λ ab −1/λ BA )/(2n) and Var(θ)=(1/λ AB   2 −1/λ BA   2 )/(4n 2 ). Consequently, the mean squared error of θ, under the exponential distribution assumptions, is  
               MSE        (     θ   ^     )       =           (       1   /     λ   AB       -     1   /     λ   BA         )     2     +     1   /     λ   AB   2       -     1   /     λ   BA   2           4        n   2                 (   10   )                               
 
         [0040]    The variance of {overscore (θ)} can be found by substituting the minimizing values of {a i}   i−1   n  and {b i } i=1   n  into (8). Alternatively, we can work directly from (7) but will need expressions for Cov(T 1 ,T 2 ) and Cov(S 1 ,S 2 ). (Note that (T 1 ,T 2 ) and (S 1 ,S 2 ) are independent implying the four cross-covariance terms are zero.) Since  
           T   2     =       ∑     i   =   1     n          X     (   i   )           ,                         
 
         [0041]    it follows that  
         Cov        (       T   1     ,     T   2       )       =         ∑     i   =   1     n                     Cov        (       X     (   1   )       ,     X     (   i   )         )         =       nVar        (     X     (   1   )       )       =     1   /       (     n                   λ   AB   2       )     .                                 
 
         [0042]    In the same way, Cov (S 1 , S 2 )=1/(nλ BA   2 ). Combining the variances and covariances of T 1 , T 2 , S 1  and S 2  with (7) gives  
               Var        (     θ   ~     )       =         1   /     λ   AB   2       +     1   /     λ   BA   2           4        n        (     n   -   1     )                   (   11   )                               
 
         [0043]    Table 1 presents illustrative results for the case where A→B Network Delays are exponential with mean=1 and B→A Network Delays are exponential with mean=5. Root Mean Squared Error (RMSE) is the square root of MSE, and it is clear that the SS estimator has significantly smaller RMSE than Paxson&#39;s estimator.  
                                     TABLE 1                               Root Mean           No. Samples   Squared Error            N   Paxson   SS               10   .324   .269       20   .162   .131       40   .0810   .0645                  
 
         [0044]    Bias Correction of {circumflex over (θ)} 
         [0045]    As pointed out above the estimator, {circumflex over (θ)}, is in general a biased estimator of θ. Since network delays in the two directions are often quite different, the bias can be significant in practice and some form of bias estimation and correction is warranted.  
         [0046]    In this section, we estimate the bias in {circumflex over (θ)} using the bootstrap technique and develop a bias-corrected estimator of θ. The bootstrap estimate of the bias of {circumflex over (θ)} yields to a closed form expression which is then used to define a bias-corrected estimator, {circumflex over (θ)} BC , of θ. It will be seen that {circumflex over (θ)} BC , like {circumflex over (θ)}, is a linear combination of the ordered sequences {X (i) } i=1   n  and {Y (i) } i=1   n .  
         [0047]    Let F and G denote the cumulative distribution functions of the variable portions of the A→B and B→A network delays, respectively. It follows that the cumulative distribution functions of X and Y are F(x)=F AB (x−θ−d), x≧θ+d, and G (y)=F BA (y+θ−d), y≧−θ+d, respectively. Furthermore, the independence of the A→B and B→A transmission delays implies the joint distribution of X and Y is H(x,y)=F(x)G(y). An iteration of n timing message exchanges yields a random sample {(X i , Y i )} i=1   n  of observations. The nonparametric estimator of H(x, y) is Ĥ(x,y)={circumflex over (F)}(x)Ĝ(y) where {circumflex over (F)}(x) and Ĝ(y) are the usual empirical cumulative distribution functions based on the  
           {     X   i     }       i   =   1     n                   and                     {     Y   i     }       i   =   1     n                           
 
         [0048]    sequences, respectively.  
         [0049]    Let B ({circumflex over (θ)}) denote the bias of {circumflex over (θ)}. Then,  
                     B        (     θ   ^     )       =           E   H          (         min     1   ≤   i   ≤   N                       X   t       -       min     1   ≤   i   ≤   N            Y   i         )       /   2     -   θ                 =         (         ∫   0   ∞              [     1   -     F        (   x   )         ]     n             x         -       ∫   0   ∞              [     1   -     G        (   y   )         ]     n             y           )     /   2     -     θ   .                     (   12   )                               
 
         [0050]    The bootstrap estimate of B({circumflex over (θ)}) is (see, e.g., Efron and Tibshirani,  An Introduction to the Bootstrap,  Chapman and Hall, 1993, section 10.2)  
                 B   ^          (     θ   ^     )       =             E     H   ^            (         min                   X   i         1   ≤   i   ≤   n       -       min                   Y   i         1   ≤   i   ≤   n         )       /   2     -     θ   ^       =         (         ∫   0   ∞              [     1   -       F   ^                (   x   )       ]     n             x         -       ∫   0   ∞              [     1   -       G   ^          (   y   )         ]     n             y           )     /   2     -       θ   ^     .                 (   13   )                               
 
         [0051]    Note that to obtain (13), the unknown joint distribution function H required by the expectation in (12) was replaced by the nonparametric estimator, Ĥ, and the unknown θ was replaced by {circumflex over (θ)}.  
         [0052]    Let {X (i) } i=1   n  and {Y (i) } i=1   n  denote the order statistics of the {X i } i=1   n  and {Y i } i=1   n  sequences, respectively. Define X (0) =Y (0) =0 and X (n+1) =Y (n+1) =∞. With I(·) as the usual indicator function, it follows that  
                   1   -       F   ^                (   x   )       =       ∑     i   =   1       n   +   1                n   -   i   +   1     n     ×     I        [       X     (     i   -   1     )       ≤   x   &lt;     X     (   i   )         ]                  
        and                        (   14   )                     1   -       G   ^                (   y   )       =       ∑     i   =   1       n   +   1                n   -   i   +   1     n     ×     I        [       Y     (     i   -   1     )       ≤   y   &lt;     Y     (   i   )         ]                  
          Combining                   (   13   )          -          (   15   )                   gives                          (   15   )                   B   ^          (     θ   ^     )       =         ∑     i   =   1     n                (       n   -   i   +   1     n     )     n          [       X     (   i   )       -     X     (     i   -   1     )       -     Y     (   i   )       +     Y     (     i   -   1     )         ]       /   2       -     θ   ^               (   16   )                               
 
         [0053]    Having an estimate of B({circumflex over (θ)}) available, a bias-corrected estimator can be formed  
                 θ   ^     BC     =         θ   ^     -       B   ^          (     θ   ^     )              
                =         2        θ   ^       -       ∑     i   =   1     n                (       n   -   i   +   1     n     )     n          [       X     (   i   )       -     X     (     i   -   1     )       -     Y     (   i   )       +     Y     (     i   -   1     )         ]       /   2              
                =       X     (   1   )       -     Y     (   1   )       -       ∑     i   =   1     n                (       n   -   i   +   1     n     )     n          [       X     (   i   )       -     X     (     i   -   1     )       -     Y     (   i   )       +     Y     (     i   -   1     )         ]       /   2.                     (   17   )                               
 
         [0054]    The closed form expression for {circumflex over (θ)} BC  warrants some accent as in many cases bootstrap estimators require evaluation via monte-carlo resampling of the empirical distribution functions. Defining w i =1/2+[(n−1)/n] n /2 and  
         [0055]    w i =−[(n−i+1)/n] n /2+[(n−i)/n] n /2, for 2≦i≦n, it is easy to verify that  
           θ   ^     BC     =       ∑     i   =   1     n            w   i          (       X     (   i   )       -     Y     (   i   )         )                               
 
         [0056]    and that  
           ∑     i   =   1     n          w   i       =       1   2     .                           
 
         [0057]    In contrast, {circumflex over (θ)} is the linear combination of the X (i)−Y   (i)  that has the first weight equal to 1/2 and all subsequent weights are equal to zero. It has been recognized (see, for example, Efron and Tibshirani 1993, supra, section 10.6) that bias-corrected estimators do not always have smaller mean squared error than the uncorrected estimator. However, we will demonstrate in what follows that for common clock offset contexts, {circumflex over (θ)} BC  does have smaller mean squared error than {circumflex over (θ)}.  
         [0058]    Mean Squared Error Comparison of {circumflex over (θ)}, {circumflex over (θ)} BC  and {circumflex over (θ)} Under Exponential Delays  
         [0059]    Previously, formulas were given in (10) and (11) for the MSE of {circumflex over (θ)} and {circumflex over (θ)}. To evaluate the MSE of {circumflex over (θ)} BC , it is convenient to use the representation  
           θ   ^     BC     =       ∑     i   =   1     n            w   i          (       X     (   i   )       -     Y     (   i   )         )                               
 
         [0060]    from which it follows that  
         E        (       θ   ^     BC     )       =       ∑     i   =   1     n            w   i          [       E        (     X     (   i   )       )       -     E        (     Y     (   i   )       )         ]                               
 
         [0061]    and  
         Var        (       θ   ^     BC     )       =       Var        (       ∑     i   =   1     n            w   i          X     (   i   )           )       +       Var        (       ∑     i   =   1     n            w   i          Y     (   i   )           )       .                             
 
         [0062]    Again using the means, variances and covariances of the {X (i)}   i=1   n  and {Y (i)}   i=1   n  sequences, it can be shown  
               E        (       θ   ^       B                 C       )       =     θ   +       (       ∑     i   =   1     n            w   i          μ     (   i   )           )     ×     (       1     λ     A                 B         -     1     λ     B                 A           )                   and               (   18   )                 Var        (       θ   ^       B                 C       )       =       (       1     λ     A                 B     2       -     1     λ     B                 A     2         )     ×       ∑     i   =   1     n            (       w   i   2     +       ∑     k   =     i   +   1       n            w   i          w   k           )            σ   i   2     .                   (   19   )                               
 
         [0063]    Combining (18) and (19), we find  
         MSE        (       θ   ^       B                 C       )       =         (       1     λ     A                 B     2       +     1     λ     B                 A     2         )     ×       ∑     i   =   1     n            (       w   i   2     +     2          ∑     k   =     i   +   1       n            w   i          w   k             )          σ   i   2           +     
              (       1     λ     A                 B         -     1     λ     B                 A           )     2                ×         (       ∑     i   =   1     n            w   i          μ     (   i   )           )     2     .                               
 
         [0064]    The MSE of the three estimators {circumflex over (θ)}, {circumflex over (θ)} BC  and {tilde over (θ)} only depend on the mean values, 1/λ AB  and 1/λ BA , of the network delay distributions and the sample size n. Tables 2-4 show the bias, standard deviation and root mean squared error (RMSE) for the cases where (1/λ AB ,1/λ BA ) is (1, 5), (1, 10) and (1, 1), respectively. (We use the notation EXP(μ) in table headings to denote an exponential distribution with mean equal to μ.) Each table shows the three cases of n equal to 10, 20 and 40. From Table 2 and Table 3, we can see the bootstrap technique is quite effective at reducing bias. Moreover, we see that while it is true that the standard deviation of {circumflex over (θ)} BC  is larger than the standard deviation of {circumflex over (θ)}, the overall RMSE is smaller. The increase in the standard deviation is more than neutralized by the reduction in the bias. Table 2 and Table 3 also show that {circumflex over (θ)} BC  is more competitive with the o-BLUE (in terms of RMSE) than is {circumflex over (θ)}. Table 4, being a case where λ AB =λ BA , is a case where all three estimators are unbiased and we have shown that {circumflex over (θ)}is the o-BLUE. We see from Table 4 that all three estimators have essentially the same standard deviation.  
                                                                                                                               TABLE 2                           A→B Network Delays are EXP(1) and B→A Network Delays are EXP(5)                Root Mean Squared                Bias   Standard Deviation   Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}                    10   −.200   −.0851   0   .255   .276   .269   .324   .289   .269       20   −.100   −.0422   0   .127   .137   .131   .162   .144   .131       40   −.0500   −.0210   0   .0637   .0686   .0645   .0810   .0718   .0645                  
 
         [0065]    [0065]                                                                                                                               TABLE 3                           A→B Network Delays are EXP(t) and B→A Network Delays are EXP(10)                Root Mean Squared                Bias   Standard Deviation   Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}                    10   −.450   −.192   0   .502   .543   .530   .675   .576   .530       20   −.225   −.0949   0   .251   .271   .258   .337   .287   .258       40   −.113   −.0472   0   .126   .135   .127   .169   .143   .127                    
         [0066]    [0066]                                                                                                                               TABLE 4                           A→B Network Delays are EXP(1) and B→A Network Delays are EXP(1)                Root Mean Squared                Bias   Standard Deviation   Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {circumflex over (θ)}   {circumflex over (θ)} BC     zθ   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}                    10   0   0   0   .0707   .0765   .0745   .0707   .0765   .0745       20   0   0   0   .0354   .0381   .0363   .0354   .0381   .0363       40   0   0   0   .0177   .0190   .0179   .0177   .0190   .0179                    
         [0067]    Robustness  
         [0068]    When the network delays are not exponentially distributed, {tilde over (θ)} is no longer the o-BLUE. Moreover, {tilde over (θ)}(just like {circumflex over (θ)}) will generally be biased unless the distributions of the A→B and B→A network delay components are identical. It is natural, therefore, to consider the bootstrap bias-corrected version of {tilde over (θ)} as a candidate estimator of clock offset. A parallel development to the section entitled “Bias Correction of {circumflex over (θ)} ”will show that the bootstrap bias-corrected estimator of {tilde over (θ)} is  
             θ   ~       B                 C       =       ∑     i   =   1     n            v   i          (       X     (   i   )       -     Y     (   i   )         )           ,                         
 
         [0069]    where  
               v   1     =       n     n   -   1            {       1   2     -     1     2        n   2         +       1   2            (       n   -   1     n     )     n         }                       v   i     =       n     n   -   1            {       -     1     2        n   2           -       1   2            (       n   -   i   +   1     n     )     n       +       1   2            (       n   -   i     n     )     n         }         ,     i   ≥   2.                                 
 
         [0070]    It is easy to verify that  
           ∑     i   =   1     n          v   i       =     1   /   2.                           
 
         [0071]    Tables 5-8, obtained from simulating 2,000 message exchange sequences, compare the RMSE of all four estimators {circumflex over (θ)}, {circumflex over (θ)} BC , {tilde over (θ)} and {tilde over (θ)} BC  in contexts where one or both of the distributions for the network delay components in the A→B and B→A directions is lognormal. (We use the notation LN(μ, σ 2 ) in the table headings to denote a lognormal distribution with mean μ and variance σ 2 .) Tables 5 and 6 correspond to the case where the network delay components in both directions have lognormal distributions. Note that the means and variances of the A→B and B→A network delays in Tables 5 and 6 are the same as in Tables 2 and 3, respectively.  
                                                                                                                                             TABLE 5                           A→B Network Delays are LN(1, 1) and B→A Network Delays are LN(5, 25)                Bias   Standard Deviation   Root Mean Squared Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC                      10   −.430   −.320   −.255   −.134   .255   .285   .272   .316   .499   .429   .373   .343       20   −.332   −.264   −.244   −.173   .174   .196   .179   .205   .374   .329   .302   .268       40   −.253   −.207   −.208   −.161   .119   .138   .121   .141   .279   .249   .241   .214                  
 
         [0072]    [0072]                                                                                                                                             TABLE 6                           A→B Network Delays are LN(1, 1) and B→A Network Delays are LN(10, 100)                Bias   Standard Deviation   Root Mean Squared Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC                      10   −.980   −.737   −.587   −316   .511   .576   .543   .635   1.105   .935   .800   .710       20   −.738   −.588   −.544   −386   .345   .393   .357   .411   .815   .707   .651   .563       40   −.569   −.466   −.468   −362   .236   .271   .240   .277   .616   .539   .526   .456                    
         [0073]    As with Tables 2 and 3, we see that the bootstrap bias-corrected estimators {circumflex over (θ)} BC  and {tilde over (θ)} reduce bias significantly. Although the standard deviation of {circumflex over (θ)} BC  and {tilde over (θ)} BC  are their corresponding uncorrected estimators, the net result is an appreciable decrease in the RMSE. We also note that {tilde over (θ)} BC  is superior to {circumflex over (θ)} BC .  
         [0074]    Tables 7 and 8 show the bias, standard deviation and RMSE when the network delay component in the A→B direction is exponential, and in the B→A direction is lognormal. The means and variances of the A→B and B→A network delays in Tables 5 and 6 are again the same as what they are in Tables 2 and 3, respectively. The conclusions that can be drawn from Tables 7 and 8 are the same as what was observed from Tables 5 and 6, viz., that the bootstrapping is quite effective at reducing bias and although the variance increases, the net result is a decrease in RMSE. Note that the rank order in Tables 7 and 8 of the four estimators with respect to RMSE is also consistent with what is shown in Tables 5 and 6. For the cases considered, it is clear that the two best estimators are {tilde over (θ)} BC  and {tilde over (θ)} in that order.  
                                                                                                                                             TABLE 7                           A→B Network Delays are EXP(1) and B→A Network Delays are LN(5, 25)                Bias   Standard Deviation   Root Mean Squared Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC                      10   −.500   −.395   −.334   −.217   .261   .292   .279   .323   .564   .491   .435   .389       20   −.385   −.313   −.300   −.224   .169   .193   .175   .202   .420   .368   .347   .302       40   −.302   −.252   −.259   −.207   .116   .133   .118   .136   .323   .285   .284   .248                  
 
         [0075]    [0075]                                                                                                                                             TABLE 8                           A→B Network Delays are EXP (1) and B→A Network Delays are LN(10, 100)                Bias   Standard Deviation   Root Mean Squared Error            n   {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC     {circumflex over (θ)}   {circumflex over (θ)} BC     {tilde over (θ)}   {tilde over (θ)} BC                      10   −1.057   −.817   −.680   −.413   .529   .597   .567   .662   1.182   1.011   .885   .780       20   −.784   −.629   −.589   −.426   .335   .380   .347   .398   .853   .735   .684   .583       40   −.611   −.502   −.512   −.399   .240   .278   .244   .283   .657   .573   .567   .490                    
         [0076]    Analysis Summary  
         [0077]    We showed that bootstrap bias correcting Paxson&#39;s estimator produces an estimator with smaller RMSE, relative to the uncorrected estimator. We derived the minimum variance unbiased estimator (i.e., the SS estimator) of clock offset (based on ordered sojurn times) under an exponential assumption for the variable part of network delays. We showed the SS estimator further reduces the RMSE, relative to the bias corrected Paxson estimator, with or without the exponential assumption. As such, the bias-corrected SS estimator is the robust estimator of choice for real applications.  
         [0078]    Illustrative Network Applications  
         [0079]    [0079]FIG. 2 shows an illustrative single-server network deployment of present inventive techniques. Specifically, FIG. 2 shows a time server  200  connected through a network  210  to a plurality of other network nodes  220 - i , i=1, 2, . . . , N. Nodes  220 - i  may be routers, switches, servers of various kinds, network end points (including terminals, workstations or computers), or any other kind of network node. Each of nodes  220 - i  has a clock and messaging facilities for exchanging messages with time server  200  in the manner described above. That is, time server  200  forms one of the pair of nodes and, in turn, one or more (typically all) of the nodes  220 - i  forms the other of the node pair for purposes of exchanging time-stamped messages and deriving offset estimates and estimate bias information in accordance with the improved offset estimator described above. While each of the nodes  220 - i  may have equal access to time server  200 , priorities may be accorded some nodes  220 - i,  or some nodes  220 - i  may be accorded access to server  200  more frequently.  
         [0080]    By exchanging messages with nodes  220 - i,  time server  200  will provide clock offset estimates and estimate bias information as described above, which information is available at nodes  220 - i  for correcting clock offset. Of course, N may have a value of 1, so that only a single network node device may interact with a particular time server. While time server  200  is shown as a separate dedicated function network node, it will be understood that the function of network node  200  may be included in a node performing other functions. Likewise, many network arrangements will have a plurality of time servers, each serving network nodes connected on a respective network or sub-network  210 .  
         [0081]    [0081]FIG. 3 shows an illustrative alternative network arrangement in which a plurality of time servers  330  and  340 - i,  i=1, 2, . . . , M, are connected in hierarchical relation through a plurality of networks  310 - i,  i=1, 2, . . . , M. In the illustrative arrangement of FIG. 3, only two levels are shown in the server hierarchy, but those skilled in the art will recognize that any number of levels of time servers may be used. Likewise, while the number of networks is shown equal to the number of nodes at the lowest hierarchical level, no such limitation is required in practicing the present invention using a hierarchical arrangement of time servers. Each of the networks  310 - i  has one or more network nodes capable of accessing the respective time server connected to the network. By way of illustration, network  310 - 1  has nodes  350 - 11  through  350 - 1 P connected to it. Likewise, network  310 -M is shown having nodes  350 -M 1  through  350 -MQ. Here, P and Q may be any integer.  
         [0082]    In operation, time server  330  exchanges time-stamped messages with each of the time servers  340 - i  to provide the latter with offset estimates and estimate bias information of the type illustrated above to permit clock correction at the illustrative (second-level) time servers  340 - i.  Each of the time servers  340 - i  then serves the clock correction requirements of respective nodes  350 - xx  in the same manner. Of course, when more than two hierarchical levels of time servers are used, each level (after the first or highest) derives clock synchronization information from a time server at the next highest level. The number of nodes will generally vary from one network  310 - i  to another, and all or some of networks  310 - i  may be sub-networks of a larger network. Some time servers may be connected to nodes such as  350 - xx  and to a next lower order node as well. Some or all time servers may be located in the same local area or distributed over a wide area (including globally) to meet load and geographic distribution requirements for clock synchronization service.  
         [0083]    Access to respective time servers by particular nodes (or subordinate time servers) may be scheduled (e.g., periodic), dependent upon availability of time server resources, dependent on prior clock offset behavior at particular nodes (or subsidiary time servers) or detected conditions at such nodes or subsidiary time servers. Exchange of messages and derivation of correction information in accordance with present inventive teachings may be initiated, in appropriate cases, by a particular time server or by a node (or subsidiary time server) for which the particular time server provides clock synchronization services. In each case, however, the time server acts as the reference source of time in determining offset using exchanges of time-stamped messages of the types described above. Thus, for example, a particular node (routinely, or in response to conditions detected at that node) may request that an associated time server initiate a synchronization sequence and supply the results to the particular node.  
         [0084]    Numerous and varied particular applications of the present inventive principles, all within the spirit of the present description and scope of the attached claims, will prove useful to those skilled in the art. For example, while variable delays have been described in some of the illustrative embodiments and applications of the present invention as having exponential or lognormal probability distributions, principles and methods of the present invention are also applicable to embodiments and contexts in which such delays are better characterized in terms of other well-known distributions, e.g., gamma or Weibull distributions.