Internet DRAFT - draft-alavarez-hamelin-tictoc-sic
draft-alavarez-hamelin-tictoc-sic
TICTOC J.I.A-H. Alvarez-Hamelin, Ed.
Internet-Draft Universidad de Buenos Aires - CONICET
Updates: none (if approved) D. Samaniego
Intended status: Standards Track A.A. Ortega
Expires: 12 May 2022 Universidad de Buenos Aires
R. Geib
Deutsche Telekom
8 November 2021
Synchronizing Internet Clock frequency protocol (sic)
draft-alavarez-hamelin-tictoc-sic-08
Abstract
Synchronizing Internet Clock (sic) Frequency specifies a new secure
method to synchronize clocks on the Internet, assuring smoothness
(i.e., frequency stability) and robustness to man-in-the-middle
attacks. This protocol is oriented to assure the quality of Internet
performance measurements, where they are frequently obtained as the
difference of timestamps, hence frequency stability is needed. In
90% of all cases, Synchronized Internet Clock Frequency is highly
accurate, with a Maximum Time Interval Error of fewer than 25
microseconds by a minute. Synchronized Internet Clock Frequency is
based on a regular packet exchange and works with commodity terminal
hardware.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 12 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The sic frequency protocol overview . . . . . . . . . . . . . 4
3. The formal definition of sic frequency protocol . . . . . . . 8
3.1. Algorithm description . . . . . . . . . . . . . . . . . . 9
3.2. Protocol definitions . . . . . . . . . . . . . . . . . . 13
3.3. Protocol packet specification . . . . . . . . . . . . . . 15
3.4. Minimum sic deployment . . . . . . . . . . . . . . . . . 16
4. Implementation of sic frequency protocol . . . . . . . . . . 17
4.1. Evaluation . . . . . . . . . . . . . . . . . . . . . . . 17
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Example of RTT to NTP servers . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
One way metric measurements [RFC7679] require synchronization of
sender and receiver clocks to obtain reliable results. This
synchronization should be smooth (i.e., operate without steps in
synchronization and detect and reach a stable operation state), offer
a precise frequency, easily implemented in any host on the Internet,
and faithful. A reliable clock frequency is needed to perform
precise differential metric measurements between any two hosts to
capture performance metrics like packet delay variation or loss of
packets exchanged between arbitrary measurement hosts. The required
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clock synchronization is designed to be implementable by software to
allow deployment with hosts in arbitrary locations. It cannot
replace or compete against commodity clock synchronization standards
(and any intent to do so is out of scope). Finally, some security
measures are needed to avoid some common types of attacks in the
Internet, given more robustness to the sytem.
There are different types of clock synchronization on the Internet.
NTP [RFC5905] remains one of the most popular because a potential
user does not need any extra hardware, and it is practically a
standard in most of the operating-systems distributions. Its working
principle relies on time servers with precise clock source, as atomic
clocks or GPS based. For most of the needs, NTP provides an accurate
synchronization. Moreover, NTP recently incorporates some strategies
oriented to avoid man-in-the-middle (MitM) attacks. NTPs potential
accuracy is in the order of tens of milliseconds..
Another proposal is the TSClocks [ToN2008], which take advantage of
the internal computers' clock. This work has been shown an
attractive solution because it is not expensive and can be used on
any computer connected to the Internet. This solution was proposed
in the beginning at LAN (Local Area Network) level, and then it has
been extended to other situations. In [ToN2008] authors report a
differential clock error of about half of hundred of microseconds for
a WAN connection with 40ms of RTT (Round Trip Time), i.e., the
absolute error is the same order that RTT.
When accuracy and stability are needed, further options arise, e.g.,
the PTP clock [RFC8173] (this mechanism was also defined as the IEEE
Std. 1588-2008). However, the PTP clock incorporates specialized
hardware to provide a highly accurate clock required in each point to
be synchronized. Also, the GPS (Global Position System) requires
specialized hardware at every point of measurement. While GPS may be
less expensive than PTP, the GPS unit requires a sky-clear view for
working. The latter may be costly or impossible in some locations
due to the antenna installation.
This document introduce the Synchronizing Internet Clock frequency
(sic frequency), which is a protocol providing synchronized
differential clocks (i.e., when the amount to measure is the elapsed
time between two timestamps) in two endpoints connected to the
Internet. While synchronized absolute clocks aim at a measurement of
exact time differences between them, synchronized differential clocks
allow measurements during identical time intervals at two locations.
This property is useful for Internet performance measurements, like
congestion, jitter, or delay variation; which are based on the
elapsed time between timestamps.
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The sic frequency design is close to TSClocks, but it takes advantage
of statistics to perform better. The sic frequency synchronization
relies on Internet-based delay measurements, including the frequent
routes' change detection. Finally, our implementation also
contemplates protecting MitM attacks, including light but a powerful
signature of measurements in each packet. The sic frequency protocol
does neither put constraints on the quality of a server's clock nor
require a limitation of synchronized end systems' physical distance.
The quality of the absolute synchronization is determined by the
stability of the reference clock and the RTT/2 as in NTP, but the sic
frequency protocol has a better performance in frequency stability
than NTP.
Finally, we mention the [ITU-G.8260] shows a methodology to measure
delays in networks. It is based on filtering that selects some
packets to perform the delay computation. The packet selection is
based on the minimum and average RTT, and we show that both of them
have some statistical problems to determine both, the average and the
minimum RTT (see Section 2).
2. The sic frequency protocol overview
Synchronizing Internet Clock frequency (sic frequency) is a protocol
providing synchronized differential clocks in two endpoints connected
to the Internet. Synchronized differential clocks allow measurements
during identical time intervals at two locations. This is useful if
congestion, packet loss or a variation in delay is to be measured.
The model of typical Internet time-measurement is shown in Figure 1.
XXXXXXXX XXXXXXXX
XXXXXX XXX X
XX XXX
+----+----------+XX XXXX
| XX XX
| X Internet XX
| XX XXX
+--+------+ XXXXXX XX+---------+------+
| | X XX |
| Client | XX XXX |
| | XX XXX XXXXX XX +---+----+
| | XXX XXXXXXX XXXXXX | |
+---------+ | Server |
| |
| |
+--------+
Figure 1: The clock synchronization of sic.--
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In this model, sic frequency performs measurements with packets in
the way shown in
Figure 2.
t2 t3
Server +---------------@-------*----------------------------->
/ \_ C_s [s]
/ \_
/ \_
/ \_
/ \_
/ \_
/ \_
/ \_
/ \_
/ \_
/ \_ C_c [s]
Client +---*------------------------------------------@------>
t1 t4
Figure 2: Time line of packets.--
Here, C_s is the server clock, C_c is the client clock and t1...t4
are timestamps.
Figure 2 shows a horizontal timeline for client and server. The
diagonal lines depict a packet traversing some physical space (wires,
routers, and switches). The packet travel times are not assumed to
be identical because routes and background load may differ in each
direction.
The difference between the client clock C_c and the server clock C_s
can be modeled as:
C_c = C_s + phi ,
phi(t) = C_c(t) - C_s(t) , (1)
where phi is the absolute clock difference. If RTT is constant (i.e.
little or no background load) and routes are symmetric in both
directions, the difference between clocks can be computed as:
phi[c->s] = t1 - ( t2 - RTT/2 ) , (2)
phi[c<-s] = t4 - ( t3 + RTT/2 ) , (3)
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and phi[c->s] = phi[c<-s]. The general equation for the RTT is:
RTT = ( t2 - t1 ) + ( t4 - t3 ) . (4)
Computing Equations 2 and 3 for this simplified case allows
calculation of phi as an RTT function. Note that if routes are not
symmetrical, it is impossible to determine the absolute clocks'
difference.
The sic frequency protocol is based on statistics, background traffic
and network behavior observations. The RTT between two endpoints
follows a heavy-tailed distribution. An alpha-stable distribution
shows as one possible model [traffic-stable]. This distribution can
be characterized by four parameters: the localization "delta," the
stretching "gamma," the tail "alpha," and the symmetry "beta,"
[alfa-stables]. The location parameter is highly related to the mode
of the distribution: delta > 0. The stretching is related to the
dispersion: gamma > 0. The symmetry, -1 <= beta <= 1, indicates if
the distribution is skewed to the right (the tail decays to the left)
for positive values or the opposite direction for negatives ones.
Finally, the tail alpha, defined in (0,2], indicates if the
distribution is Gaussian one when alpha=2, a power-law without
variance for alpha <2, and without statistic mean for alpha<1. The
alpha-stable distribution is the generalization of the Central Limit
Theorem for any distribution (i.e., it includes the cases without
variance or mean).
Then, the phi(t) estimation involves the subtraction of two alpha-
stable random variables, which yields on another alfa-stable
distribution but symmetrical [alfa-stables]. Due to this result's
characteristic, i.e., a fixed mode and symmetry, a good estimator of
the mode is the median.
Therefore, sic performs periodic measurements to infer the difference
of two clocks on the Internet, taking advantage of the empiric
observations. The periodicity of RTT measurements is set to 1
second.
The parameters of the simple skew model [ToN2008] are estimated by
the following equation:
phi(t) = K + F * t , (5)
where phi(t) = C_c - C_s, K is a constant representing the absolute
difference of time of client clock C_c and server clock C_s, and F is
the rate parameter. As sic frequency is a differential clock, we
only estimate the frequency parameter "F."
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Note that the "K" parameter cannot be estimated using just endpoints
measurements. Assessing the "K" parameter accurately is out of
scope, and we use K=min(RTT)/2, as it is used in several
synchronization protocols under the assumption of symmetric paths,
e,g, NTP. Considering the following asymmetry definition,
t[c->s]
A = 1 - --------- , (6)
t[c<-s]
where t[c->s] is the minimum delay measured from the client to the
server. The maximum asymmetry A of equation 6 is A=1, which is
unlucky, and this establishes the hardbound for the error of K as
min(RTT): if t[c->s] approaches RTT, t[c->s] approaches zero. The
difference between the two is phi (t), and this difference hence is
close to min(RTT), if A=1. In our experiments (see Section 4.1), the
error in estimation phi(t) was always less than min(RTT)/2.
Another problem with most of the synchronization protocols is the
minimum RTT estimation, which depends upon the time-window within
which the RTT is captured. A minimum RTT can only be measured in the
absence of any cross traffic. In the first step, the minimum RTT
measured during a window of 10 minutes (mRTT10m) is captured. Based
on these values, the minimum RTT over a week (mRTTw) is determined.
RTTee is defined as mRTT10m - mRTTw. Figure 3 shows the the RTT
estimation error captured during an experiment where the minimum the
latency between probes was 9431 microseconds during one week, i.e.,
mRTTw=9431 microseconds. Notice that mRTT10m varies a lot, and the
observed values can be more than 450 microseconds above the minimum
RTT over a week. This error is a consequence of the statistical
behavior of the RTT, which can be modeled by the alfa-stable
distribution.
Finally, it is mostly believed there always exist NTP servers at less
than five hops with few milliseconds of RTT because of the NTP
deployment. In Appendix A we show a typical case in Latin America
region where the RTT differs notably from a host in the same city
(Buenos Aires). This example reveals that some countries could not
have this desired situation, and other synchronization tools are
needed.
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Error of the min(RTT)
[micro-seconds]
500 +------|-------|------|------|------|-------|------|------+
| + + + O + + + + |
| * |
400 |-+ ** O +-|
| * * O O ** O O |
| O * * ** * ** ** ** |
300 |-+ * O*O * O O* * O*O * * O O *-|
| O* O O * * * O * * * *|
| O * O ** * * O * * * O** O|
200 |-* * * * * O * O * O*O * O +-|
|** O O * ** * * * * O |
|O * *** O * * * |
100 |-+ O O O * O +-|
| ** |
| ** |
0 |-+ + + O + + + + + +-|
+------|-------|------|------|------|-------|------|------+
0 50 100 150 200 250 300 350 400
time [minutes]
Figure 3: Min RTT error, estimated every 10 minutes along 7 hours.--
The sic frequency protocol estimates phi(t) of Equation 5 using
measurement statistics and taking advantage of the inherent RTT
properties, i.e., the heavy tail distribution and its alfa-stable
distribution model. The basic sic frequency operation is to
periodically send packets, estimate phi(t), and correct the local
clock with:
t_c = t + phi(t) , (7)
where t_c is the corrected time and t the local clock time (notice
that phi(t) is calculated according to Equation 1).
The sic protocol also detects route changes by seeking a non-
negligible difference between the minimum RTT of the actual and past
round trip measurement. The next section also discusses different
mechanisms to detect route changes by RTT evaluation.
3. The formal definition of sic frequency protocol
Section 3.1 presents the sic frequency algorithm. In addition,
parameters and their definitions are introduced. Finally, formal
packet formats are provided.
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The sic frequency protocol MUST sign the packets with the
deterministic Elliptic Curve Digital Signature Algorithm (ECDSA)
specified by [RFC6979] to protect sic frequency from MitM attacks.
To avoid delays when a packet is signed, sic frequency signs them in
a deferred fashion. That is, in each packet carries the signature of
the previous packet (see algorithms in Figure 6 and Figure 5).
3.1. Algorithm description
A sic frequency implementations MUST support the formal description
specified by this section. Once activated, the sic frequency
protocol MUST operate permanently while a client and a receiver
exchange measurement packets. The sic frequency works with three
states: NOSYNC, PRESYNC and SYNC. These states are triggered by the
variables errsync, presync, and synck.
Lines 1 to 4 of the pseudocode in Figure 4 initialize the required
data structures needed and set the sic frequency state to NOSYNC. In
NOSYNC state, a complete measurement window estimates phi's by
Equation 2 (see line 8). Notice that also Equation 3 can be used, or
an average of both Equations. During the experiments, using a single
equation only resulted in estimations with a smaller error. The
possible explanation is that measurements are affected by the same
type of traffic.
The median of the measurement window is computed in line 9, while
lines 10-12 are used to verify if the path changed during the
measurements. When an appreciable difference is detected (bounded by
errRTT) in line 13, the "else" clause is executed and the systems re-
initiates the cycle (see lines 17-22). Notice that line 13 verifies
if the minimum RTTs' absolute value is lower than a percentage of
minimum over the complete RTT window.
The sic three tables of pseudocode present frequency algorithm
specification. The parameters are explained after the third table.
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=======================================================================
| sic frequency algorithm |
=======================================================================
1 Wmedian <-0, Wm <-0, WRTT <-0, actual_m <-0, actual_c <-0
2 presync <- INT_MAX - P, epochsync <- INT_MAX - P, n_to <-0
3 synck <- false, errsync <- epoch, set(0, 0, NOSYNC), e_prev<-epoch
4 send_sic_packet(SERVER_IP, TIMEOUT)
5 for each timer(RUNNING_TIME) == 0
6 | (epoch, t1, t2, t3, t4, to) <- send_sic_p(SERVER_IP,TIMEOUT)
7 | if (to == false) then
8 | | Wm <- t1 - t2 + (t2 - t1 + t4 - t3)/2
9 | | Wmedian <- median(Wm)
10 | | WRTT <- t4 - t1 size(W)
11 | | RTTf <- min(WRTT[size(WRTT)/2,size(WRTT)])
12 | | RTTl <- min(WRTT[0,size(WRTT)/2])
13 | | if ((|RTTf - RTTl| <= errRTT * min(WRTT)) then
14 | | | if (epoch >= presynck + P)) then
15 | | | | presynck <- true
16 | | | end if
17 | | else
18 | | | synck <- false, Wmedian <- 0
19 | | | Wm <- 0, errsync <- epoch, n_to <- 0
20 | | | epoch_sync <- INT_MAX - P, pre_sync <- INT_MAX - P
21 | | | set(0, 0, NOSYNC)
22 | | end if
23 | | if ((synck == true) && (epoch >= epochsync + P)) then
24 | | | (m, c) <- linear_fit(Wmedian)
25 | | | actual_c <- c
26 | | | actual_m <- (1-alpha) * m + alpha * actual_m
27 | | | epochsync <- epoch, n_to <- 0
28 | | | set(actual_m, actual_c, SYNC)
29 | | else
30 | | | if (epoch == errsync + MEDIAN_MAX_SIZE) then
31 | | | | presync <- epoch
32 | | | end if
33 | | | if (epoch >= presync + P) then
34 | | | | (actual_m, actual_c) <- linear_fit(Wmedian)
35 | | | | synck <- true , epoch_sync <- epoch
36 | | | | set(actual_m, actual_c, PRESYNC)
37 | | | end if
38 | | end if
39 | else
40 | | to <- false
41 | end if
42 end for
=======================================================================
Figure 4: Formal description of sic.--
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Several conditions should be verified to pass from NOSYNC to PRESYNC.
First, the "else" condition of line 29 should occur, and also the
elapsed time between errsync and actual epoch should be
MEDIAN_MAX_SIZE (30-32). Therefore, when it also P time is passed
form presync, the condition on line 33 is true, and the system
arrives at PRESYNC, providing an initial estimation of phi.
Then, if there is no route change, the condition in line 14 will be
true when the time was increased in another P period. Then, the
system is in the SYNC state, and it provides the estimation of phi(t)
in line 28. Notice that every P time, the estimation of phi(t) is
computed unless a route change occurs (lines 13 and 17-22).
The function in line 6: (epoch, t1, t2, t3, t4, to) <-
send_sic_packet(SERVER_IP, TIMEOUT), has a special treatment. It
sends the packets specified in Section 3.3, which have signatures.
To avoid the processing delay caused by the signature computation, we
implemented a policy to send the signature of the previous packet,
and if an error is detected, we can stop the synchronization just one
loop ahead.
Figure 5 illustrates how the client-side MUST implement the function
send_sic_p (SERVER_IP, TIMEOUT). This function computes the
timestamp t1 in line 1 and builds and sends the UDP packet in lines
2-3. Then, if there is no timeout, it calculates the t4 timestamp
(line 5), and if no packets were lost, verifies the signature of the
previous one in lines 8-18. If the signature is not valid with the
received certificate, then the system MUST change to NOSYNC state
immediately (see line 11). NOSYNC state MUST also be set if the
limit of time without receiving packets MAX_to is reached. Finally,
it stores the received packet into prev_rcv_pck (a global variable)
to use in the next packet (line 19). Notice that n_to, the lost
packets, is a global variable, as well as the epoch of the previous
packet: e_prev.
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=======================================================================
| function: send_sic_p(server, TIMEOUT) |
=======================================================================
1 t1 <- get_timestamp()
2 sic_P <- sic_pck(t1, 0, 0, prev_sig)
3 (to, rcv_sic_pck) <- send(sic_P,UDP_PORT, SERVER_IP, TIMEOUT)
4 if (to == false) then
5 | t4 <- get_timestamp()
6 | epoch <- trunc_to_seconds(t1)
7 | prev_sig <- get_signature(sic_P)
8 | if (epoch - e_prev <= RUNNING_TIME) then
9 | | if (n_to < MAX_to) then
10 | | | if (verify(prev_rcv_pck,rcv_sic.CERT) == false) then
11 | | | | set(0, 0, NOSYNC)
12 | | | else
13 | | | | n_to <- 0, e_prev <- epoch
14 | | | end if
15 | | else
16 | | | set(0, 0, NOSYNC)
17 | | end if
18 | end if
19 | prev_rcv_pck <- rcv_sic_pck
20 | t2 <- rcv_sic_pck.t2
21 | t3 <- rcv_sic_pck.t3
22 else
23 | n_to <- n_to + 1
24 end if
25 return (epoch, t1, t2, t3, t4, to)
=======================================================================
Figure 5: The send_sic_p function.--
The server sic algorithm is presented in Figure 6. It uses
prev_sic_P{}, which is a structure to store the received previous
signatures, indexed by the IP client addresses (CLIENT_add contains
its IP and UDP port), and the same for prev_sig{} with the previously
sent signatures. Line 6 verifies either signature is null because it
is the first packet or a valid signature. In both cases, the
algorithm process the packet computing t3, building up the sic
frequency packet, sending it, and computing its signature (stored to
send in the next reply) in lines 7-11. Next, the actual packet is
stored in the prev_sic_P{} structure, line 13.
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=======================================================================
| sic Server algorithm |
=======================================================================
1 prev_sic_P{} <- null, prev_sig{} <-- null
2 while (RUNNING == true) then
3 | if (receive() == true) then
4 | | t2 <- get_timestamp()
5 | | prev_sig <- get_signature(prev_sic_P{receive().CLIENT_add})
6 | | if (prev_sig == null) ||
| | (verify(prev_sig, CLIENT_add.CERT) == true) then
7 [ | | t3 <- get_timestamp()
8 | | | sic_P<-sic_pack(t1, t2, t3, prev_sig)
9 | | | send(sic_P, CLIENT_add.UDP, CLIENT_add.IP, TIMEOUT)
10 | | | prev_sig <- get_signature(sic_P)
11 | | | prev_sig{receive().CLIENT_add} <- prev_sig
12 | | end if
13 | | prev_sic_P{receive().CLIENT_add} <- receive().sic_pack
14 | end if
15 end while
=======================================================================
Figure 6: Algorithm sic for the Server.--
3.2. Protocol definitions
We provide a formal definition of each used constant and variables;
the RECOMMENDED values are displayed in parentheses at the end of the
description. These constant and variables MUST be represented in a
sic frequency implementation. All the types MUST be respected. They
are expressed in "C" programming language running on a 64-bit
processor.
a. Constants used for the sic frequency algorithm (Figure 4)
1. RUNNING_TIME: is the period between sic packets are sent (1
second).
2. MEDIAN_MAX_SIZE: is the window size used to compute the
median of the measurements (600).
3. P: is the period between phi's estimation (60).
4. alpha: is a float in the [0,1], the coefficient of the
autoregressive estimation of the slope of phi(t) (0.05).
5. TIMEOUT: is the maximum time in seconds that a sic packet
reply is expected (0.8 seconds).
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6. SERVER_IP: is the IP address of the server (@IP in version 4
or 6).
7. errRTT: is a float that bounds the maximum difference to
detect a route change (0.2).
8. MAX_to: is an integer representing the maximum number of
packet lost (P/10).
9. CERT: is a public certificate of the other end, it is used
to verify signs of the packets.
10. UDP_PORT: is an integer with the port UDP where the service
is running on the server. (4444)
11. SERVER_IP: is the IP address of the server.
12. CLIENT_IP: is the IP address of the client.
b. States used for the sic frequency algorithm (Figure 4)
1. NOSYNC: a boolean indicates that it is not possible to
correct the local time.
2. PRESYNC: an integer indicates that sic is almost (P
RUNNING_TIME) seconds from the synchronization.
3. SYNC: a boolean indicates that sic is synchronized.
c. Variables used for the sic frequency algorithms (Figure 4,
Figure 5 and Figure 6)
1. errsync: is an integer with the UNIX timestamp epoch of the
initial NOSYNC cycle. It is used to complete the window or
measurements (Wm) to compute their medians.
2. presync: is an integer with the UNIX timestamp epoch of the
initial PRESYNC cycle. It is used to wait until (P
RUNNING_TIME) seconds to the linear fit of phi(t).
3. synck: is an integer with the UNIX timestamp epoch of the
initial SYNC cycle. Every P RUNNING_TIME) seconds the
phi(t) function is estimated.
4. epochsync: is an integer with the last UNIX timestamp epoch
of synchronization. It is used to compute a new estimation
of phi(t), every (P RUNNING_TIME) seconds.
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5. epoch: is an integer with UNIX timestamp in seconds. It
carries the initial epoch of each sic measurement packet.
6. t1, t2, t3, t4: are long long integers to store the t UNIX
timestamps in microseconds.
7. actual_m : is a double with the slope for the phi(t)
estimation.
8. actual_c: is a double with the intercept for the phi(t)
estimation.
9. Wm: is an array of doubles of MEDIAN_MAX_SIZE. It stores
the instantaneous estimates of phi(t).
10. Wmedian: is an array of doubles of P size. It saves the
computed medians of Wm every RUNNING_TIME.
11. WRTT: is an array of doubles of (2 P) size. It stores the
calculated RTT of last measurements.
12. RTTl: is a double with the minimum of last P RTTs. It is
used to detect changes on the route from the client to the
server.
13. RTTf: is a double with the minimum of previous P RTTs. It
is used to detect changes on the route from the client to
the server.
14. n_to: is an integer representing the number of lost packets
in the actual synchronization window P.
15. e_prev: is an integer with the UNIX timestamp epoch of the
last valid packet.
16. prev_rcv_pck: is a sic packet structure, the previous
received one.
3.3. Protocol packet specification
The sic frequency uses UNIX microsecond format timestamps. Regarding
Figure 2, the client takes a timestamp t1 just before it sends the
packet. When the server receives the packet, it immediately computes
t2, and just before it is sent back to the client, it computes t3.
When the client receives the packet, it calculates t4.
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The server does not need the timestamp t1 because the proposed
protocol synchronizes a client with the server clock. This
information could, however, be useful for the server for future use.
The packets respect the NTP4 format as they are defined in the
Section A.1.2 of [RFC5905] and the signature of the fields, shown in
Figure 7. We use the time formats of 64 bits with seconds and a
fraction of seconds. They MUST be sent as UDP data, and it MUST have
the mentioned fields.
The client and server certificates SHOULD be valid and signed ones
(only for experimentation, the user MAY use autogenerated ones).
+-----------------------------------------------------------+
| NTP_packet (t1_c,t2_s,t3_s) | Sig_r n-1 | Sig_s n-1 |
+-----------------------------------------------------------+
Client --> Server
+-----------------------------------------------------------+
| NTP_packet (t1_c,t2_s,t3_s) | Sig_r n-1 | Sig_s n-1 |
+-----------------------------------------------------------+
Server --> Client
Figure 7: Packet format for the sic protocol.--
3.4. Minimum sic deployment
To deploy the sic frequency algorithm, as a minimum, a Server and one
Client are needed. The Server can support multiple clients. The
maximum number of clients is for further study. The Server clock is
considered the master one, and all clients synchronize with it. The
Server-side runs sic frequency as a server with a UDP_PORT number, as
specified by the algorithm shown in Figure 6.
Client sic runs the algorithm shown in Figure 4 and also SHOULD
provide the corrected time as
t = actual_c + actual_m * timestamp (8)
Figure 8
Different ways of doing this task are possible:
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Providing a client capable of reading the variables actual_m and
actual_c in shared memory and producing the result of Equation 8.
Providing a service in a UDP port answering the correct timestamp
queries with Equation 8.
Other solution.
4. Implementation of sic frequency protocol
In this section we present the prove of the sic concept through some
test that we already performed, and the current implementation of sic
in C language. Our implementation is publicly available
[sic-implementation].
This protocol implements protection against MitM attacks. The
identity of endpoints is guaranteed by signed certificates using the
deterministic Elliptic Curve Digital Signature Algorithm (ECDSA)
specified in the [RFC6979]. Server and Client should use signed and
valid ECDSA certificates to ensure their identity, and each side has
responsible to verify the public certificate of the other side before
to run the algorithm in Figure 4.
4.1. Evaluation
To verify the sic proposal, we tested it using three hosts with GPS
units. The first two were located in Buenos Aires, and the third at
Los Angeles. We slightly modified the algorithm in Figure 4 to
trigger each measurement using the PPS (pulse per second) the signal
provided by the GPS unit. Then, recording the client and server
clocks with the PPS signal, we can determine the real phi function of
Equation 1, within the GPS error (it is several orders of magnitude
smaller than the error of the sic frequency protocol).
We use MTIE defined as follows (Maximum Time Interval Error, see
[ToIM1996]):
MTIE = max [phi(t')] - min [phi (t)] , (9)
for every t' and t in the interval [t,t+s]; and we chose s=60
seconds. We first used two host (RaspBerriesPI-2) connected back to
back to analyze the minimum achievable precision, yielding an MTIE of
15.8 microseconds for the 90 percentile. Then, we selected two real
cases of study, one national and the other international. In
Figure 9 we show the result of the MTIE, evaluated in 60 seconds
intervals, for the experiment Buenos Aires-Buenos Aires (RTT of 10ms)
and Buenos Aires-Los Angeles (RTT of 198ms). The percentile 90
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corresponds to 18.35 microseconds for the Buenos Aires case, and 25.4
microseconds for the Los Angeles case. The percentile 97.5
corresponds to 30 microseconds for the Buenos Aires case, and 42
microseconds for the Los Angeles case. We display the quartiles in
Figure 10. These measurements were performed during a week in each
case.
CDF
+--|-------|-------|-----|-------|----------|------|------+
1 |-+ + + + #########*#*#*#*#*#*#*#*#******|
| ##### ******* |
| #### **** |
0.8 |-+ ## *** +-|
| ### ** |
| ## *** |
0.6 |-+ ## ** +-|
| ## ** |
| ## ** |
0.4 |-+ ## ** +-|
| ## ** |
| ## ** |
0.2 |-+ ## *** +-|
| #### *** Buenos Aires ####### |
| #### *** Los Angeles ******* |
0 |##******* + + + + + + + +-|
+--|-------|-------|-----|-------|-----|----|------|------+
7 10 15 20 30 40 50 70 100
[micro-seconds]
Figure 9: Cumulative distribution function of the MTIE (60s).--
|Buenos Aires (10ms) | Los Angeles (198ms) |local NTP4 (12ms)
====+====================+=====================+=================
Q3 | 14.69 | 19.29 | 9059
----+--------------------+---------------------+-----------------
Q2 | 11.60 | 14.93 | 5245
----+--------------------+---------------------+-----------------
Q1 | 9.41 | 12.26 | 3338
Figure 10: Table with MTIE quartiles for two RTT sic cases, and
for a local NTP4 system (the numbers indicate microseconds).--
We also conducted another test for NTP4 in Buenos Aires, Argentina.
We used a host with GPS, whose PPS signal triggered a process to log
actual timestamps. This host was also running NTP4 with the server
time.afip.gov.ar, also located in Buenos Aires city, with an average
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RTT of 12ms. Applying the same process of the previous cases, we
obtained that the following quartiles Q3: 9.1ms, Q2: 5.2ms, and Q1:
3.3ms for the MTIE of the NTP4 measurements (also reported in
Figure 10). Finally, percentile 90 of the NTP4's MTIE is 11.1ms.
The comparison of NTP4 with frequency sic shows that this new method
performs two orders of magnitude better.
5. Conclusions
This document presents the sic frequency protocol, employed to
synchronize host clock frequency across the Internet, and which is
also resistant to MitM attacks. The main field of application is the
Internet performance analysis, e.g., to measure congestion, jitter,
or delay variations parameters; all of them are based on a difference
of timestamps. It shows the complete specification, implementation,
and experiment results that support its working principle. In
particular, sic frequency provides a clock rate stability of less
than 1ppm for most of the time.
6. Security Considerations
Following [RFC7384] enumeration of Time Protocols in packet-switched
networks, the proposed encryption of timing packets, based on a
mechanism of secure key distribution, provides the following
characteristics:
3.2.1 Packet Manipulation: Prevented by packet signature.
3.2.2 Spoofing: Prevented by packet signature and secure key
distribution.
3.2.3 Replay Attack: Prevented by chain signing of packets.
3.2.4 Rogue Master Attack: Prevented by secure key distribution.
3.2.5 Packet Interception and Removal: If several packets are
removal, the protocol does not arrive at SYNC state.
3.2.6 Packet Delay Manipulation: Not prevented. Future versions
may prevent this using over-specification of timing (using
redundant masters)
3.2.7 L2/L3 DoS attacks: Not prevented. This attack can be
prevented in future versions using over-specification of timing
and redundant masters time servers.
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3.2.8 Cryptographic performance attacks: Not an issue in ECDSA.
3.2.9 DoS attacks agains the time protocol: Prevented by secure
key distribution.
3.2.10 Grandmaster Time source attack (GPS attacks): Not
prevented. Future versions may prevent this using over-
specification of timing (using several time servers) .
3.2.11 Exploiting vulnerabilities in the time protocol: Not
prevented, future vulnerabilities are unknown.
3.2.12 Network Reconnaissance: Not prevented in this version. No
countermeasures were done in node anonymization.
The Packet Delay manipulation is one of the hardest problems to solve
because there exist some smart ways to attack any synchronization
protocol. Even thou, the sic frequency protocol can protect itself
because it can identify several attacks of this type, i.e., it is
challenging to mimic traffic behavior. This emulation of behavior
can be easily overcome regarding the RTTs' distribution, which has to
be a heavy-tailed one. This way, the analysis should be studied to
ensure the implementation of a robust and light test of the raw data.
7. IANA Considerations
This memo makes no requests of IANA.
8. Acknowledgements
The authors thank to Ethan Katz-Bassett, Zahaib Akhtar, the USC and
CAIDA for lodging the testbed of the sic frequency protocol.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
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[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[RFC7679] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Delay Metric for IP Performance Metrics
(IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
2016, <https://www.rfc-editor.org/info/rfc7679>.
[RFC8173] Shankarkumar, V., Montini, L., Frost, T., and G. Dowd,
"Precision Time Protocol Version 2 (PTPv2) Management
Information Base", RFC 8173, DOI 10.17487/RFC8173, June
2017, <https://www.rfc-editor.org/info/rfc8173>.
9.2. Informative References
[alfa-stables]
Uchaikin, V.V. and V.M. Zolotarev, "Chance and stability:
stable distributions and their applications.", Walter de
Gruyter. (Book), 1999.
[ITU-G.8260]
ITU, T. S. S. O., "Definitions and terminology for
synchronization in packet networks (Recommendation ITU-T
G.8260)", August 2015.
[sic-implementation]
Samariego, E., Ortega, A., and J.I. Alvarez-Hamelin,
"Synchronizing Internet
Clocks", https://github.com/CoNexDat/SIC, February 2018.
[ToIM1996] Bregni, S., "Measurement of maximum time interval error
for telecommunications clock stability
characterization", Bregni S. Measurement of maximum time
interval error for telecommunicIEEE transactions on
instrumentation and measurement. 45(5):900-906, October
1996.
[ToN2008] Veitch, D., Ridoux, J., and S.D. Korada, "Robust
synchronization of absolute and differential clocks over
networks.", IEEE.ACM Transactions on Networking (TON)
17.2 (2009): 417-430, 2009.
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[traffic-stable]
Carisimo, E., Grynberg, S.P., and J.I. Alvarez-Hamelin,
"Influence of traffic in stochastic behavior of
latency.", 7th PhD School on Traffic Monitoring and
Analysis (TMA), Ireland, Dublin,, June 2017.
Appendix A. Example of RTT to NTP servers
This appendix shows an experiment to measure the RTT and the distance
in hops from four different points to a time server in Buenos Aires
city (the capital of Argentina). We did the measures two times from
the four points, and we used one hundred packets to determine some
statistical parameters. Next traceroute measurements show that the
number of hops and RTT are very different from each point also
changes a lot. For instance, taking a distinctive look at the STD,
average, and maximum is possible to detect huge variations. We
provide here a case in Argentina, trying to reach an NTP server from
4 different points at the Buenos Aires city.
-------------------------------------------------------------------------
host1$ mtr -r -c 100 time.afip.gov.ar
Start: Tue Mar 27 19:03:51 2018
HOST: raspbian-server Loss% Snt Last Avg Best Wrst StDev
1.|-- gw-vlan-srv.innova-red.ne 0.0% 100 2.2 2.8 2.1 37.7 4.9
2.|-- rnoc5.BUENOS-AIRES.innova 0.0% 100 2.3 3.8 2.1 55.8 7.9
3.|-- 10.5.10.2 0.0% 100 2.5 2.6 2.2 3.1 0.0
4.|-- 200.0.17.104 0.0% 100 3.1 3.1 2.4 13.7 1.1
5.|-- 172.18.2.53 0.0% 100 4.8 5.7 3.8 12.4 1.7
6.|-- time.afip.gob.ar 0.0% 100 5.2 5.2 3.8 12.0 1.3
host1$ mtr -r -c 100 time.afip.gov.ar
Start: Tue Mar 27 18:57:06 2018
HOST: raspbian-server Loss% Snt Last Avg Best Wrst StDev
1.|-- gw-vlan-srv.innova-red.ne 0.0% 50 2.4 2.8 2.0 34.2 4.5
2.|-- rnoc5.BUENOS-AIRES.innova 0.0% 50 2.1 3.8 2.1 52.8 7.7
3.|-- 10.5.10.2 0.0% 50 2.7 2.9 2.2 15.6 1.8
4.|-- 200.0.17.104 0.0% 50 2.8 3.0 2.3 3.9 0.0
5.|-- 172.18.2.53 0.0% 50 4.5 5.8 3.8 17.8 2.2
6.|-- time.afip.gob.ar 0.0% 50 4.7 9.9 4.2 238.5 33.0
-------------------------------------------------------------------------
host2$ mtr -r -c 100 time.afip.gov.ar
Start: Tue Mar 27 19:03:47 2018
HOST: ws-david Loss% Snt Last Avg Best Wrst StDev
1.|-- 10.10.96.1 0.0% 100 0.3 0.2 0.2 0.3 0.0
2.|-- 200.16.116.171 0.0% 100 1.0 5.9 0.6 158.4 22.9
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3.|-- static.33.229.104.190.cps 1.0% 100 1.6 2.5 1.5 80.6 8.0
4.|-- static.129.192.104.190.cp 0.0% 100 2.1 1.9 1.8 3.0 0.1
5.|-- 200.0.17.104 1.0% 100 2.0 2.2 1.8 9.4 0.7
6.|-- 172.18.2.53 0.0% 100 3.2 4.2 3.1 12.0 1.5
7.|-- auth.afip.gob.ar 0.0% 100 4.2 4.5 3.3 9.8 1.2
host2$ mtr -r -c 100 time.afip.gov.ar
Start: Tue Mar 27 18:57:00 2018
HOST: ws-david Loss% Snt Last Avg Best Wrst StDev
1.|-- 10.10.96.1 0.0% 50 0.3 0.3 0.2 0.7 0.0
2.|-- 200.16.116.171 0.0% 50 0.9 6.7 0.7 196.5 29.1
3.|-- static.33.229.104.190.cps 2.0% 50 1.6 1.7 1.5 2.2 0.0
4.|-- static.129.192.104.190.cp 0.0% 50 2.1 2.0 1.7 2.4 0.0
5.|-- 200.0.17.104 0.0% 50 2.0 2.1 1.8 2.6 0.0
6.|-- 172.18.2.53 0.0% 50 4.8 4.3 3.2 9.1 1.3
7.|-- time.afip.gob.ar 0.0% 50 4.3 9.4 3.3 234.9 32.7
-------------------------------------------------------------------------
host3$ mtr -r -c 100 time.afip.gov.ar
Start: 2018-03-27T19:03:51-0300
HOST: aleph.local Loss% Snt Last Avg Best Wrst StDev
1.|-- 10.17.71.254 0.0% 100 4.7 30.3 3.5 280.4 52.4
2.|-- 10.255.254.250 0.0% 100 2.5 31.1 1.8 285.4 59.2
3.|-- 209.13.133.10 0.0% 100 30.5 43.9 2.3 237.2 64.9
4.|-- host169.advance.com.ar 3.0% 100 36.0 64.8 3.1 404.4 86.9
5.|-- 200.32.33.33 2.0% 100 106.9 70.6 2.8 315.0 80.4
6.|-- 200.32.34.66 5.0% 100 93.1 56.8 2.7 336.1 74.5
7.|-- 200.32.33.38 7.0% 100 42.8 58.0 2.9 357.8 76.7
8.|-- 209.13.139.211 4.0% 100 46.2 56.7 2.8 298.8 69.9
9.|-- 209.13.139.209 1.0% 100 84.5 57.1 2.6 277.7 72.3
10.|-- 209.13.166.211 1.0% 100 43.4 63.5 3.2 390.6 78.7
11.|-- 200.32.34.137 2.0% 100 68.7 64.1 3.7 259.5 75.5
12.|-- 200.32.33.37 0.0% 100 4.1 56.9 3.3 249.6 64.3
13.|-- 200.32.34.121 3.0% 100 10.9 65.0 4.1 415.7 85.1
14.|-- 200.32.33.241 2.0% 100 252.6 61.8 3.8 355.9 74.5
15.|-- 200.16.206.198 3.0% 100 188.0 54.6 3.1 461.7 74.9
16.|-- 172.18.2.53 2.0% 100 133.4 53.1 4.3 402.1 69.2
17.|-- time.afip.gob.ar 4.0% 100 72.5 54.1 4.9 343.2 66.9
host3$ mtr -r -c 100 time.afip.gov.ar
Start: 2018-03-27T18:57:05-0300
HOST: aleph.local Loss% Snt Last Avg Best Wrst StDev
1.|-- 10.17.71.254 0.0% 50 125.6 88.1 3.7 392.4 79.3
2.|-- 10.255.254.250 0.0% 50 62.1 54.8 2.1 333.2 68.0
3.|-- 209.13.133.10 0.0% 50 4.0 33.9 2.4 280.8 51.3
4.|-- host169.advance.com.ar 2.0% 50 4.1 21.3 2.9 236.7 40.4
5.|-- 200.32.33.33 2.0% 50 4.5 32.2 3.2 341.3 69.4
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6.|-- 200.32.34.66 4.0% 50 7.7 26.0 3.5 278.8 55.8
7.|-- 200.32.33.38 2.0% 50 4.8 29.4 3.0 221.3 43.4
8.|-- 209.13.139.211 0.0% 50 84.8 40.3 3.1 250.4 53.0
9.|-- 209.13.139.209 0.0% 50 25.1 35.0 2.8 249.2 55.4
10.|-- 209.13.166.211 0.0% 50 3.7 33.5 2.6 188.9 54.3
11.|-- 200.32.34.137 0.0% 50 5.6 28.2 3.7 224.3 51.1
12.|-- 200.32.33.37 0.0% 50 3.7 24.2 3.5 189.5 44.9
13.|-- 200.32.34.121 0.0% 50 4.7 30.8 4.0 213.2 51.6
14.|-- 200.32.33.241 0.0% 50 14.4 33.1 3.9 364.6 67.2
15.|-- 200.16.206.198 0.0% 50 5.0 58.2 3.1 300.7 88.5
16.|-- 172.18.2.53 0.0% 50 9.4 117.8 4.4 315.1 103.4
17.|-- time.afip.gob.ar 0.0% 50 199.6 120.2 5.2 484.0 96.2
-------------------------------------------------------------------------
host4$ mtr -r -c 100 time.afip.gov.ar
Start: 2018-03-27T19:03:51-0300
HOST: cnet Loss% Snt Last Avg Best Wrst StDev
1.|-- 157.92.58.1 0.0% 100 6.6 2.8 0.3 12.8 2.5
2.|-- ??? 100.0 100 0.0 0.0 0.0 0.0 0.0
3.|-- ??? 100.0 100 0.0 0.0 0.0 0.0 0.0
4.|-- host98.131-100-186.static 0.0% 100 5.7 5.6 1.5 9.4 2.2
5.|-- host130.131-100-186.stati 0.0% 100 6.5 6.3 2.5 10.3 2.2
6.|-- 200.0.17.104 0.0% 100 2.4 2.7 2.3 15.6 1.4
7.|-- ??? 100.0 100 0.0 0.0 0.0 0.0 0.0
8.|-- time.afip.gob.ar 0.0% 100 4.9 7.6 3.9 243.0 23.9
host4$ mtr -r -c 100 time.afip.gov.ar
Start: Tue Mar 27 18:41:40 2018
HOST: cnet Loss% Snt Last Avg Best Wrst StDev
1.|-- 157.92.58.1 0.0% 50 4.0 1.6 0.3 9.1 1.6
2.|-- ??? 100.0 50 0.0 0.0 0.0 0.0 0.0
3.|-- ??? 100.0 50 0.0 0.0 0.0 0.0 0.0
4.|-- host98.131-100-186.static 0.0% 50 4.7 5.5 1.5 10.9 2.4
5.|-- host130.131-100-186.stati 0.0% 50 8.4 6.5 2.6 10.5 2.2
6.|-- 200.0.17.104 0.0% 50 2.5 2.8 2.3 11.0 1.2
7.|-- ??? 100.0 50 0.0 0.0 0.0 0.0 0.0
8.|-- time.afip.gob.ar 0.0% 50 4.9 9.2 3.8 226.7 31.4
--------------------------------------------------------------------------
Authors' Addresses
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Jose Ignacio Alvarez-Hamelin (editor)
Universidad de Buenos Aires - CONICET
Av. Paseo Colon 850
C1063ACV Buenos Aires
Argentina
Phone: +54 11 5285-0716
Email: ihameli@cnet.fi.uba.ar
URI: http://cnet.fi.uba.ar/ignacio.alvarez-hamelin/
David Samaniego
Universidad de Buenos Aires
Av. Paseo Colon 850
C1063ACV Buenos Aires
Argentina
Phone: +54 11 5285-0716
Email: dsamanie@fi.uba.ar
Alfredo A. Ortega
Universidad de Buenos Aires
Av. Paseo Colon 850
C1063ACV Buenos Aires
Argentina
Phone: +54 11 5285-0716
Email: ortegaalfredo@gmail.com
Ruediger Geib
Deutsche Telekom
Heinrich-Hertz-Str. 3-7
64297 Darmstadt
Germany
Phone: +49 6151 5812747
Email: Ruediger.Geib@telekom.de
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