3.7. Clock Synch hronisation

Transcription

1 Chapter 3.7 Clock-Synchronisation hronisation 3.7. Clock Synch 1 Content Introduction Physical Clocks - How to measure time? - Synchronisation - Cristian s Algorithm - Berkeley Algorithm - NTP / SNTP - PTP IEEE 1588 Logical Clocks Ordering Event counting Lampert s algorithm Vector Clocks 2 1

2 Introduction (1) Clock synchronization for what? Temporal ordering of events produced by concurrent processes Synchronization between senders and receivers of messages Coordination of joint activity Serialization of concurrent access for shared objects Time stamps for authentication, avoiding duplication of transactions 3 Introduction (2) Logical clock keeps track of event ordering among related (causal) events Physical clocks are useful to have the system keep good real time Set to time of day Consistent across systems 4 2

3 Introduction (3) When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time. 5 Content Introduction Physical Clocks - How to measure time - Synchronisation - Cristian s Algorithm - Berkeley Algorithmen - NTP / SNTP - PTP IEEE 1588 Logical Clocks Ordering Event counting Lampert s algorithm Vector Clocks 6 3

4 How to measure time? Use a periodic process or a time dependent process hronisation 3.7. Clock Synch 7 Physical Clocks (1) Mean Solar Second Computation of the mean solar day. hronisation 3.7. Clock Synch 8 4

5 Physical Clocks (2) TAI: International Atomic Time Atomic Clock (1948), more than 50 different Clocks, Mean-Value Number of ticks of the cesium 133 clock since midnight on Jan divided by ,0 9 Braunschweig Paris Physical Clocks (3) UTC: Universal Coordinated time TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun. Offered by: shortwave-radio, satellites, 10 5

6 Computer-Clocks (1) CMOS clock (counter) circuit driven by a quartz oscillator battery backup to continue measuring time when power is off OS generally programs a timer circuit to generate an interrupt periodically e.g., 60 or 100 interrupts per second Programmable Interrupt Controller (PIC) Interrupt tservice procedure adds 1 to a counter in memory 11 Computer Clocks (2) Oszillator (i) TC Counter(i) Clock C( t) = α H( t) +β i Hi(t) : local hardware-clock i Ci(t) : local system time 12 6

7 Synchronization / Problems System 1 System 2 Getting two systems to agree on time Two clocks hardly ever agree Quartz oscillators oscillate at slightly different frequencies Clocks tick at different rates Create ever-widening gap in perceived time Clock Drift Difference between two clocks at one point in time Clock Skew 13 Ideal Clock too fast C(t) too slow t: UTC 14 7

8 Dealing with Drift (1) synchronization point too fast C(t) t: UTC Time goes back! 15 Dealing with Drift (2) synchronization point too fast Modify rate! C(t) t: UTC 16 8

9 Clock Synchronization Algorithms The relation between clock time and UTC when clocks tick at different rates. dc 1 ρ 1+ ρ dt ρ maximum drift rate Error δ (Δt after Sync) δ = 2ρΔt resync time : Δt = δ 2ρ 17 Requirements to clocks Requirements to Hardware-Clock (1 ρ)( t a t b ) H ( t a ) H ( t b ) (1 + ρ)( t a t b ) Requirements to local system time: monoton a b a ( t > t ) C( t ) > C( t b ) 18 9

10 Getting accurate time Different technical clocks will give different times: -> Synchronize to a better clock! 19 External Synchronisation: Time will be compared with UTC (Coordinated Universal Time ) given by a UTC-Timesource S(t) { 1,2 N} S( t) Ci ( t) D i,... D: Synchronizationerror Time Server: machine that provides time information Synchronization Algorithms: Ensure that condition is always fulfilled. Simplest solution: RPC - Issue RPC to obtain time -Set time Client what is the time? 3:59:48:12 Time Server Does not account for network or processing latency 20 10

11 Cristian's Algorithm Getting the current time from a time server. Compensate for delays, Note times: request sent: T0 reply received: T1 Assume network delays are symmetric T 1 T T T 0 Client = Server Error Bounds hronisation 3.7. Clock Synch 22 11

12 Cristian s algorithm: example hronisation 3.7. Clock Synch 23 Internal Synchronisation Different technical clocks will give different times: Ensure that clocks inside a network show the same time Internal Synchronisation: Time will be compared with the time of other computers in the network. Real physical time ist not neceassary to know. D: Synchronisationerror { 1,2 N} C j ( t) Ci ( t) D i, j,

13 Berkeley algorithm Gusella & Zatti, 1989 Assumes no machine has an accurate time source Obtains average from participating computers Synchronizes all clocks to average 25 Berkeley algorithm Each machine has a time daemon. One machine master, all other slaves The time daemon master asks all the other machines for their clock values, The machines answer The time daemon master calculates mean value and tells everyone how to adjust their clock 26 13

14 Berkeley Algorithm: example hronisation 3.7. Clock Synch 27 NTP: Network Time Protocol 1991, 1992 Internet Standard, version 3: RFC 1305 Enable clients across Internet to be accurately synchronized to UTC despite message delays Use statistical techniques to filter data and gauge quality of results Provide reliable service Survive lengthy losses of connectivity Redundant paths Redundant servers Enable clients to synchronize frequently offset effects of clock drift Provide protection against interference Authenticate source of data 28 14

15 NTP: Network Time Protocol Distribution of universal time "Despite the large and variable message delays encountered" NTP uses UDP port 123 as its transport layer. It is designed particularly to resist the effects of variable latency. in use since 1985, NTP was originally designed by Dave Mills of the University of Delaware, who still maintains it, along with a team of volunteers. The NTP daemon is a user-level process that runs continuously on a machine that supports NTP, and most of the protocol is implemented in this use. 29 NTP Servers Arranged in strata 1st stratum: machines connected directly to accurate time source 2nd stratum: machines synchronized from 1 st stratum machines SYNCHRONIZATION SUBNET 30 15

16 NTP Synchronization Modes Multicast mode for high speed LANS Lower accuracy but efficient Procedure call mode Similar to Cristian s algorithm Symmetric mode Intended d for master servers Pair of servers exchange messages and retain data to improve synchronization over Time All messages delivered unreliably with UDP 31 NTP Messages Procedure call and symmetric mode Messages exchanged in pairs NTP calculates: l Offset for each pair of messages Estimate of offset between two clocks Delay Transmit time between two messages Filter Dispersion Estimate of error quality of results Based on accuracy of server s clock and consistency of network transit time Use this data to find preferred server: lower stratum & lowest total dispersion 32 16

17 NTP Message Structure Leap second indicator Last minute has 59, 60, 61 seconds Version number Mode (symmetric, unicast, broadcast) Stratum (1=primary reference, 2-15) Poll interval Maximum interval between 2 successive messages, nearest power of 2 Precision of local clock Nearest power of 2 33 NTP message structure Root delay Total roundtrip delay to primary source (16 bits seconds, 16 bits decimal) Root dispersion Nominal error relative to primary source Reference clock ID Atomic, NIST dial-up, radio, LORAN-C navigation system, GOES, GPS, Reference timestamp Time at which clock was last set (64 bit) Authenticator (key ID, digest) Signature (ignored in SNTP) 34 17

18 NTP Message structure T1: originate timestamp Time request departed client (client s time) T2: receive timestamp Time request arrived at server (server s time) T3: transmit timestamp Time request left server (server s time) 35 NTP s validation tests Timestamp provided last timestamp received duplicate message? Originating timestamp in message consistent with sent data Messages arriving in order? Timestamp within range? Originating and received timestamps 0? Authentication disabled? Else authenticate Peer clock is synchronized? Don t sync with clock of higher stratum # Reasonable data for delay & dispersion 36 18

19 SNTP Simple Network Time Protocol Based on Unicast mode of NTP Subset of NTP, not new protocol Operates in multicast or procedure call mode Recommended for environments where server is root node and client is leaf of synchronization subnet Root delay, root dispersion, reference timestamp ignored RFC SNTP hronisation 3.7. Clock Synch 38 19

20 SNTP example hronisation 3.7. Clock Synch 39 PTP: Precesion Time Protocol Defined in IEEE 1588 Local networks (Ethernet) Multicast-messages Distributed automation systems with synchronisation better than 1 μs 40 20

21 PTP: The Problem hronisation 3.7. Clock Synch 41 PTP: Synchronisation hronisation 3.7. Clock Synch 42 21

22 PTP: Synchronization Example hronisation 3.7. Clock Synch 43 PTP: Time-Stamping hronisation 3.7. Clock Synch 44 22

23 PTP: Boundary Clock hronisation 3.7. Clock Synch 45 PTP - Sequences hronisation 3.7. Clock Synch 46 23

24 PTP: Tree-Structures hronisation 3.7. Clock Synch 47 Content Introduction Physical Clocks - How to measure time? - Synchronisation - Cristian s Algorithm - Berkeley Algorithm - NTP / SNTP - PTP IEEE Questions Logical Clocks Ordering Event counting Lampert s algorithm Vector Clocks 48 24

25 Questions (1) What is the difference between TAI und UTC? Name at least three sources of delay, that can be introduced between a broadcast time source (Radio, Satellite, ) and the processors in distributed systems, setting there time-clocks. Consider two machines in a distributed system. Both clocks are supposed to tick 1000 times per millisecond. One of them does, but the other does only 990 times/ms. If a UTC Update arrives once a minute, what is the maximum clock skew that will occur? 49 Questions (2) For a clock in a Processor the accuracy is given as 0.005%. The Hardware-clock gives ticks/ms. Is it possible to get a synchronization better than 1 ms? What would be the condition? Christian s algorithm shall be implemented in a distributed CAN System running at 1 Mbit/s. Give an description, how this can be done. Calculate the best case Error-Bound that can be achieved. Why is a timestamping done near the Hardware- Interface in PTP? What is the advantage related to time-stamping at the application layer? 50 25

26 Content Introduction Physical Clocks - How to measure time? - Synchronisation - Cristian s Algorithm - Berkeley Algorithm - NTP / SNTP - PTP IEEE 1588 Logical Clocks Ordering Event counting Lampert s algorithm Vector Clocks 51 Logical Clocks Assign sequence numbers to events All cooperating processes can agree on order of events vs. physical clocks: time of day Assume no central time source Each system maintains its own local clock No total ordering of events No concept of happened-when h 52 26

27 Lamport s Happened before Processes agree, in which order events occur Lamport s happened-before notation a b: event a happened before event b e.g.: a: message being sent, b: receipt Transitive: if a b and b c then a c 53 Example (1) Three systems: P1, P2, P3 Events a, b, c, Local event counter on each system Systems occasionally communicates 54 27

28 Lamport Timestamps Assign time value to each event if a b then clock(a) < clock(b) since time cannot run backwards If a and b occur on different processes that do not exchange messages, then neither a b nor b a are true These events are concurrent 55 Lampert s algorithm Each message carries a timestamp of the sender s clock When a message arrives: if receiver s clock < message timestamp set system clock to (message timestamp + 1) else do nothing clock must be advanced between any two events Algorithm allows us to maintain time ordering among related events Partial ordering 56 28

29 Lamport s algorithm A 6 D B 24 C A 6 D B 24 C P1 P2 P3 P1 P2 P3 Lamport:Problem 1 a b, b c, : local events are sequenced j c, f k, g d, : Lamport imposes a send receive relationship Concurrent events (e.g., k, i) may have the same timestamp or not 58 29

30 Creating unique timestamps (total ordering) Idea: If two events never happen exactly at the same time We can force each timestamp to be unique Define global logical timestamp (Ti, i) Ti represents local Lamport timestamp i represents process number (globally unique), E.g. (host address, process ID) Compare timestamps: (Ti, i) < (Tj, j) if and only if Ti < Tj or Ti = Tj and i < j 59 Lamport: Problem 2 Detecting causal relations If L(e) < L(e ) Cannot conclude that e e e Looking at Lamport timestamps Cannot conclude which events are causally related Solution: use a vector clock 60 30

31 Vector clocks 61 Define: Vi[i]: Number of Events so far occured in Process i known by process i Vi[j]: Number of events so far occured in Process j known by process i Rules: 1. Vector initialized to 0 at each process Pi Vi[j]= 0 for i,j=1,, N 2. Process increments its element of the vector in local vector before timestamping event: Vi[i] = Vi[i] +1 (No. of events so far occured) 3. Message is sent from process Pi with Vi attached to it 4. When Pj receives message, compares vectors element by element Sets local vector to higher of two values Vj[i] = max(vi[i], Vj[i]) for i=1,, N Comparing vector timestamps Define V=V if V [i] = V [i] for i=1,, N V V if V [i] V [i] for i=1,, N For any two events e, e if e e then V(e) < V(e ) Just like Lamport s algorithm if V(e) < V(e ) then e e Two events are concurrent if neither V(e) V(e ) nor V(e ) V(e) 62 31

32 Vector-Timestamp V1 [1,2,3] (1,0,0) (000) (0,0,0) a V2 [1,2,3] (0,0,0) (2,0,0) b (2,1,0) c (2,2,0) d P1 P2 V3 [1,2,3] (0,0,0) (0,0,1) e (2,2,2) f P3 63 Vector-Timestamp (0,0,0) (0,0,0) (1,0,0) a (2,0,0) b (2,1,0) c (2,2,0) d P1 P2 (0,0,0) (0,0,1) e concurrent events (2,2,2) f P