WIGOWIN 2010
Collision avoidance using a wandering
token in the PTP protocol
_____________________________
Augusto Ciuffol...
PTP?
● PTP stands for Precision Time Protocol
● The nickname for IEEE1588-2008
● It addresses clock synchronization in pac...
Clock Synchronization - Why?
● Control of industrial plants requires sub
microsecond coordination.
● Telecom networks need...
Synchronization over Packet - Why?
● Link level synchronization solutions exist (SONET,
SyncE …)
● Packet based solutions ...
The PTP solution
● Messages can travel across layers (UDP/IP, Eth,
DeviceNet)
● One master, many slaves
● The master perio...
PTP - Syntonization
● Sync-Sync protocol:
– Invariant: (t1
-t0
) = (t3
-t2
)
– The slave tunes clock frequency
● Assumptio...
PTP - Synchronization
● Delay_Req – Delay_Resp protocol
– Invariant: PacketDelay =[(t1
-t0
) – (t3
-t2
)]/2
– The slave co...
Delay_Req – Delay_Resp
● Delay_Req timing is critical (Delay_Resp is not)
● Delay_Req/Delay_Resp traffic grows with system...
Coordinate Delay_Req to avoid
collisions
● PRO:
– Protocol overall more reliable (no collisions)
– Improved network utiliz...
One token to rule them all
● Token circulation to control the production of a
Delay_Req
● Randomized token circulation fro...
Limit return time
● Introduce some global knowledge in the master
(already a single point of failure) to avoid starvation
...
Token passing operation
● Token “bounces”
on the master
● Master learns
slaves timeouts
passively
● Token de-routed
to sta...
A dynamic neighborhood
● Neighborhood
initially just legal
● Token source
observes token
routing
● Selected destination
sw...
Token passing operation
● The token travels
embedded within
PTP messages
● Delay_Req carries
selected dest
● Delay_Resp
ca...
Future work
● Implementation using the UDP open source PTP
(sourceforge) (thesis)
● Application to wireless ad-hoc network...
Upcoming SlideShare
Loading in …5
×

Collision avoidance using a wandering token in the PTP protocol

1,177 views

Published on

Slides presented during the 2010 WIGOWIN Workshop at the Department of Computer Science in Pisa - May 26.
Full paper available at http://eprints.adm.unipi.it

Published in: Technology
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
1,177
On SlideShare
0
From Embeds
0
Number of Embeds
9
Actions
Shares
0
Downloads
10
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Collision avoidance using a wandering token in the PTP protocol

  1. 1. WIGOWIN 2010 Collision avoidance using a wandering token in the PTP protocol _____________________________ Augusto Ciuffoletti
  2. 2. PTP? ● PTP stands for Precision Time Protocol ● The nickname for IEEE1588-2008 ● It addresses clock synchronization in packet networks ● Substantially improves the Network Time Protocol (IETF NTP) ● Addresses converging users (telecom, industrial plant control)
  3. 3. Clock Synchronization - Why? ● Control of industrial plants requires sub microsecond coordination. ● Telecom networks need precise synchronization to manage media playback applications. ● Network monitoring applications need synchronization preciser than packet delay. ● TDM (Time Division Multiplexing) over IP is a solution for many problems, but needs synchronized clocks.
  4. 4. Synchronization over Packet - Why? ● Link level synchronization solutions exist (SONET, SyncE …) ● Packet based solutions have many advantages: – Scalability – Adaptability – Maintainability – Interoperability ● Packet based solutions have one (big) problem: – Sensitive to packet delay variations
  5. 5. The PTP solution ● Messages can travel across layers (UDP/IP, Eth, DeviceNet) ● One master, many slaves ● The master periodically sends a syntonization message (Sync) ● Each slave responds after a delay with a Delay_Req message ● The master replies with a Delay_Resp
  6. 6. PTP - Syntonization ● Sync-Sync protocol: – Invariant: (t1 -t0 ) = (t3 -t2 ) – The slave tunes clock frequency ● Assumption: M-S packet delay is constant (!) Master Slave Sync(t0 ) Sync Sync Sync Sync t0 t1 t2 t3
  7. 7. PTP - Synchronization ● Delay_Req – Delay_Resp protocol – Invariant: PacketDelay =[(t1 -t0 ) – (t3 -t2 )]/2 – The slave compensates time of the day offset ● Assumption: M-S packet delay is symmetric (!) Master Slave Sync(t0 ) Delay_Req(t3 ) Delay_Resp(t1 ) t0 t1 t2 t3
  8. 8. Delay_Req – Delay_Resp ● Delay_Req timing is critical (Delay_Resp is not) ● Delay_Req/Delay_Resp traffic grows with system size (Sync traffic does not) ● The PTP protocol arranges Delay_Req to be distributed uniformly on time, but uncoordinated ● Clock synchronization is allocated a small, protected (virtual) channel ● Packet collisions may occur that disrupt protocol consistency
  9. 9. Coordinate Delay_Req to avoid collisions ● PRO: – Protocol overall more reliable (no collisions) – Improved network utilization ● Beware of: – Extra complexity, extra problems (KISS) – Fault tolerance – Footprint (traffic, processing) – Standard boundaries
  10. 10. One token to rule them all ● Token circulation to control the production of a Delay_Req ● Randomized token circulation from slave to slave to avoid complex and slow ring maintenance ● Each slave maintains a limited number of neighbors to choose from ● But: – How to limit the token return time? – How to effectively maintain a neighborhood? – Do we need additional messages to carry the token?
  11. 11. Limit return time ● Introduce some global knowledge in the master (already a single point of failure) to avoid starvation ● Accumulate global knowledge passively observing the system (no footprint) ● Activate only when needed (minimize impact of inconsistencies of global knowledge)
  12. 12. Token passing operation ● Token “bounces” on the master ● Master learns slaves timeouts passively ● Token de-routed to starving slave ● Master usually transparent Source Selected destinationStarving slave
  13. 13. A dynamic neighborhood ● Neighborhood initially just legal ● Token source observes token routing ● Selected destination swapped with real destination Token de-routed to D A – B – C Selected address is B A – D – C Random neighborhood of token source After token passing
  14. 14. Token passing operation ● The token travels embedded within PTP messages ● Delay_Req carries selected dest ● Delay_Resp carries final dest ● All slaves observe Delay_Resp msgs Source Final destination Delay_req (unicast) Delay_resp (multicast) Token path PTP messages
  15. 15. Future work ● Implementation using the UDP open source PTP (sourceforge) (thesis) ● Application to wireless ad-hoc networks (simulation, experiments with low cost devices) ● Use of the PTP in virtual networking environments (IEEE803.1Q) ● ...and more... These slides available at “http://www.slideshare.net/AugustoCiuffoletti”

×