WIGOWIN 2010

Collision avoidance using a wandering token in the PTP protocol

_____________________________

Augusto Ciuffoletti

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)

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.

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

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

PTP - Syntonization

● Sync-Sync protocol:

– Invariant: (t1-t

0) = (t

3-t

2)

– The slave tunes clock frequency

● Assumption: M-S packet delay is constant (!)

Master

Slave

Sync(t0) Sync Sync Sync Sync

t0

t1

t2

t3

PTP - Synchronization

● Delay_Req – Delay_Resp protocol

– Invariant: PacketDelay =[(t1-t

0) – (t

3-t

2)]/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

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

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

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?

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)

Token passing operation

● Token “bounces” on the master

● Master learns slaves timeouts passively

● Token de-routed to starving slave

● Master usually transparent

Source

SelecteddestinationStarving

slave

A dynamic neighborhood

● Neighborhood initially just legal

● Token source observes token routing

● Selected destination swapped with real destination

Token de-routedto D

A – B – C

Selected addressis B

A – D – C

Randomneighborhood

of token source

After token passing

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

Finaldestination

Delay_req(unicast)

Delay_resp(multicast)

Token path PTP messages

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”