Hi all,

I have recently been working time distribution in the
presence of strong asymmetry,

and have come across a method that helps in certain ca=
ses.

I am sure that you all remember the CTP algorithm that=
I
have brought up before

(and presented at IETF-74).

The same academic group has an earlier article that I =
had previously
overlooked.

I am talking about :

Gurewitz O, Sidi M.

Estimating One-Way Delays from Cyclic Path Delay Measurements.

16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF]

This article gives a procedure for determining one-way
delays

based purely on round-trip delay measurements (i.e., w=
hat we
would call T4-T1),

knowledge of topology, and the assumption of additivit=
y of
propagation delays.

The idea is that nodes measure round-trip times to var=
ious
other nodes,

knowing which nodes are traversed.

For example, assume three nodes connected in a triangl=
e

=
1

=
; /
\

=
; /
\

=
2 -------- 3

and we measure the times for the following paths

1 2 3

2 3 2

3 1 3

1 2 3 1

We thus have 4 equations for 6 variables

(since the links are not assumed symmetric, the variab=
les
are D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ).

Using additivity and non-negativity it turns out that =
one
can solve an optimization problem

which minimizes the error of these equations.

The solution requires a centralized server to do the m=
ath
PCE-style,

but solves a problem that I don't know any other way t=
o
solve.

Comments ?

Y(J)S

Thanks =
Yaakov,

This is an =
interesting idea. It
does require the that complete paths be known and controlled. =
Perhaps it could
be used in conjunction with MPLS.

//Doug

**From:=
**
tictoc-bounces@ietf.org [mailto:tictoc-bounces@ietf.org] **On Behalf Of =
**Yaakov
Stein

**Sent:** Wednesday, January 06, 2010 9:16 AM

**To:** tictoc@ietf.org

**Subject:** [TICTOC] interesting article on a global mechanism for =
one-way
delay measurement

Hi all,

I have recently been working time distribution in =
the
presence of strong asymmetry,

and have come across a method that helps in certain =
cases.

I am sure that you all remember the CTP algorithm =
that I have
brought up before

(and presented at IETF-74).

The same academic group has an earlier article that =
I had
previously overlooked.

I am talking about :

Gurewitz O, Sidi M.

Estimating One-Way Delays from Cyclic Path Delay Measurements.

16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF]

This article gives a procedure for determining =
one-way
delays

based purely on round-trip delay measurements =
(i.e., what we
would call T4-T1),

knowledge of topology, and the assumption of =
additivity of
propagation delays.

The idea is that nodes measure round-trip times to =
various
other nodes,

knowing which nodes are traversed.

For example, assume three nodes connected in a =
triangle

&=
nbsp;
1

/ =
\

/ =
\

&=
nbsp;
2 -------- 3

and we measure the times for the following =
paths

1 2 3

2 3 2

3 1 3

1 2 3 1

We thus have 4 equations for 6 variables =

(since the links are not assumed symmetric, the =
variables
are D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ).

Using additivity and non-negativity it turns out =
that one
can solve an optimization problem

which minimizes the error of these =
equations.

The solution requires a centralized server to do =
the math
PCE-style,

but solves a problem that I don't know any other =
way to
solve.

Comments ?

Y(J)S

Doug

Yes, the topology has be=
known,
although it can change (reroute events) as long as we are informed of this.=

For ToD distribution it =
is a bit
less elegant than running full CTP,

but it gives you a bette=
r
feeling for what is happening.

I think it could go well=
with
MPLS-TP, or even better with MPLS and a PCE box

(which not only knows th=
e
topology, but could optimize the timing paths).

Y(J)S

**From:** Doug Arnold
[mailto:darnold@symmetricom.com]

**Sent:** Wednesday, January 06, 2010 20:22

**To:** Yaakov Stein; tictoc@ietf.org

**Subject:** RE: [TICTOC] interesting article on a global mechanism for
one-way delay measurement

Thanks Yaakov,

This is an interesting
idea. It does require the that complete paths be known and
controlled. Perhaps it could be used in conjunction with MPLS.

//Doug

**From:**
tictoc-bounces@ietf.org [mailto:tictoc-bounces@ietf.org] **On Behalf Of Yaakov
Stein
Sent: Wednesday, January 06, 2010 9:16 AM
To: tictoc@ietf.org
Subject: [TICTOC] interesting article on a global mechanism for one-=
way
delay measurement**

Hi all,

I have recently been working time distribution in the
presence of strong asymmetry,

and have come across a method that helps in certain ca=
ses.

I am sure that you all remember the CTP algorithm that=
I
have brought up before

(and presented at IETF-74).

The same academic group has an earlier article that I =
had
previously overlooked.

I am talking about :

Gurewitz O, Sidi M.

Estimating One-Way Delays from Cyclic Path Delay Measurements.

16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF]

This article gives a procedure for determining one-way
delays

based purely on round-trip delay measurements (i.e., w=
hat we
would call T4-T1),

knowledge of topology, and the assumption of additivit=
y of
propagation delays.

The idea is that nodes measure round-trip times to var=
ious
other nodes,

knowing which nodes are traversed.

For example, assume three nodes connected in a triangl=
e

=
1

/ &n=
bsp;
\

/ &n=
bsp;
\

=
2 -------- 3

and we measure the times for the following paths

1 2 3

2 3 2

3 1 3

1 2 3 1

We thus have 4 equations for 6 variables

(since the links are not assumed symmetric, the variab=
les
are D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ).

Using additivity and non-negativity it turns out that =
one
can solve an optimization problem

which minimizes the error of these equations.

The solution requires a centralized server to do the m=
ath
PCE-style,

but solves a problem that I don't know any other way t=
o
solve.

Comments ?

Y(J)S

What do we mean by optimize the timing=20
paths?

is this least hops, minimised=20
jitter?

When we say the paths are known and =
controlled i am=20
assuming we need to know the underlying architecture in detail and =
minimise the=20
hop / jitter etc.

Regards,

*Mike*

Doug

Yes, the topology =
has be known,=20
although it can change (reroute events) as long as we are informed of=20
this.

For ToD distribution =
it is a bit=20
less elegant than running full CTP,

but it gives you a =
better=20
feeling for what is happening.

I think it could go =
well with=20
MPLS-TP, or even better with MPLS and a PCE box

(which not only =
knows the=20
topology, but could optimize the timing paths).

Y(J)S

**From:** Doug =
Arnold=20
[mailto:darnold@symmetricom.com] **Sent:** Wednesday, January 06, =
2010=20
20:22**To:** Yaakov Stein; tictoc@ietf.org**Subject:** RE: =
[TICTOC] interesting article on a global mechanism for one-way delay=20
measurement

Thanks=20
Yaakov,

This is an =
interesting=20
idea. It does require the that complete paths be known and=20
controlled. Perhaps it could be used in conjunction with=20
MPLS.

//Doug

**From:**=20
tictoc-bounces@ietf.org [mailto:tictoc-bounces@ietf.org] **On Behalf Of =
**Yaakov Stein**Sent:** Wednesday, January 06, 2010 9:16=20
AM**To:** tictoc@ietf.org**Subject:** [TICTOC] interesting =
article=20
on a global mechanism for one-way delay=20
measurement

Hi all,

I have recently been working time distribution in =
the=20
presence of strong asymmetry,

and have come across a method that helps in certain =
cases.

I am sure that you all remember the CTP algorithm =
that I have=20
brought up before

(and presented at IETF-74).

The same academic group has an earlier article that =
I had=20
previously overlooked.

I am talking about :

Gurewitz=20
O, Sidi M.

Estimating One-Way Delays from Cyclic Path Delay=20
Measurements.

16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF]

This article gives a procedure for determining =
one-way=20
delays

based purely on round-trip delay measurements =
(i.e., what we=20
would call T4-T1),

knowledge of topology, and the assumption of =
additivity of=20
propagation delays.

The idea is that nodes measure round-trip times to =
various=20
other nodes,

knowing which nodes are traversed.

For example, assume three nodes connected in a=20
triangle

&=
nbsp; =20
1

=20
/ =
=20
\

=20
/ =
=20
\

&=
nbsp; =20
2 -------- 3

and we measure the times for the following=20
paths

1 2 3

2 3 2

3 1 3

1 2 3 1

We thus have 4 equations for 6 variables =

(since the links are not assumed symmetric, the =
variables are=20
D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ).

Using additivity and non-negativity it turns out =
that one can=20
solve an optimization problem

which minimizes the error of these =
equations.

The solution requires a centralized server to do =
the math=20
PCE-style,

but solves a problem that I don't know any other =
way to=20
solve.

Comments ?

Y(J)S

Mike

Yes, the idea for real c=
ases
would be employ the usual filtering algorithms for the end-point pair
round-trip times,

and to use these results=
in the
optimization procedure.

You need to be able to s=
pecify that
the path from A to B went through C and D (in that order)

in order to be able to s=
ay that
the time from A to B equals the time from A to C plus the time

from C to D plus the tim=
e from D
to B. This additivity is the major assumption.

However, there is no ass=
umption
of symmetry and no need to use intermediate timestamps.

Instead we find the asym=
metric
time delays, and then can directly compute the ToD corrections.

Y(J)S

**From:** mike.gilson@b=
t.com
[mailto:mike.gilson@bt.com]

**Sent:** Tuesday, January 12, 2010 13:20

**To:** Yaakov Stein; darnold@symmetricom.com; tictoc@ietf.org

**Subject:** RE: [TICTOC] interesting article on a global mechanism for
one-way delay measurement

What do we mean by optimize the timing paths?

is this least hops, minimised jitter?

When we say the paths are known and controlled i am assuming we
need to know the underlying architecture in detail and minimise the hop /
jitter etc.

Regards,

*Mike*

**From:** tictoc-bounces@ietf.org
[mailto:tictoc-bounces@ietf.org] **On Behalf Of **Yaakov Stein

**Sent:** 11 January 2010 07:49

**To:** Doug Arnold; tictoc@ietf.org

**Subject:** Re: [TICTOC] interesting article on a global mechanism for
one-way delay measurement

Doug

Yes, the topology has be=
known,
although it can change (reroute events) as long as we are informed of this.=

For ToD distribution it =
is a bit
less elegant than running full CTP,

but it gives you a bette=
r
feeling for what is happening.

I think it could go well=
with
MPLS-TP, or even better with MPLS and a PCE box

(which not only knows th=
e
topology, but could optimize the timing paths).

Y(J)S

**From:** Doug Arnold
[mailto:darnold@symmetricom.com]

**Sent:** Wednesday, January 06, 2010 20:22

**To:** Yaakov Stein; tictoc@ietf.org

**Subject:** RE: [TICTOC] interesting article on a global mechanism for
one-way delay measurement

Thanks Yaakov,

This is an interesting
idea. It does require the that complete paths be known and
controlled. Perhaps it could be used in conjunction with MPLS.

//Doug

**From:**
tictoc-bounces@ietf.org [mailto:tictoc-bounces@ietf.org] **On Behalf Of Yaakov
Stein
Sent: Wednesday, January 06, 2010 9:16 AM
To: tictoc@ietf.org
Subject: [TICTOC] interesting article on a global mechanism for one-=
way
delay measurement**

Hi all,

I have recently been working time distribution in the
presence of strong asymmetry,

and have come across a method that helps in certain ca=
ses.

I am sure that you all remember the CTP algorithm that=
I
have brought up before

(and presented at IETF-74).

The same academic group has an earlier article that I =
had
previously overlooked.

I am talking about :

Estimating One-Way Delays from Cyclic Path Delay Measurements.

16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF]

This article gives a procedure for determining one-way
delays

based purely on round-trip delay measurements (i.e., w=
hat we
would call T4-T1),

knowledge of topology, and the assumption of additivit=
y of
propagation delays.

The idea is that nodes measure round-trip times to var=
ious
other nodes,

knowing which nodes are traversed.

For example, assume three nodes connected in a triangl=
e

=
1

/ &n=
bsp;
\

/ &n=
bsp;
\

=
2 -------- 3

and we measure the times for the following paths

1 2 3

2 3 2

3 1 3

1 2 3 1

We thus have 4 equations for 6 variables

(since the links are not assumed symmetric, the variab=
les
are D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ).

Using additivity and non-negativity it turns out that =
one
can solve an optimization problem

which minimizes the error of these equations.

The solution requires a centralized server to do the m=
ath
PCE-style,

but solves a problem that I don't know any other way t=
o
solve.

Comments ?

Y(J)S