Subject: Re: [Pals] Comments of draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

From: "Lizhong Jin" <[email-masked]>
Date: Wed, 12 Nov 2014 10:00:33 +0800

Hi Weiqiang,

Thanks for the reply, see inline below.

Lizhong

From: 程伟强 [mailto:[email-masked]] 
Sent: 2014年11月12日 9:21
To: 'Lizhong Jin'
Cc: [email-masked]
Subject: RE: [Pals] PALS IETF 91 Slot Requests - Honolulu - SLIDES

Hi  Lizhong,

Thank you very much for your comments. Jie and I answered your questions
inline.

[BTW: Some mails were not post on the WG board, because “Too many
recipients to the message”. In Following mail, I summarized all comments
and answers ] 

B.R.

Weiqiang Cheng

From: Lizhong Jin [mailto:[email-masked]] 
Sent: Tuesday, November 11, 2014 6:04 PM
To: 'Dongjie (Jimmy)'; '程伟强'; 'David Sinicrope'; [email-masked]; 'Shahram
Davari'
Cc: 'Stewart Bryant'; 'Andrew Malis'; [email-masked];
[email-masked]; 'Hui Deng'
Subject: Comments of draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

Hi,

Have a quick review of draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01.

In section 3.2:

After the exchange of service PW state and switching request, both

   dual-homing PEs could determine the Active/Standby forwarding status

   of the working and protection service PWs.

The “switching request” and “switchover request” refer to the switchover
request by LDP defined in RFC6870, right? In that case, you need to describe
the interaction between the mechanism defined in RFC6870 and the DHC defined
in your document. One sentence as above is not enough. E.g., in case
Two-side Dual-homing Scenario, if independent mode is enabled between two
PEs, and the service PW is done, then when PE1 will switchover, after
waiting for protection PW to be active, or send packets to PE2, and let PE2
to drop packets first.

[Jie] Here the switching request refers to the “Dual-Node Switching TLV”
in DHC message exchanged between the dual-homing PEs, it does not rely on
RFC6870. 

[Jie] I guess the “PE2” in your above example means the remote side PE
(not the other dual-homing PE in the same side), right? If so, the
switchover coordination between PE1 and PE2 can be achieved using PSC
defined in RFC6378. 

[Lizhong] I see. Could you say that explicitly, and the relationship with
RFC6378? 

[Weiqiang] Agree. We will add one sentence to describe the relationship 

And I suggest to not use terminology “standby”. You could use, e.g.,
normal state on protection PW. A PW in standby state is clearly defined in
RFC6870.

[Weiqiang] Agree. Is that OK if We use “hot-standby” to replace the
“standby” in the draft?

[Lizhong] personally, I like “protection state”, or “normal state”
defined in RFC6378. Anyway, depends on you.

Another concern from me is in the second presentation, slide 9, if failure 3
and 5 happened simultaneously (or failure 1 and 3), it is possible that one
packet from CE1 will be send to PE2 by PE1, and PE2 will send back to CE1
(the AC2 is active because of the failure5), then that looped packet may
influence the MAC table of CE switch. This is a corner case, but it seems it
is possible. If the CE switch allows MAC moving, this looped packet may lead
to traffic loss.

[Weiqiang] The forwarding behavior of the dual-homing PE nodes is determined
by the forwarding state machine as shown in the following  table:

          +-----------+---------+--------+---------------------+

          |Service PW |   AC    | DNI PW | Forwarding Behavior |

          +-----------+---------+--------+---------------------+

          |  Active   | Active  |   Up   |Service PW <-> AC    |

          +-----------+---------+--------+---------------------+

          |  Active   | Standby |   Up   |Service PW <-> DNI PW|

          +-----------+---------+--------+---------------------+

          |  Standby  | Active  |   Up   |    DNI PW <-> AC    |

          +-----------+---------+--------+---------------------+

          |  Standby  | Standby |   Up   |  Drop all packets   |

          +-----------+---------+--------+---------------------+

When the scenario you mentioned happens, the PE2’s state = (Service
PW=Active, AC= Standby, DNI PW= UP) => the forwarding behavior will be only
Service PW<->DNI PW which means the packets from DNI PW can only be
forwarded to PE2’s Service PW. 

[Lizhong] In the scenario I mean failure 3 and 5 happen simultaneously
(e.g., link of working PW and AC1 share same risk in deployment), then on
PE2, the protection PW is still standby (for the signaling reason from PE1
or to PE3), and AC2 is already active. When PE2 receiving packets from DNI
PW, it will forward the packets back to CE1 via AC2. PE1 will not wait for
PE2 to be state ready, and forward the packets from AC1 directly to PE2.Did
I misunderstanding the state machine? 

Regards

Lizhong

---

From: "Dongjie (Jimmy)" <[email-masked]>
Date: Wed, 12 Nov 2014 02:53:56 +0000

Hi Lizhong,

Please see my further replies inline.

Many thanks,
Jie

From: Pals [mailto:[email-masked]] On Behalf Of Lizhong Jin
Sent: Wednesday, November 12, 2014 10:01 AM
To: '程伟强'
Cc: [email-masked]
Subject: Re: [Pals] Comments of draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

Hi Weiqiang,

Lizhong

From: 程伟强 [mailto:[email-masked]]
Sent: 2014年11月12日 9:21
To: 'Lizhong Jin'
Cc: [email-masked]<mailto:[email-masked]>
Subject: RE: [Pals] PALS IETF 91 Slot Requests - Honolulu - SLIDES

Hi  Lizhong,

Thank you very much for your comments. Jie and I answered your questions inline.
[BTW: Some mails were not post on the WG board, because “Too many recipients to the message”. In Following mail, I summarized all comments and answers ]

B.R.
Weiqiang Cheng
From: Lizhong Jin [mailto:[email-masked]]
Sent: Tuesday, November 11, 2014 6:04 PM
To: 'Dongjie (Jimmy)'; '程伟强'; 'David Sinicrope'; [email-masked]<mailto:[email-masked]>; 'Shahram Davari'
Cc: 'Stewart Bryant'; 'Andrew Malis'; [email-masked]<mailto:[email-masked]>; [email-masked]<mailto:[email-masked]>; 'Hui Deng'
Subject: Comments of draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

Hi,
The “switching request” and “switchover request” refer to the switchover request by LDP defined in RFC6870, right? In that case, you need to describe the interaction between the mechanism defined in RFC6870 and the DHC defined in your document. One sentence as above is not enough. E.g., in case Two-side Dual-homing Scenario, if independent mode is enabled between two PEs, and the service PW is done, then when PE1 will switchover, after waiting for protection PW to be active, or send packets to PE2, and let PE2 to drop packets first.

[Jie] Here the switching request refers to the “Dual-Node Switching TLV” in DHC message exchanged between the dual-homing PEs, it does not rely on RFC6870.

[Jie] I guess the “PE2” in your above example means the remote side PE (not the other dual-homing PE in the same side), right? If so, the switchover coordination between PE1 and PE2 can be achieved using PSC defined in RFC6378.
[Lizhong] I see. Could you say that explicitly, and the relationship with RFC6378?

And I suggest to not use terminology “standby”. You could use, e.g., normal state on protection PW. A PW in standby state is clearly defined in RFC6870.

[Weiqiang] Agree. Is that OK if We use “hot-standby” to replace the “standby” in the draft?
[Lizhong] personally, I like “protection state”, or “normal state” defined in RFC6378. Anyway, depends on you.
[Jie] We will discuss with co-authors and have some update in next revision.

Another concern from me is in the second presentation, slide 9, if failure 3 and 5 happened simultaneously (or failure 1 and 3), it is possible that one packet from CE1 will be send to PE2 by PE1, and PE2 will send back to CE1 (the AC2 is active because of the failure5), then that looped packet may influence the MAC table of CE switch. This is a corner case, but it seems it is possible. If the CE switch allows MAC moving, this looped packet may lead to traffic loss.

[Weiqiang] The forwarding behavior of the dual-homing PE nodes is determined by the forwarding state machine as shown in the following  table:

          +-----------+---------+--------+---------------------+
          |Service PW |   AC    | DNI PW | Forwarding Behavior |
          +-----------+---------+--------+---------------------+
          |  Active   | Active  |   Up   |Service PW <-> AC    |
          +-----------+---------+--------+---------------------+
          |  Active   | Standby |   Up   |Service PW <-> DNI PW|
          +-----------+---------+--------+---------------------+
          |  Standby  | Active  |   Up   |    DNI PW <-> AC    |
          +-----------+---------+--------+---------------------+
          |  Standby  | Standby |   Up   |  Drop all packets   |
          +-----------+---------+--------+---------------------+
When the scenario you mentioned happens, the PE2’s state = (Service PW=Active, AC= Standby, DNI PW= UP) => the forwarding behavior will be only Service PW<->DNI PW which means the packets from DNI PW can only be forwarded to PE2’s Service PW.
[Lizhong] In the scenario I mean failure 3 and 5 happen simultaneously (e.g., link of working PW and AC1 share same risk in deployment), then on PE2, the protection PW is still standby (for the signaling reason from PE1 or to PE3), and AC2 is already active. When PE2 receiving packets from DNI PW, it will forward the packets back to CE1 via AC2. PE1 will not wait for PE2 to be state ready, and forward the packets from AC1 directly to PE2.Did I misunderstanding the state machine?
[Jie] I see this is a multi-failure scenario (PW failure + AC failure). In this case PE1 would notify PE2 using DHC message that the working PW is down, so that protection PW would become active. At the same time the AC redundancy mechanism would change the state of AC2 to active. Then according to the above forwarding state machine, the forwarding behavior of PE2 will be protection PW<->AC2. I understand maybe your concern is about the dual-homing transition period, during which some packets may have to be dropped. IMO this happens for all protection mechanisms. The important thing is after switchover, traffic will be forwarded normally.

Regards
Lizhong

---

From: "[email-masked]" <[email-masked]>
Date: Wed, 12 Nov 2014 23:50:52 +0800

[Lizhong] yes, this is a multi-failure scenario. But the key point is, after switchover, the traffic may be dropped by CE. For example, when the packet sent by CE is looped back via AC2, then CE may learn the source MAC with AC2 as the output interface. In that case, the packet to the learned source MAC will be droped (because of source port filtering) until the MAC address is relearned again via the other correct port. This is a corner case, but if happend, will lead to severe problem.

Regards
Lizhong

Regards
Lizhong

---

From: "Lizhong Jin" <[email-masked]>
Date: Thu, 13 Nov 2014 10:38:59 +0800

Hi Weiqiang,

OK, then you should detail the mechanism you described below in the draft,
and integrate the DHC message state into your state machine. The state
transition diagram should be also described. For the current state machine
in table1, I could only see that the packet forwarding is only controlled by
the state of local PW or AC. 

Thanks

Lizhong

From: 程伟强 [mailto:[email-masked]] 
Sent: 2014年11月13日 9:01
To: [email-masked]; 'jie.dong'
Cc: 'pals'
Subject: RE: [Pals] Comments of
draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

From: Pals [mailto:[email-masked]] On Behalf Of [email-masked]
Sent: Wednesday, November 12, 2014 11:51 PM
To: jie.dong; chengweiqiang
Cc: pals
Subject: Re: [Pals] Comments of
draft-cheng-pwe3-mpls-tp-dual-homing-coordination-01

 Another concern from me is in the second presentation, slide 9, if failure
3 and 5 happened simultaneously (or failure 1 and 3), it is possible that
one packet from CE1 will be send to PE2 by PE1, and PE2 will send back to
CE1 (the AC2 is active because of the failure5), then that looped packet may
influence the MAC table of CE switch. This is a corner case, but it seems it
is possible. If the CE switch allows MAC moving, this looped packet may lead
to traffic loss.

[Jie] I see this is a multi-failure scenario (PW failure + AC failure). In
this case PE1 would notify PE2 using DHC message that the working PW is
down, so that protection PW would become active. At the same time the AC
redundancy mechanism would change the state of AC2 to active. Then according
to the above forwarding state machine, the forwarding behavior of PE2 will
be protection PW<->AC2. I understand maybe your concern is about the
dual-homing transition period, during which some packets may have to be
dropped. IMO this happens for all protection mechanisms. The important thing
is after switchover, traffic will be forwarded normally.

[Lizhong] yes, this is a multi-failure scenario. But the key point is, after
switchover, the traffic may be dropped by CE. For example, when the packet
sent by CE is looped back via AC2, then CE may learn the source MAC with AC2
as the output interface. In that case, the packet to the learned source MAC
will be droped (because of source port filtering) until the MAC address is
relearned again via the other correct port. This is a corner case, but if
happend, will lead to severe problem.

[Weiqiang] The Key point is PE1’s state machine also need time to transit.
When the state machine is on the transition period, any traffic packets have
to be dropped. Let’s consider the scenario, you mentioned: We set an
example: AC failure is detected first:

Before PE1 detect the issue on the AC1 failure, the AC1 traffic will be
forwarded to working PW, no packets from AC1 go to PE2.

Once the PE1 detect the issue on the AC failure, the state machine will be
on transition state, all the packets (include buffered packets)  from AC1
will be dropped. And the notification will be send to PE2, the PE1 got the
Ack, and then finished the state transition. So when the PE1 begins to send
packet to PE2, there is no packet from AC1 left in PE1.

Regards

Lizhong

Regards

Lizhong