Subject: Re: [PWE3] [Fwd: Proposed text for SAToP congestion section]

From: Stewart Bryant <[email-masked]>
Date: Wed, 28 Dec 2005 15:02:44 +0000

Joe Merritt wrote:

>Stewart,
>
>IMHO the proposed text appears to run counter to OAM methodology used in
>the TDM networks. SAToP is about transport versus services. The
>transport of the network has well established OAM methodology even when
>using packet transport (i.e. reference ITU-T I.610) which is harmonized
>with the way TDM equipment works and established Fault Management
>systems.
>
>Perhaps, I am not fully appreciating the proposed text however I am
>concerned about the approach being discussed based on 15 years
>experience with DS1/E1 and packet switched access equipment.
>
>  
>
Joe

Please could you elaborate your concerns.

The text was written to address the concern that a SAToP PW
MUST NOT drive the underlying IP network into congestion collape.
Therefore when it is detected that the such a collapse is
incipient the PW must stop sending packets. In a well engineered
network this mechanism will not be invoked, but it is needed
as a backstop.

Stewart

>Regards,
>Joe
>
>-----Original Message-----
>From: [email-masked] [mailto:[email-masked]] On Behalf Of
>Stewart Bryant
>Sent: Tuesday, December 27, 2005 5:14 AM
>To: pwe3
>Subject: [PWE3] [Fwd: Proposed text for SAToP congestion section]
>
>Following discussion with the transport ADs, we propose that text of the
>following form be added to the TDM drafts.
>
>I think that we need to add similar text to our other drafts to cover
>the case where they are used to carry non-TCP friendly streams.
>
>The obvious cases is where this is needed is ATM cell mode.
>
>- Stewart
>
>
>8.  Congestion Control  
> 
>As explained in [PWE3-ARCH], the PSN carrying the PW may be subject to
>congestion, with congestion characteristics depending on PSN type,
>network architecture, configuration, and loading. During congestion the
>PSN may exhibit packet loss and PDV that will impact the timing and data
>integrity of the SAToP PW.
>During intervals of accute congestion SAToP will not be able to maintain
>service.
>In addition, since SAToP PWs carry constant bit rate (CBR) services
>across the PSN, they cannot behave in a TCP-friendly manner prescribed
>by [RFC2914].
>In the presence of services that reduce transmission rate, SAToP PWs
>will thus consume more than their fair share (although being CBR, the
>percentage of total bandwidth they consume remains constant) and SHOULD
>be halted.
> 
>Whenever possible, SAToP PWs should be run over traffic-engineered PSNs
>providing bandwidth allocation and admission control mechanisms.
>IntServ-enabled domains providing the Guaranteed Service (GS) or
>DiffServ-enabled domains using EF (expedited forwarding) are examples of
>traffic-engineered PSNs. Such PSNs will minimize loss and delay while
>providing some degree of isolation of the SAToP PW's effects from
>neighboring streams.
> 
>The PEs SHOULD monitor for congestion (by using explicit congestion
>notification, VCCV, or by measuring packet loss) in order to ensure that
>the SAToP service may be maintained. Note that SAToP packet loss rate
>may always be measured unintrusively since the expected packet arrival
>rate is known and fixed.
>When significant congestion is detected the SAToP service SHOULD be
>terminated. Because PWs are inherently birectional, the detecting PE can
>terminate the SAToP service without explicitly communicating with the
>remote PE. The PE initiating termination simply ceases transmission of
>SAToP packets. The other PE, even if previously unaware of a problem,
>will of necessity detect that packets are not arriving and MUST
>similarly cease packet transmission.
>If the PW has been set up using the PWE control protocol, then
>procedures specified in [PWE3-CONTROL] for label Withdrawal MAY be used
>as well. The PW may be restarted by manual intervention, or by automatic
>means after an appropriate waiting time.
>
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>
>  
>

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