Subject: [PWE3] Proposed SAToP congestion text

From: Stewart Bryant <[email-masked]>
Date: Mon, 30 Jan 2006 11:45:07 +0000

This note contains the proposed new congestion section for SAToP.
I am sending it to the PWE3 WG for review.

We propose sending to Alison for her review and signoff at the
end of the week.

- Stewart

8. Congestion Control  
 
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion. SAToP PWs represent inelastic constant bit-rate (CBR) flows
and cannot respond to congestion in a TCP-friendly manner prescribed by
[RFC2914], although the percentage of total bandwidth they consume
remains constant. 
 
Unless appropriate precautions are taken, undiminished demand of
bandwidth by SAToP PWs can contribute to network congestion that may
impact network control protocols. 
 
Whenever possible, SAToP PWs SHOULD be carried across traffic-
engineered PSNs that provide either bandwidth reservation and admission
control or forwarding prioritization and boundary traffic conditioning
mechanisms. IntServ-enabled domains supporting Guaranteed Service (GS)
[RFC2212] and DiffServ-enabled domains [RFC2475] supporting Expedited
Forwarding (EF) [RFC3246] provide examples of such PSNs. Such
mechanisms will negate, to some degree, the effect of the SAToP PWs on
the neighboring streams. In order to facilitate boundary traffic
conditioning of SAToP traffic over IP PSNs, the SAToP IP packets SHOULD
NOT use the DiffServ Code Point (DSCP) value reserved for the Default
PHB[RFC2474].
 
If SAToP PWs run over a PSN providing best-effort service, they SHOULD
monitor packet loss in order to detect "severe congestion". If such a
condition is detected, a SAToP PW SHOULD shut down bi-directionally for
some period of time as described in Section 6.5 of [RFC3985]. 
 
Note that:
 
   1. The SAToP IWF can inherently provide packet loss measurement
      since the expected rate of arrival of SAToP packets is fixed and
      known
   2. The results of the SAToP packet loss measurement may not be a
      reliable indication of presence or absence of severe congestion
      if the PSN provides enhanced delivery, e.g.:
      a) If SAToP traffic takes precedence over non-SAToP traffic,
         severe congestion can develop without significant SAToP
         packet loss
      b) If non-SAToP traffic takes precedence over SAToP traffic,
         SAToP may experience substantial packet loss due to a
         short-term burst of high-priority traffic
   3. The TDM services emulated by the SAToP PWs have high
      availability objectives (see [G.826]) that MUST be taken into
      account when deciding on temporary shutdown of SAToP PWs.
 
This specification does not define the exact criteria for detecting
"severe congestion" using the SAToP packet loss rate or the specific
methods for bi-directional shutdown the SAToP PWs (when such severe
congestion has been detected) and their consequent re-start after a
suitable delay. This is left for further study. However, the following
considerations may be used as guidelines for implementing the SAToP
severe congestion shutdown mechanism:
 
1. SAToP Performance Monitoring techniques (see Section 6.4)
   provide entry and exit criteria for the SAToP PW "Unavailable"
   state that make it closely correlated with the "Unavailable"
   state of the emulated TDM circuit as specified in [G.826]. Using
   the same criteria for "severe congestion" detection may decrease
   the risk of shutting down the SAToP PW while the emulated TDM
   circuit is still considered available by the CE.

2. If the SAToP PW has been set up using either PWE3 control
   protocol [PWE3-CONTROL] or L2TPv3 [RFC 3931], the regular PW
   teardown procedures of these protocols SHOULD be used. 

3. If one of the SAToP PW end points stops transmission of packets
   for a sufficiently long period, its peer (observing 100% packet
   loss) will necessarily detect "severe congestion" and also stop
   transmission, thus achieving bi-directional PW shutdown.

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