Subject: [PCN] PCN architecture - revised text on ECMP

From: <[email-masked]>
Date: Wed, 8 Aug 2007 23:13:13 +0100

This is to flag up that I wrote some revised text on ECMP (in Section 6,
Design goals and challenges):

   The following are open issues.  They are taken from

   [I-D.briscoe-tsvwg-cl-architecture] which also describes some

   possible solutions (potential solutions are out of scope for this

   document).  Note that some may be considered unimportant in general

   or in specific deployment scenarios.

   o  ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion

      is measured on a specific ingress-egress-aggregate.  However, if

      the PCN-domain runs ECMP, then traffic on this ingress-egress-

      aggregate may follow several different paths - some of the paths

      could be pre-congested whilst others are not.  There are two

      potential problems:

      1.  over-admission: a new flow is admitted (because the pre-

          congestion level measured by the PCN-egress-node is

          sufficiently diluted by unmarked packets from non-congested

          paths that a new flow is admitted), but its packets travel

          through a pre-congested PCN-node

      2.  ineffective termination: flows are terminated, however their

          path doesn't travel through the (pre-)congested router(s).

      The overall PCN solution for flow termination must solve the

      second problem, since flow termination is a 'last resort'.  For

      flow admission, the risk of slight over-admission may be

      acceptable (particularly with flow termination as a fall-back), at

      least for some operators.

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From: "Jozef Babiarz" <[email-masked]>
Date: Tue, 14 Aug 2007 16:57:42 -0400

Please see my comment below denoted with [Joe].

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

________________________________

From: [email-masked] [mailto:[email-masked]] 
Sent: August 8, 2007 6:13 PM
To: [email-masked]
Subject: [PCN] PCN architecture - revised text on ECMP

 

      potential problems:

[Joe] Actual there are 3 or 4 potential problems. [end]

[Joe] 2.  under-admission: a new flow is blocked (because the
pre-congestion level measured by the PCN-egress-node is sufficiently
diluted by marked packets from pre-congested PCN-nodes that a new flow
is blocked admission), but its packets would travel through a
un-congested path. [end]

[Joe] The above requires that we only terminate marked flows. In the
"Flow termination" section of the architecture draft there is no mention
that only flows travelling through congested router(s) are to be
terminated. I believe we need to add statement that only flows as
indicated by packet(s) marking indicating passing over link(s) where the
PCN-upper-rate was exceeded as requirement for ECMP. For flow
termination to work with ECMP, only flows that pass over link(s) where
PCN-upper-rate was exceeded should be terminated.

[Joe] I believe we should state that flow termination provides
protection to the network should over-admission occur. However,
over-admission followed by termination of some over-admitted flows is
not desirable as it leads to dissatisfied users.

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From: <[email-masked]>
Date: Mon, 20 Aug 2007 08:54:42 +0100

In-line

phil

-----Original Message-----
From: Jozef Babiarz [mailto:[email-masked]] 
Sent: 14 August 2007 21:58
To: Eardley,PL,Philip,CXR9 R; [email-masked]
Subject: RE: [PCN] PCN architecture - revised text on ECMP

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

 

      potential problems:

[phil] ok, will add.

[phil] the start of this section of open issues says "potential
solutions are out of scope for this document". However, I think it would
be worth explicitly adding in a statement that says the characteristic
that a solution would have (which is essentially the opposite of the
problem statements)

[phil[ will add something about the risk of dissatisfied users