Subject: RFC2873 manually setting precedence.

From: Phillip Grasso-Nguyen <[email-masked]>
Date: Sun, 2 Jul 2000 14:23:52 +1000

Hi,

Since most DSCP and QoS functions are normally handled within a single
network domain, what impact would it have to manually set precedence at the
edge.

for example,
CE = customer edge, or host systems (TCP endpoints)
PE = Provider edge,
P   = Provider Core, 

CE - PE - P - PE - CE

If the provider PE edge manually sets the precedence outbound to the CE what
affect will this have on the TCP session.
the TCP RST condition should happen when their is a ["lower precedence" or
any change in precedence] if both ends where set to the same precedence by
the PE router for example precedence or dscp of "0". Does tcp security have
a specific requirement of precedence, what happens if the tcp stack
initiates a connection with a precedence of 1 and during transmission it
gets reassigned with precedence of "0" does this screw the TCP
session/connection? is manually setting precedence or DSCP to the a single
value outbound to the CE or TCP endpoints a possible work around?

Regards
	Phillip.

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Davnet Telecommunications
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"Leave complexity at the 'edges' and keep the network 'core' simple"

---

From: Alan Hannan <[email-masked]>
Date: Sun, 2 Jul 2000 11:43:18 -0700

Phillip,

> CE = customer edge, or host systems (TCP endpoints)

  I'll modify as:

  CE1 -- PE1 -- P -- PE2 -- CE2

> affect will this have on the TCP session.

  As I think you know, the primary catalyst for this modification
  was observed operational behaviour.  We saw that when mid-point
  devices [P] changed TOS/DSCP and the observed response packets
  [at CE1] within the TCP session varied, CE1 would reset the packet.

> the PE router 

  Although a viable solution, this is difficult to implement in
  practice.  The need this presents for PE1 and PE2 to synchronize by
  session and policy is great.  I believe this demand would severely
  impede DHCP/TOS and internet deployment.

  Another implementation workaround would maintain state as below,
  but this is only a partial resolution to the problem.

> for example precedence or dscp of "0". Does tcp security have
> a specific requirement of precedence, 

  In practice, no.  Per a strict interpretation of the RFC793, yes.
  We saw that mainly some [10%] of MacTCP (apple) TCP/IP stacks
  enforced this, as well as some old IBM stacks.

> what happens if the tcp stack
> session/connection? 

  Yes, because what CE1 sends "1" is changed by {PE1,P,PE2, or CE2}
  and is not what he expected.

> is manually setting precedence or DSCP to the a single

  It might be, except that it either:

  	requires sychnornization by CE's or PE's

  	or

  	disallows tcp-based DSCP delivery.  One could set things to a
  	default value [say, 0] and remember what it got on a
  	flow-by-flow basis.  However, this would negate end-to-end
  	ability of DSCP/TCP; which is certainly a desirable behaviour
  	we do not wish to negate.

   -alan