Subject: Re: pilc minutes for IETF 48

From: Vernon Schryver <[email-masked]>
Date: Fri, 6 Oct 2000 18:28:35 -0600 (MDT)

> From: Reiner Ludwig <[email-masked]>

> ...
> To avoid that problem, you can allow the link layer to do out-of-order 
> delivery of packets belonging to different flows. You should only enforce 
> in-order delivery for packets belonging to the same TCP flow to not falsely 
> trigger fast retransmit / fast recovery.
>
> Caveat: the link layer must be given the opportunity to distinguish flows.

Why go to any trouble to prevent out-of-order delivery?
We are talking only about IP, right?  Not PPP or various bridging protocols?

What is the cost of out-of-order delivery?  As far as I know, IP packet
re-ordering has at most the following costs:

  - broken IP fragmentation code goes crazy
      this is surely not a consideration

  - broken UDP/IP applications go crazy
      they are also surely irrelevant.

  - TCP goes slightly slower, and with sufficient consecutive out-of-order
     segments, can send duplicate ACKs which can cause retransmissions.

Is that last the concern?

Vernon Schryver    [email-masked]

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From: Reiner Ludwig <[email-masked]>
Date: Mon, 09 Oct 2000 13:30:53 +0200

>Is that last the concern?

Yes, it is.

As soon as the link's capacity goes beyond 4x MTU, any link layer 
retransmission would cause a *spurious* fast retransmit / fast recovery in 
TCP. That assumes that (1) the following 3 packets do not incur a link 
layer retransmission and (2) out-of-order delivery is permitted even within 
a flow.

For some wireless networks this could become a real pain ...

///Reiner

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From: Bill Sommerfeld <[email-masked]>
Date: Mon, 09 Oct 2000 13:04:59 -0400

>       this is surely not a consideration

related, but slightly different:
	- broken packet filtering code goes crazy when fragments are
	  reordered.