Subject: RE: Silent Packet Loss & Header Compression

From: "Chen, David" <[email-masked]>
Date: Thu, 24 May 2001 17:17:08 -0400

There is a factor to decide, if the received packets are out of order or
dropped.
As the following example:
Let's say, the receiving packets (segments) in temporal sequence as
 1 2 4 5 6 7 8 9 10 3

At 4, the device will know 3 is missing but
not until after 10 it receive 3 knows that the packet 3 is reordered.

Therefore the device has to buffer (4 5 6 7 8 9 10) packets and insert 3 at
front.
However, if the 3 never show up for next xx packets from 4 then
the device will declare the 3 is dropped even it arrived after xx.

The re-ordering and dropping is sensitive to time and buffer (as always)
tradeoffs.
The more buffering the more jitter due to the transmission will be more
burst then
received.

This is true to a single link.

--- David

-----Original Message-----
From: James Carlson [mailto:[email-masked]]
Sent: Thursday, May 24, 2001 4:55 PM
To: Ken Peirce
Cc: Ramana Gollamudi; [email-masked]'
Subject: RE: Silent Packet Loss & Header Compression

Ken Peirce writes:
> For MP, your can reorder,

No.  You can misorder *among* links, but you cannot misorder *within*
a link (see RFC 1990 section 4.1 for the "increasing sequence number"
requirement).  Since IP underneath L2TP can sometimes misorder within
a flow, that means you need protection to avoid breaking MP.

> but you must then buffer those out-of-order
> fragments. This results in increased buffering capacity requirements for
the
> MP bundle owner as well as increased latency and jitter for the data. 

True between links, but not for sequential packets on a single member
link.

-- 
James Carlson, Internet Engineering       <[email-masked]>
SUN Microsystems / 1 Network Drive         71.234W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.497N   Fax +1 781 442 1677

---

From: James Carlson <[email-masked]>
Date: Thu, 24 May 2001 17:30:42 -0400 (EDT)

> There is a factor to decide, if the received packets are out of order or

Right; that's a good explanation of that part of the problem.

For MP, there's more to it than that.  MP multiplies the loss rate by
the fragmentation factor.  In other words, if you fragment a packet
into (say) three MP fragments, then a loss of *any* of those fragments
results in the loss of the complete packet and a delay in reassembly
of the subsequent packets as well.  In other words, the packet loss
rate is multiplied by 3.

Worse still, the L2TP optimization you're talking about plays into
this.  At some point you have to give up waiting for a packet even
though it's just been delayed (and misordered) by a congested link
somewhere in the network.  When you do give up, you create additional
artificial losses.

Even worse, there's no control over the arrival skew between the
links, and MP's reassembly latency (and buffer size) is completely
dependent on this skew.  This latency will affect TCP's performance,
because it's not bounded in typical implementations.

So, with MP, you're vulnerable in more places, and with L2TP, you
create more lossage and lower performance.  That's why I wouldn't say
that MP over L2TP over the general Internet is a great idea.