Subject: RE: Silent Packet Loss & Header Compression

From: Ramana Gollamudi <[email-masked]>
Date: Wed, 23 May 2001 17:46:14 -0400

A newbie question on this topic. Is sequencing in the data plane used in
existing popular implementations.  I wonder if there are significant
performance implications due to sequencing?

Ramana

-----Original Message-----
From: James Carlson [mailto:[email-masked]]
Sent: Wednesday, May 23, 2001 12:04 PM
To: Ignacio Goyret
Cc: [email-masked]; Jonathan Goodchild; [email-masked]'
Subject: Re: Silent Packet Loss & Header Compression

Ignacio Goyret writes:
> That would fail fatally if the client is using MP since no single LAC is
> guaranteed that it will have access to all the channels composing an MP
> link, which is one of many reasons why LACs should not do anything like
that.

Running MP over L2TP over a lossy or delay-prone network is a bad idea
to begin with.  Having the MP links bundled in a LAC-local manner (as
with Bay's MMP) and then representing the bundle as a single PPP
session to the LNS is a much better answer.

Note that because of the Endpoint-Discriminator issues, MP over L2TP
where the LAC endpoints are *not* reasonably local to each other just
doesn't work anyway.

-- 
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: Vernon Schryver <[email-masked]>
Date: Thu, 24 May 2001 09:01:52 -0600 (MDT)

> From: James Carlson <[email-masked]>
> To: Ramana Gollamudi <[email-masked]>
> Cc: "'Ken Peirce'" <[email-masked]>, "[email-masked]'" <[email-masked]>

> ...
> The problem with reordering, from TCP's perspective, is that it kicks
> you out of the fast path in most implementations.  If you're going to
> delay packets in the hope the you can resolve the ordering later,
> you're already negating the gain seen by being in the fast path in the
> first place.

Reordering kicks TCP into fast retransmits which wastes bandwidth
throughout the path and reduces the congestion window, which reduces the
amoung of data in flight.  Each out of order TCP segment is required to
generate a duplicat Ack.  Three consecutive Acks cause a fast retransmit.
See RFC 2001.

In my ancient experiements with what I shipped and called "BF&I"
PPP multilink and others called "round robin" and other things,
the small ordering inevitable in running two independing PPP pipes
reduced TCP throughput on 9600-38400 bit/sec wires by about 20%.

Vernon Schryver    [email-masked]

---

From: "Chen, David" <[email-masked]>
Date: Thu, 24 May 2001 11:25:18 -0400

The IP service does not quarantee packet delivery in sequence.
Since L2tp encapsulate PPP and that encapsulate IP (most of the time), 
it does not make sense ask L2tp provide packet delivery in sequece.

On the other hand, when the UDP that l2tp use as media,
the UDP/IP does not quarantee packet delivery in sequence.

Since no flow control in l2tp data channel, the Nr and Ns bits in l2tp
header 
are only for detecting packet drop (or re-ordering) and 
do nothing about it (in protocol view)

--- David

-----Original Message-----
From: Ramana Gollamudi [mailto:[email-masked]]
Sent: Wednesday, May 23, 2001 6:16 PM
To: 'Ken Peirce'
Cc: [email-masked]'
Subject: RE: Silent Packet Loss & Header Compression

Agreed. However, most of internet seems to work reasonably well without
intermediate nodes doing providing any support for ensuring ordered
delivery. Best effort seems to enough. The argument in this mail chain
seemed to indicate that MP PPP spread over a multiple LACs is the driver for
sequencing in L2TP. So, I wondered how common this usage is, and what does
it really solve?

I would really appreciate any pointers to old discussions on this.

Ramana

-----Original Message-----
From: Ken Peirce [mailto:[email-masked]]
Sent: Wednesday, May 23, 2001 6:03 PM
To: 'Ramana Gollamudi'
Subject: RE: Silent Packet Loss & Header Compression

TCP has an intense dislike for resequencing.

> From: Ramana Gollamudi [mailto:[email-masked]]
> Sent: Wednesday, May 23, 2001 2:46 PM
> To: 'James Carlson'
> Cc: [email-masked]'
> Subject: RE: Silent Packet Loss & Header Compression
> 
>

---

From: James Carlson <[email-masked]>
Date: Thu, 24 May 2001 16:54:40 -0400 (EDT)

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.