Subject: Re: [multipathtcp] MPTCP ndiffports parameter

From: Christoph Paasch <[email-masked]>
Date: Sun, 6 Jul 2014 11:50:14 +0200

Hello,

On 06/07/14 - 03:15:01, Abhishek Verma wrote:
> Hi,
> 
> I came across the ndiffports parameter in
> http://multipath-tcp.org/pmwiki.php?n=Main.Release88
> 
> The idea here is to have parallel TCP sessions between a given pair of
> source and dest. The advantage would be that each TCP session will
> send/recv packets based on their individual tcp_rmem/tcp_wmem values. Each

the tcp_rmem/wmem values specify the hard limits for the
send/receive-window. The send/receive-window in MPTCP is handled at the
MPTCP-layer and not on a per-subflow level.

> would go through its slow start algorithm, but the total data transferred
> in each phase would multiply be the number of TCP sessions.
> 
> This would work as long as the pipe between the two end points is wide
> enough to carry all the combined traffic since if this link gets congested
> then all the parallel TCP Sessions will get affected and each will reduce
> its window size.

The idea behind ndiffports is that ECMP may be in the network, sending the
traffic across distinct bottlenecks, by doing L4 load-balancing.

If there is no ECMP, thus all subflows go through the same bottleneck, there
is probably no benefit you will observe with MPTCP.

Cheers,
Christoph

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From: Abhishek Verma <[email-masked]>
Date: Sun, 6 Jul 2014 20:33:49 +0530

Hi Christoph,

> is probably no benefit you will observe with MPTCP.

I disagree to this.

Even if all subflows of the MPTCP go via the same path it would still offer
some benefit as the total throughput achieved is n times the throughput
achieved with a single TCP session, where n is the number of subflows.

Basically, in that case it would behave like "n" parallel TCP sessions
where some traffic is sent over each TCP session. For example, when
transferring a file, each subflow would carry a part of the file.

Regards, Abhishek

---

From: Gregory Detal <[email-masked]>
Date: Mon, 7 Jul 2014 09:03:10 +0200

Hello,

This is true only if you don't use the coupled congestion control (and its
variants) which tries to be fair with regular TCP when establishing
multiple subflows over the same bottleneck. See
http://tools.ietf.org/html/rfc6356.

Regards,
Gregory

On Sun, Jul 6, 2014 at 5:03 PM, Abhishek Verma <[email-masked]>
wrote:

>  Even if all subflows of the MPTCP go via the same path it would still
> offer some benefit as the total throughput achieved is n times the
> throughput achieved with a single TCP session, where n is the number of
> subflows.
>