[LARTC] TEQL question

[bert hubert ahu@ds9a.nl]

<PRE>On Sun, Dec 17, 2000 at 02:00:45AM +0100, Rene 'Lynx' Pfeiffer wrote:

&gt;<i> uniformly under load. The same is true for UDP floods. When using single TCP
</I>&gt;<i> transfers (for example SCP from the SSH package) not all 8 links are used. The
</I>&gt;<i> packets use 3 or 4 links of the 8.
</I>
I think this is how it is supposed to work, I'm afraid. What happens is
this. The outgoing side initially queues a few packets, but not too much.
That is how TCP works - by receiving ACKnowledgement packets from the
remote, the sending side knows how much data it can send.

It will now wait until the remote ACKs the first three packets, say, and it
will then send a couple more. The problem is that while your bandwidth has
gone up, your latency hasn't. 

This means that in order to use all 8 of your links, the sending side should
ship 8 packets 'into the blind', without acknowledgement. You might make
this possible by raising the window size on the receiving side.

However, then there is the dark problem of packet reordering. Lets say 8
packets are sent simultaneously, they can arrive in any order and it is
highly unlikely that this order is 1 2 3 4 5 6 7 8.

Lets say the order is 1 3 8 2 6 5 4 7, the receiving side might interpret it
as follows:

	1: ok -&gt; ack up to 1
	3: missed packet 2 -&gt; ack up to 1
	8: missed packet 2, 4, 5, 6, 7 -&gt; ack up to 1
	2: missed packet 4, 5, 6, 7 -&gt; ack up to 3
	6: missed packet 4, 5, 7 -&gt; ack 3
	5: missed packet 4, 7 -&gt; ack up to 3
	4: missed packet 7 -&gt; ack up to 6
	7: ack up to 7

The sending side receives the following:

ack 1, ack 1, ack 1, ack 3, ack 3, ack 3, ack 6, ack 7

This will probably be interpreted as 'packet 1 got lost, lets resend it',
'packet 3 got lost, lets resend it'. This is not the case however, but all
this resending makes the sending kernel decide to slow down, and lower the
'congestion window'.

This means that the kernel will not allow a lot of packets to be 'in flight'
simultaneously.

&gt;<i> Any ideas? I would like to distribute the load over all 8 links.
</I>
If you have a lot of separate streams going, it gets better. Also, if the
receiving side is Linux, it might get less confused by this packet
reordering. I think regular Linux is able to reorder up to 3 out of place
packets.

&gt;<i>  GNU/Linux Manages! - Support, Administration, Consulting
</I>&gt;<i>   RP3191-RIPE       - Networking, Programming, Installation
</I>
Good luck!

Regards,

bert hubert

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</PRE>