[RndTbl] aggregating dsl lines

Adam Thompson athompso at athompso.net
Tue Sep 29 13:52:16 CDT 2015

I disagree with Theo on this... thus illustrating that it's not a 
"solved" problem in the industry!

What I've encountered using ECMP for point-to-point connections over 
non-deterministic paths is packet-ordering problems so bad that I'm 
better off using only one of the two links in the first place! (Remember 
that one packet arriving too soon or too late nullifies the entire TCP 
in-flight window...)
The following sentence is equally applicable to LACP as it is to ECMP, 
by the way...

WiFi links are the worst for bonding via ECMP, other types of wireless 
second-worst, DSL third-worst, and cable modems fourth-worst.  Over 
plain old Ethernet, of course, ECMP works beautifully nearly all the time.

Then you have to deal with the fact that not all ECMP implementations 
are equal; the Linux kernel still either hashes the traffic (thus NOT 
giving you a 2x speed boost) or treats is more like an active/passive 
failover pair (again, no speed boost).  As far as routers go, MikroTik 
appears to be the worst offender... it *looks* like it does full ECMP 
but it doesn't.  Cisco and Juniper routers, naturally, do it right 
99.999% of the time... at least you get /something/ for your money there.

Then you have to run a dynamic routing protocol to make use of ECMP, 
which drastically increases the solution complexity.

Pushing redundancy (and multi-path, and complexity in general) as far 
down the OSI stack as it can go has worked for me much better in all 
cases *EXCEPT* where it's impossible, and then pushing the multi-path 
capability right up to the application becomes necessary.  Doing it in 
the middle... tends to break, in my experience.


On 15-09-29 11:28 AM, Theodore Baschak wrote:
> To get a single TCP connection/thread going over two or more equal 
> bandwidth connections you just need equal cost routing to use all paths.
> An actual router (not firewall, or nat gateway) with two equal cost 
> routes will load balance packets over both connections as long as 
> you're not firewalling. If you're trying to do this multi-connection 
> extension at layer2 though instead of at layer3, you'll probably have 
> a lot more hair-pulling fun, and less success.
> Theo
>> On Sep 29, 2015, at 8:09 AM, Adam Thompson <athompso at athompso.net 
>> <mailto:athompso at athompso.net>> wrote:
>> This is an active area of research, particularly with the advent of 
>> multi-path TCP.
>> Presently, however, you have to hide the two-link-ness from the TCP 
>> layer, and essentially from the IP layer as well.
>> ECMP would work, as long as both lines are the same (this does not 
>> hold true as a dynamic assertion with DSL technology, *ever*).
>> LACP will *not* work.
>> If you have Linux boxes at both ends, you can use mod_bonding in its 
>> round-robin mode... I've done that in the past and it does work.
>> Far more effective, however, would be to upgrade to a symmetric VDSL2 
>> setup that supports DSL bonded pairs.
>> That'll set you back around $600+ per end, IIRC, replaces both the 
>> DSLAM and the DSLR, but makes your problems go away by turning all 
>> the copper into a single Ethernet link.
>> I just worked with someone else on this kind of setup, I'll see if I 
>> can find the links...
>> -Adam
>> On September 29, 2015 4:18:54 AM CDT, Trevor Cordes 
>> <trevor at tecnopolis.ca <mailto:trevor at tecnopolis.ca>> wrote:
>>     Is it possible to aggregate DSL lines, to combine them to get X-times the
>>     bandwidth on a single link?  In this situation, I control both ends, the
>>     DSLAM and the DSL modem side on the other end of some POTS runs (CAT3-ish
>>     I assume, or worse).
>>     Note, I don't want load balancing or fancy routing/sharing.  I need double
>>     (or more) the bandwidth for a single application (single TCP connection).
>>     If required, we can have linux/bsd boxes we control at either end of the
>>     links.
>>     If it's not possible, does anyone have any other ideas for somehow getting
>>     better bandwidth out of 500m POTS wires (quantity 4)?
>>     Thanks!
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