[nanomsg] Re: Load balancing / redundancy among connections
- From: "Garrett D'Amore" <garrett@xxxxxxxxxx>
- To: "nanomsg@xxxxxxxxxxxxx" <nanomsg@xxxxxxxxxxxxx>
- Date: Thu, 7 Aug 2014 00:19:44 -0700
Well, there is a brief handshake that nanomsg performs... but as it is just
on top of TCP, that shouldn't matter. TCP guarantees a reliable connection
*once the connection is established*. The performance may be in the
toilet, and its possible that its bad enough that it takes longer to get a
correct packet thru than the req/rep timers.
In fact, I'm almost sure that if it isn't TCP itself, its going to be the
req timing out -- if it doesn't get a reply in "n" milliseconds (don't
recall what n is, I think its tunable), it resubmits, which effectively
"drops" the previous effort.
How on this green earth you'd expect to have functional networking with 25%
of your packets falling on the floor is beyond me. You'd have better
reliability with carrier pigeons. :-) (There's an RFC for that, too! :-)
- Garrett
On Wed, Aug 6, 2014 at 10:47 PM, Drew Crawford <drew@xxxxxxxxxxxxxxxxxx>
wrote:
> It’s not TCP. I benchmarked other TCP-based protocols that actually make
> connections under those circumstances.
>
> My theory was at the time—and again, I’ve never investigated this, it’s
> just a theory—that something inside nanomsg’s transport or protocol layer
> has some extra steps in its handshake, and so the combined TCP + nanomsg
> handshake somehow exceeds the combined TCP + otherprotocol transport for
> various other protocols in my benchmark.
>
>
> On Aug 7, 2014, at 12:43 AM, Garrett D'Amore <garrett@xxxxxxxxxx> wrote:
>
> That's going to be TCP. TCP melts down badly under extreme pressure. At
> 25% packet loss, your sessions don't live long enough to successfully
> survive to the point of actually exchanging user data. (The 3-way
> handshake probably doesn't complete a substantial amount of the time.)
>
> 1% packet loss is considered a very bad network. Well before 10% loss you
> start looking for people to fire or equipment to replace.
>
>
> On Wed, Aug 6, 2014 at 9:30 PM, Drew Crawford <drew@xxxxxxxxxxxxxxxxxx>
> wrote:
>
>> For whatever it’s worth, I benchmarked nanomsg req/rep in my “very bad
>> network lab" and it did very poorly in a packet loss scenario. I think
>> when packet loss rose above 25% or so it was impossible to transmit a
>> single message.
>>
>> The problem wasn’t critical enough at that time to merit any further
>> investigation from me, but if there’s interest from somebody else in
>> submitting some patches I’d be happy to benchmark them on what is a pretty
>> robust test environment.
>>
>> Drew
>>
>>
>> On Aug 6, 2014, at 3:09 PM, Alex Elsayed <eternaleye@xxxxxxxxx> wrote:
>>
>> > Name Withheld wrote:
>> >
>> >> I have two linux machines (X and Y), with 2-20 extremely unreliable IP
>> >> connections between them. TCP is more reliable even without the its
>> >> reliability control than UDP in this setting, because the various ISPs
>> >> along the way apparently drop UDPs when they are congested, but not
>> TCP.
>> >> The connections use different media (frame relay, GPRS, 3G, 4G, WiFi,
>> >> mesh networks, you name it), and are mostly there although each goes
>> >> away for few minutes to a few hours every few days. A jungle, no doubt.
>> >>
>> >> I want to use all the connections available at a given moment to
>> >> increase the bandwidth, and since I can modify the applications running
>> >> on both machines - I wondered if I could use nanomsg for that? I can
>> >> deal with reordering and duplicate messages, but not with missing
>> >> messages ("at least once" delivery is needed)
>> >>
>> >> From reading the documentation, it sounds like two pipeline connections
>> >> (X push -> Y pull, X pull <- Y push) would give me the load balancing,
>> >> as long as I can use different IP addresses to guarantee the
>> connections
>> >> are going out through the different connections (which I can! each
>> >> machine has 20 IP addresses). However, I can't figure out from the docs
>> >> if there is retransmit if a connection dies while a message is "in
>> >> flight" . Also, I can't figure out from the docs how a broken transport
>> >> is detected - will it have to wait until the TCP connection died
>> >> (>1minute), or is there in inner timeout I can control?
>> >>
>> >> So, the question is:
>> >>
>> >> Is my understanding correct, and pipeline is the way to go? Or is there
>> >> a better solution? (Or, is nanomsg totally not the right tool for me?)
>> >>
>> >> I've considered using the linux bonding interface, and doing a tcp
>> >> connection above that; However, this would introduce crazy latencies
>> and
>> >> retransmits because tcp tries to keep packet order, which I can do
>> >> without.
>> >>
>> >> Thanks in advance.
>> >
>> > One thing I'd suggest looking into is MPTCP[1] (Multipath TCP) - it's
>> > designed for basically this exact use case.
>> >
>> > If you can't (or don't want to) build kernel modules, then the MPTCP
>> > proxy[2] (which runs in userspace using netfilter/iptables) may be of
>> > interest.
>> >
>> > Either option would be essentially transparent underneath nanomsg, due
>> to
>> > the design of MPTCP.
>> >
>> > Another option might be SCTP (since it supports multihoming), although
>> that
>> > would likely require adding an SCTP transport to nanomsg.
>> >
>> > [1] http://www.multipath-tcp.org/
>> > [2]
>> http://www.ietf.org/mail-archive/web/multipathtcp/current/msg01934.html
>> >
>> >
>>
>>
>>
>
>
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