Re: [multipathtcp] mptcp quick Q: MP-TCP for address redundancy

Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be> Thu, 11 June 2020 20:19 UTC

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Reply-To: Olivier.Bonaventure@uclouvain.be
To: Toerless Eckert <tte@cs.fau.de>
Cc: multipathtcp@ietf.org
References: <20200611162904.GO16371@faui48f.informatik.uni-erlangen.de> <c3a6428e-7351-fab4-31c7-3cf909bdd6e9@uclouvain.be> <20200611174810.GR16371@faui48f.informatik.uni-erlangen.de> <fbd4fb80-b608-d81a-f292-ef26d41ab665@uclouvain.be> <20200611195823.GU16371@faui48f.informatik.uni-erlangen.de>
From: Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be>
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Subject: Re: [multipathtcp] mptcp quick Q: MP-TCP for address redundancy
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Toerless,
> 
> Not quite sure from your reply what applies to non-backup vs. backup subflows,
> you did not say.

MPTCP only uses a backup subflow if there is no available non-backup 
subflow.

> Also: Is it necessary to avoid loosing all sub-flows, or does
> that only speed up recovery ?

That speeds up the recovery but is not required. MPTCP supports both 
make before break and break before make.
> 
> Aka: If i start with one subflow, learn additional addresses, but
> not create additional subflows (just announce addresses), and
> now one of the peer has an interface failure and this only
> sub-flow fails. Would it then still be able to create a new
> subflow from a different address ?

Yes. Subflows can be created at any time from any local address to any 
of the addresses learned from the peer. Note that the client does not 
need to advertise its addresses to be able to use them to reach a 
server. For example, a smartphone has only Wifi enabled and connects to 
a server. It leaves out of reach of the Wifi, the subflow fails. It then 
enables its cellular interface, obtains an IP address and use it to 
create a new subflow to the server.
> 
> Sorry, even to me these questions sound 101, but really hard to read
> asnwers from the spec to the uninitiated...

There are a few tutorials on MPTCP where this is partially discussed:

https://www.usenix.org/publications/login/october-2012-volume-37-number-5/overview-multipath-tcp
https://dl.acm.org/doi/10.1145/2578508.2591369
http://www.sigcomm.org/content/ebook (chapter 2)
https://www.youtube.com/watch?v=PkvLq_kCv4o


Hope this helps


Olivier