Re: [multipathtcp] The point about multiple listening sockets at theclient side

"Ford, Alan" <alan.ford@roke.co.uk> Thu, 12 November 2009 07:27 UTC

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Date: Thu, 12 Nov 2009 07:27:03 -0000
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From: "Ford, Alan" <alan.ford@roke.co.uk>
To: William Herrin <bill@herrin.us>
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Subject: Re: [multipathtcp] The point about multiple listening sockets at theclient side
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Hi Bill, all,

> Plot the middlebox use scenarios:
> 
> TS: TCP SYN-SYN/ACK-ACK from the server to the client which includes
> the multipath TCP subflow join option.
> 
> MJ: TCP packet without SYN but containing the multipath TCP join
> option and expecting the same sort of JOIN-JOIN/ACK-ACK handshake.
> 
> Scenarios:
> 
> No firewall: TS succeeds. MJ succeeds.
> Packet filter: TS fails. SYN to a random client port? Don't think so!
> MJ succeeds.
> Legacy Stateful firewall: TS fails. MJ fails.
> Legacy "DSL router": TS fails. MJ fails.
> Multipath-aware stateful firewall: TS fails. SYN to a random client
> port? Bad plan! MJ succeeds.
> Multipath-aware "DSL router": TS fails. SYN to a random client port?
> Come on! MJ succeeds.

So the only cases here where TS != MJ is when you're talking about
Multipath-aware routers or firewalls? In order for them to let through
MJ they need the existence of the Join option to override the natural
rejection of such a packet due to coming from an unknown address. In
which case, they could do the same for TS! (Secondary point is that TS
is likely to be going to a port on the client which is already in use,
even though it's from the ephemeral range. The multipath-aware middlebox
may well be aware of this).

Regards,
Alan