[multipathtcp] Multipath TCP Address advertisement 1/5 - Load Balancing

[Fabien Duchêne <[e-mail]>]

Hello,

As agreed in Berlin during IETF96, I'm sending a series of emails to
discuss the different contributions proposed
inhttps://datatracker.ietf.org/doc/draft-duchene-mptcp-add-addr/

To allow Multipath TCP to operate for servers residing behind unmodified
layer 4 load balancers we propose to define the "B" flag in the
MP_CAPABLE option.

This flag can only be used during the three-way handshake. When this
flag is set, it indicates that the source IP address of the segment
containing this flag does not accept the establishment of additional
subflows for this connection. Otherwise, the source IP address of this
segment can be used as destination address to create additional subflows.

There are two use cases for this flag :

- servers behind a load balancer can use this flag in the SYN+ACK to
indicate that the IP address of the load balancer should not be used to
establish subflows. Those servers will typically announce one or more
other addresses with the ADD_ADDR option.
- clients behind firewalls or NAT can use this flag in the SYN. If set
in the SYN, the flag MUST also be set in the third ACK to support
stateless servers.

We would appreciate your feedback on this proposition.

Thanks!

Fabien