Multipath TCP - The path to multipath

1. MultiPath TCP
The path to multipath
Diogo Mónica (@diogomonica)

2. motivation
User streaming
radio
4G BS
WiFI AP
User streaming
radio
4G BS
WiFI AP

3. motivation
Unnecessary
Collision

4. motivation
• Adds Redundancy and Persistence
• Connection stays up when one link fails
• Reduces Congestion
• Ability to steer traffic away from
congested links
• Increases Efficiency
• Takes advantage of additional
interfaces, parallel paths

5. what
• Multipath TCP is an evolution of TCP
that can effectively use multiple paths
within a single transport connection
• RFC6824
• 30 (decimal) in the TCP kind field

6. features
• Supports unmodified network and
applications
• Copes with NATs on the path
• Defensive fall back to normal TCP
(strictly better than normal TCP)

7. how
• Replaces the lower-level socket
implementations
• Three independent implementations
• Working linux kernel implementation
• iPhones currently use MPTCP for Siri
connections

8. why not SCTP?
• Does not support middle boxes
• Requires application changes

9. Connection establishment
• Three-way handshake with
MP_CAPABLE options set
Syn
Syn/Ack
Ack

10. Connection establishment
Syn
MP_CAPABLE, A's key, flags
Syn/Ack
MP_CAPABLE, B's key, flags
Ack
MP_CAPABLE, A's key, B's key, flags

11. Connection establishment
• Optional TCP checksums
• Crypto algorithm negotiation based
on optional flags
• Currently only supports SHA-1

12. Subflow creation
Socket
MTCP
Connection
Socket
MTCP
Connection

13. Subflow creation
Syn
MP_JOIN, B's token, A's nonce,
A's Address ID, flags
Syn/Ack
MP_JOIN, B's HMAC, B's nonce,
B's Address ID, flags
Ack
MP_JOIN, A's HMAC
Ack
token = H(key || nonce)[:32]
Address ID = Unique
(Implementation
specific)

14. data transfer
• 64-bit data sequence number (DSN)
to number all data sent over the
MPTCP connection
• Each subflow has its own 32-bit
sequence number space

15. data transfer
• All subflows share the same receive
buffer and advertise the same receive
window.
• Two level of ACKs, regular ACKs on
each subflow and connection-level
ACKs for the data sequence space

16. data transfer
• Initial sequence number is generated
from the key (deterministic, but
random).
• ISDN-A = H(Key-A)[:64]
• Receiver can ensure no gaps in
sequence space at the start of the
connection

17. path priorities
• Hosts can indicate whether they wish
a subflow to be used as a regular or
backup path
• A backup path is only used if there
are no regular paths available.
• Final bit of the MP_JOIN option

18. security
considerations
• Time-Shifted Hijacking Attacks become
harder
• Flooding attacks still have a stateless
solution
• Possibility of using other crypto algorithms
in the future
• Security of MTCP is “no worse” than regular
TCP

19. Conclusion
• MPTCP is strictly better than TCP*
* this claim is totally implementation dependent

20. References
• http://tools.ietf.org/html/rfc6824
• http://tools.ietf.org/html/rfc6181
• http://www.bsdcan.org/2013/schedule/
attachments/250_BSDCan2013_MPTCP.pdf
• https://www.usenix.org/conference/nsdi12/how-
hard-can-it-be-designing-and-implementing-
deployable-multipath-tcp
• http://perso.uclouvain.be/olivier.bonaventure/blog/
html/2013/09/18/mptcp.html