Multipath TCP (MPTCP)

Wei Wang

04/19/2011

CISC856 University of Delaware

Reference Materials

• Draft-ietf-mptcp-multiaddressed-03

• Draft-ietf-mptcp-architecture-05

• Draft-ietf-mptcp-congestion-02

Motivation

• Growing number of multihomed hosts– IPad and Smart Phones with 3G + WIFI– Laptops with wired and wireless connections

• When TCP encounters multihomed host– Inefficient (Throughput)– One interface, one connection (Reliability)

Possible Scenario: Mobile Client

Mobile Client

3G Celltower

Server

Scenario: Mobile Client (2)

Mobile ClientServer

Wifi

Wifi

Scenario: Mobile Client (3)

Mobile ClientServer

Wifi

Wifi

Scenario: Mobile Client (4)

Mobile Client

Server

Wifi

Wifi

MPTCP in the Networking Stack

Standard TCP vs. MPTCP Protocol Stack

Application

TCP

IP

Application

MPTCPSubflow(TCP) … Subflow(TCP)

IP

MPTCP Option

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Kind LengthSub-type

(Subtype-specific data--------------------

------------------------------------------------- Variable length)

Symbol Name Subtype Value

MP_CAPABLE Multipath Capable 0x0

MP_JOIN Join Connection 0x1

DSSData Sequence Signal

0x2

… … …

Example Usage Scenario

Host A Host BA1 A2 B1 B2

Initial Connection Setup

Additional Subflow Setup

Connection Initiation

• Single path

A (Initiator)

B (Listener)

A’s Key (SYN)

B’s Key (ACK+SYN)

A’s Key & B’s Key (ACK)

Connection Initiation (2)

• MP_CAPABLE option– 64-bit key to authenticate the addition of future

subflows– The mapping– Initial Data Sequence Number (64-bit

truncation of hash of the key)– Used in the first subflow of a connection

Kind LengthSub-type

Ver-sion

C (reserved) S

Sender’s Key(64 bits)

Receiver’s Key (64 bits)(if Length == 20)

Connection

Token (KeyA,KeyB)

Starting a New Subflow

• SYN/ACK Exchange w/ MP_JOIN optionHost A HostB

A1 A2 B1

SYN+MP_CAPABLE (Key-A)

SYN/ACK+MP_CAPABLE(Key-B)

ACK+MP_CAPABLE(Key-A,Key-B)

ACK+MP_JOIN(MAC-A)

SYN/ACK+MP_JOIN(MAC-B,R-B)

SYN+MP_JOIN(Token-B,R-A)

Starting a New Subflow (2)

• MP_JOIN option (initial SYN)– Token

• Identify the MPTCP connection• Mapped to 5-tuples after arrival• Demultiplexing using 5-tuples upon successful

subflow setup• Cryptographic hash of the receiver’s key

– Random number • Prevent replay attacks on authentication

Kind Length = 12 Subtype B Address ID

Receiver’s Token (32 bits)

Sender’s Random Number (32 bits)

Starting a New Subflow (3)

• MP_JOIN option (responding SYN + ACK)– MAC(Key, Msg)

• Key from initial handshake, Msg from Random Numbers

• MAC-B = MAC (Key=(Key-B+Key-A), Msg=(R-B+R-A))

Kind Length = 12Sub-type

B Address ID

Sender’s Truncated MAC (64bits)

Sender’s Random Number (32 bits)

Starting a New Subflow (4)

• MP_JOIN option (third ACK)– MAC-A = MAC (Key=(Key-A+Key-B),

Msg=(R-A+R-B))

Kind Length = 12Sub-type

B

Sender’s MAC (160 bits SHA-1)

Sequence Numbers

• PDUs go multiple paths– Need sequence numbers to put them back in

sequence– Need sequence numbers to infer loss on a single path

• Options– One sequence space shared across all paths?– One sequence space per path, plus an extra one to

put data back in the correct order at the receiver?

Single Sequence Space

• Stripe the data sequence numbers across subflows

• Use data cumulative ack

1 2 3 4 5 6 1 2 3 4 5 6

5 3 1

6 4 2

ACK: 1, 3, 5

ACK: 2, 4, 6

Lost PDU

• Problem– Cannot tell which subflows lost data

1 2 3 4 5 6 1 3 4 5 6

5 3 1

6 4 2

ACK: 1, 1, 1

ACK: 1, 1, 1

Multiple Sequence Spaces

• Each subflow has its own sequence number space

• Data sequence numbers are mapped on the subflow that sends them (DSN)

• Use cumulative ack on each subflow for simplicity

(Explicit) Data Sequence Mapping

1 2 3 4 5 6 1 2 3 4 5 6

3,5 2,3 1,1

3,6 2,4 1,2

ACK: 1, 2, 3

ACK: 1, 2, 3

Subflow sequence number

Data sequence number

Lost PDU

4,1

1 2 3 4 5 6 1 2 3 4 5 6

3,5 2,3 1,1

3,6 2,4 1,2

ACK: 1, 1, 1

ACK: 1, 2, 3, 4

Subflow sequence number

Data sequence number

2 3 4 5 6

ACK: 1, 2, 3

Data ACK

• Rationale– Deadlock conditions: acked at subflow level

but failed to reach application– Freedom to drop segments at subflow level– Left edge of the advertised receive window

• Shared by all subflows• Relative to Data ACK

Closing a Connection

• FIN in MPTCP only affects the subflow on which it is sent

• DATA FIN – Decoupled from subflow FIN– Included in the Data-level Length, not at subflow

level– Once acked, remaining subflows close w/ standard

FIN exchanges

• Connection closed after both host’s DATA FIN acked

A segment with DSN 80, and length 11, with DATA FIN set, would be acked with a DATA

ACK of 91.

Acknowledgement

• Multipath TCP Implementors Workshop, Maastricht et al

• Work in progress (MPTCP), Mark Handley et al

• Designing a Multipath Transport Protocol, Costin Raiciu & Mark Handley

Backup Slides

MPTCP Terminology

• Path

• Subflow

• MPTCP Connection

• Data-level (Connection-level)

• Token

• Host

Summary of Goals

• Improve throughput

• Be “fair”

• Improve resilience

• Application compatibility

• Network compatibility

• Fallback to regular TCP

Summary of Design Decisions

• Improve throughput & Be “fair”– Congestion control: coupled increases algorithm

• Improve resilience– Either end can add paths– Re-transmit on any path

• Application compatibility– TCP API – no mods to apps– Modify TCP stack

• Network compatibility– Subflows look like regular TCP– Connection & subflow sequence spaces, acks…– Signal “MPTCP capable” with TCP option on SYN

• Fallback to regular TCP

Graceful Degradation (Fallback)

• Connection Initiation– Page 13