Multipath TCP (MPTCP)

Multipath TCP (MPTCP)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

Specification:
- RFC 6824
Purpose:
- Provide the ability to simultaneously use multiple paths between peers (to increase bandwidth or reliability/robustness)
Constraints:
- Be backward compatible with TCP
- Work when one or both hosts are multi-addressed

Terminology

Path:
- A sequence of links between a sender and receiver that is defined by a 4-tuple containing the source and destination addresses and ports
Subflow:
- A flow of TCP segments over a regular TCP connection
MPTCP Connection:
- A set of one or more subflows

Overview

Getting Started:
- An MPTCP connection begins as a regular TCP connection using the 3-way handshake (but the SYN, SYNACK, and ACK packets include the MP_CAPABLE option)
Adding a Subflow:
- An additional TCP connection is created by either the client or server (after the client signals that it has additional addresses) and logically combined with the existing connection
- The SYN, SYNACK and ACK packets in this handshake include the MP_JOIN option and a token that indicates which MPTCP connection it is joining
Removing a Subflow:
- Subflows can be terminated using the normal 4-way process
Terminating a Connection:
- Peer A sends a DATA FIN in the Data Sequence Signal
- Once all of the data has been received, Peer Z sends a DATA ACK in the Data Sequence Signal

Providing Reliability

Sequence Numbers:
- MPTCP uses a 64-bit Data Sequence Number (DSN)
- Each subflow has its own 32-bit sequence number
The Data Sequence Signal:
- Contans the Data Sequence Mapping from the subflow sequence space to the data sequence space
Acknowledgments:
- Regular TCP acknowledgments are used on each subflow
- MPTCP also uses acknowledgments for the data sequence space (that track the advancement of the overall byte stream which is maintained in one buffer)

MPTCP Messages

Recall:
- TCP packets include a variable length header field for options
MPTCP Option Format:
- Kind (8 bits)
- Length (8 bits)
- Subtype (4 bits)
- Subtype-specific data (variable)

MP_CAPABLE Option

Purpose:
- Used in the initial TCP handshaking
Subtype-Specific Data:
- Version (4 bits)
- Flags (8 bits)
- Option Sender's Key (64 bits)
- Option Receiver's Key (64 bits)

MP_JOIN Option

Purpose:
- Start a new subflow
Subtype-Specific Data for SYN:
- Flags (4 bits)
- Address ID (8 bits)
- Receiver's Token (32 bits)
- Sender's Random Number (32 bits)
Subtype-Specific Data for SYNACK:
- Flags (4 bits)
- Address ID (8 bits)
- Senders Truncated Hash-based Message Authentication Code (64 bits)
- Sender's Random Number (32 bits)
Subtype-Specific Data for ACK:
- Reserved (12 bits)
- Sender's Hash-based Message Authentication Code (160 bits)

Data Sequence Signal (DSS) Option

Purpose:
- Map between the subflow sequence numbers and the data sequence numbers
Subtype-Specific Data:
- Reserved (6 bits)
- Flags (6 bits)
- Data ACK (32 or 64 bits depending on flags)
- Data sequence number (32 or 64 bits depending on flags)
- Subflow sequence number (32 bits)
- Data-level length (16 bits)
- Checksum (16 bits)