1. AMP: A Better Multipath TCP for Data Center Networks
Morteza Kheirkhah
University of Edinburgh UK
January 2017

2. Two Key Characteristics of Data Centre Networks (DCN)
Diverse applications with diverse communication patterns and requirements
Some apps are bandwidth hungry (online file storage)
Other apps are latency sensitive (online search)
Short flow dominance
Majority of flows are short-lived with deadline in flow completion time (FCT).
Majority of data volumes come from a few long flows.
Data centers exhibit a highly dynamic network

3. Two Types of Network Congestion
Persistent Congestion: Two or more long flows collide on a link (due to poor load-splitting of ECMP routing)
Low overall network throughput
Transient Congestion: Many short flows collide on a link
High queuing delays and packet drop probability
Latency sensitive short flows miss their deadlines

4. Persistent Congestion
Existing Approaches
Transient Congestion
DCTCP
(SIGCOMM ’10)
D2TCP
(SIGCOMM ’12)
Persistent Congestion
MPTCP
(SIGCOMM ’11)
Hedera
(NSDI ’10)
XMP
(coNEXT ’13)
Low latency
for short flows
High throughput
for long flows
XMP attempts to provide a good balance between the latency-throughput trade-off

5. Problems with XMP TCP Incast Last Hop Unfairness (LHU)
XMP is not robust against the TCP Incast problem
More subflows -> more packets -> buffer easily overflow -> higher chance of experiencing a retransmission timeout
Last Hop Unfairness (LHU)
XMP does not preserve network fairness when all its subflows compete with single-path flows at a shared link
More subflows -> more throughput
These problems are not only linked to XMP, they exist in any ECN-based variants of MPTCP

6. Incast (showcase)
Multipath protocols use 4 subflows
Flow size is 128KB
Link rate 10Gbps
Link delay 5us (RTT 20us)
Switch buffer size is 100 packets
The y-axies is log-scaled
Data Center MultiPath (DCM) is another ECN-Capable MPTCP variants that we also proposed in this paper. The idea is to combine MPTCP with DCTCP.
Multipath protocols complete their flows by 1-2 orders of magnitude longer than DCTCP

7. Last Hop Unfairness (example)
Let’s assume:
Propagation delay is zero
Marking threshold (K) at switches sets to 4 packets (K=4)
Minimum congestion window size sets to one packet (cwndmin=1)
Normal situation
Two single-path flows share the link fairly. Each flow generating two packets per RTT on average
Persistent buffer inflation
A new arriving packet always finds the queue size equal to K. Each flow is thus forced to reduce its cwnd to one packet
Last Hop Unfairness
The multipath flow (S5) with 4 subflows sending four times more packets than single-path flows
The LHU leads to severe unfairness and escalates the likelihood of persistent buffer inflation significantly

8. Last Hop Unfairness (showcase)
8 DCTCP flows and one XMP flows
As the number of XMP’s subflows increases, the impact of LHU problem increases

9. AMP: Adoptive Multipath TCP
Key observation:
When all subflows of a multipath flow have the smallest cwnd value, it is a good indicator that the subflows are at the same bottleneck link
AMP’s subflow suppression/release algorithms:
Suppression: When the minimum window state across all subflows remains for a small time period (e.g., 2 RTTs), AMP deactivate all subflows but one
Release: When AMP no longer receives ECN-marked packets for some time period (e.g., 8 RTTs), it reactivates all suspended subflows
AMP behaves like a single-path flow once it detects an Incast-like condition

10. AMP Simplifies Congestion Control Operation
We make two key observations:
RTT measurements of subflows are unnecessary for updating their cwnd (when ECN is used in a DCN)
DCTCP-like window reduction slows down traffic shifting

11. AMP is robust against the TCP incast problem
AMP under Incast
Flow Size of 128KB
AMP is robust against the TCP incast problem

12. AMP under LHU No. of multipath flows = 1 No. of subflow = 4
No LHU
Severe LHU

13. Summary Existing multipath congestion control schemes fail to handle:
The TCP incast problem that causes temporal switch buffer overflow due to synchronized traffic arrival
The last hop unfairness that causes persistent buffer inflation and serious unfairness
We designed AMP to effectively overcome these problems:
AMP adoptively switches its operation between a multiple-subflow and single-subflow mode

14. AMP Paper and Source Code
NS-3 implementation:

15. Thank You!