[PATCH v2 2/3] nvme-tcp: support specifying the congestion-control

[Mingbao Sun]

Before answering the questions, I’d like to address the motivation
behind this patchset.

You know, InfiniBand/RoCE provides NVMe-oF a lossless network
environment (that is zero packet loss), which is a great advantage
to performance.

In contrast, 'TCP/IP + ethernet' is often used as a lossy network
environment (packet dropping often occurs). 
And once packet dropping occurs, timeout-retransmission would be
triggered. But once timeout-retransmission was triggered, bandwidth
would drop to 0 all of a sudden. This is great damage to performance.

So although NVMe/TCP may have a bandwidth competitive to that of
NVMe/RDMA, but the packet dropping of the former is a flaw to
its performance.

However, with the combination of the following conditions, NVMe/TCP
can become much more competitive to NVMe/RDMA in the data center.

  - Ethernet NICs supporting QoS configuration (support mapping TOS/DSCP
    in IP header into priority, supporting adjusting buffer size of each
    priority, support PFC)

  - Ethernet Switches supporting ECN marking, supporting adjusting
    buffer size of each priority.

  - NVMe/TCP supports specifying the tos for its TCP traffic
    (already implemented)

  - NVMe/TCP supports specifying dctcp as the congestion-control of its
    TCP sockets (the work of this feature)

So this feature is the last item from the software aspect to form up the
above combination.