Re: [PATCH RFC net-next 0/2] tcp: Redundant Data Bundling (RDB)

Yuchung Cheng
2015-10-24T06:11:00Z

On Fri, Oct 23, 2015 at 1:50 PM, Bendik Rønning Opstad
<bro.devel@gmail.com> wrote:
>
> This is a request for comments.
>
> Redundant Data Bundling (RDB) is a mechanism for TCP aimed at reducing
> the latency for applications sending time-dependent data.
> Latency-sensitive applications or services, such as online games and
> remote desktop, produce traffic with thin-stream characteristics,
> characterized by small packets and a relatively high ITT. By bundling
> already sent data in packets with new data, RDB alleviates head-of-line
> blocking by reducing the need to retransmit data segments when packets
> are lost. RDB is a continuation on the work on latency improvements for
> TCP in Linux, previously resulting in two thin-stream mechanisms in the
> Linux kernel
> (https://github.com/torvalds/linux/blob/master/Documentation/networking/tcp-thin.txt).
>
> The RDB implementation has been thoroughly tested, and shows
> significant latency reductions when packet loss occurs[1]. The tests
> show that, by imposing restrictions on the bundling rate, it can be made
> not to negatively affect competing traffic in an unfair manner.
>
> Note: Current patch set depends on a recently submitted patch for
> tcp_skb_cb (tcp: refactor struct tcp_skb_cb: http://patchwork.ozlabs.org/patch/510674)
>
> These patches have been tested with as set of packetdrill scripts located at
> https://github.com/bendikro/packetdrill/tree/master/gtests/net/packetdrill/tests/linux/rdb
> (The tests require patching packetdrill with a new socket option:
> https://github.com/bendikro/packetdrill/commit/9916b6c53e33dd04329d29b7d8baf703b2c2ac1b)
>
> Detailed info about the RDB mechanism can be found at
> http://mlab.no/blog/2015/10/redundant-data-bundling-in-tcp, as well as in the paper

What's the difference between RDB and TCP repacketization
(http://flylib.com/books/en/3.223.1.226/1/) ?

Reading the blog page, I am concerned the amount of
change (esp on fast path) just to bundle new writes during timeout &
retransmit, for a specific type of application? why not just send X
packets with total bytes < MSS on timeout..

> "Latency and Fairness Trade-Off for Thin Streams using Redundant Data
> Bundling in TCP"[2].
>
> [1] http://home.ifi.uio.no/paalh/students/BendikOpstad.pdf
> [2] http://home.ifi.uio.no/bendiko/rdb_fairness_tradeoff.pdf
>
>
> Bendik Rønning Opstad (2):
> tcp: Add DPIFL thin stream detection mechanism
> tcp: Add Redundant Data Bundling (RDB)
>
> Documentation/networking/ip-sysctl.txt | 23 +++
> include/linux/skbuff.h | 1 +
> include/linux/tcp.h | 9 +-
> include/net/tcp.h | 34 ++++
> include/uapi/linux/tcp.h | 1 +
> net/core/skbuff.c | 3 +-
> net/ipv4/Makefile | 3 +-
> net/ipv4/sysctl_net_ipv4.c | 35 ++++
> net/ipv4/tcp.c | 19 ++-
> net/ipv4/tcp_input.c | 3 +
> net/ipv4/tcp_output.c | 11 +-
> net/ipv4/tcp_rdb.c | 281 +++++++++++++++++++++++++++++++++
> 12 files changed, 415 insertions(+), 8 deletions(-)
> create mode 100644 net/ipv4/tcp_rdb.c
>
> --
> 1.9.1
>