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

Jonas Markussen
2015-10-24T08:00:41Z

> On 24 Oct 2015, at 08:11, Yuchung Cheng <ycheng@google.com> wrote:
>
> 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..

Repacketization is only on retransmissions; RDB bundles previously sent segments with the next “normal” transmission instead.

This makes the flow recover the lost segment before a retransmission is triggered by an RTO or fast retransmit.

>> "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