2025年9月29日(月) 16:28 Max Inden <[email protected]>: > For what it is worth, also referencing previous discussion on this list: > > "Why isn't QUIC growing?" > > https://mailarchive.ietf.org/arch/msg/quic/RBhFFY3xcGRdBEdkYmTK2k926mQ/ >
Reading the old thread, I'm reminded that people often assume QUIC performs better than TCP. However, that is true only when the QUIC stack is implemented, configured, and deployed correctly. One bug I've seen in multiple stacks - one that significantly affects benchmark results - is the failure to auto-tune the receive window as aggressively as the sender's Slow Start allows. Based on my understanding, Google Quiche implements receive window auto-tuning as follows: * Send MAX_DATA / MAX_STREAM_DATA when 50% of the credit has been consumed. * Double the window size when these frames are frequently. Several other stacks have adopted this approach. The problem with this logic is that it's too conservative and causes the sender to become flow-control-blocked during Slow Start. Consider the following example: 1. The receiver advertises an initial Maximum Data of W. 2. After receiving 0.5W bytes, the receiver sends Maximum Data=2.5W along with ACKs up to W/2. The next Maximum Data will be sent once the receiver has received 1.5W bytes. 3. The receiver receives bytes up to W and ACKs them. 4. At this point, the sender's Slow Start permits transmission up to 2W bytes, but the advertised receive window is only 1.5W. As a rsult, the connection becomes flow-control-blocked. There are several ways to address this issue: * Send MAX_DATA / MAX_STREAM_DATA no later than when 33% (i.e., 1/3) of the credit is consumed. * Instead of doubling (x2) the window size, increase it by a larger factor (e.g., x4). * disable auto tuning entirely (it's needed only for latency-sensitive applications). Because it is difficult to estimate the sender's initial window and how quicly it ramps up - especially with algorithms like Careful Resume, which don't use Slow Start - my preference is to disable auto tuning by default. In fact, this is also the choice made by Chromium, which is why it is not affected by this bug! For reference, Tatshiro addressed this issue in ngtcp2 in the follwing PRs; * https://github.com/ngtcp2/ngtcp2/pull/1396 - Tweak threshold for max_stream_data and max_data transmission * https://github.com/ngtcp2/ngtcp2/pull/1397 - Add note for window auto-tuning * https://github.com/ngtcp2/ngtcp2/pull/1398 - examples/client: Disable window auto-tuning by default However, I suspect the bug may still exist in other stacks. On 29/09/2025 05.38, Lars Eggert wrote: > > Hi, > > pitch for a discussion at 124. > > https://radar.cloudflare.com/ > <https://radar.cloudflare.com/adoption-and-usage?dateRange=52w> and > similar stats have had H3 around 30% for a few years now, with little > changes since the first quichbram up to that level. > > Topic: why is that and is there anything the WG or IETF can do to change > it (upwards, of course)? > > Thanks, > Lars > -- > Sent from a mobile device; please excuse typos. > > -- Kazuho Oku
