| Message ID | 20220104003722.73982-1-ivan@cloudflare.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [net] tcp: note that tcp_rmem[1] has a limited range | expand |
On Mon, 3 Jan 2022 16:37:22 -0800 Ivan Babrou <ivan@cloudflare.com> wrote: > The value of rcv_ssthresh is limited to tcp_rwin, which is limited > to 64k at the handshake time, since window scaling is not allowed there. > > Let's add a note to the docs that increasing tcp_rmem[1] does not have > any effect on raising the initial value of rcv_ssthresh past 64k. > > Link: https://lkml.org/lkml/2021/12/22/652 > Signed-off-by: Ivan Babrou <ivan@cloudflare.com> > --- > Documentation/networking/ip-sysctl.rst | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst > index 2572eecc3e86..16528bc92e65 100644 > --- a/Documentation/networking/ip-sysctl.rst > +++ b/Documentation/networking/ip-sysctl.rst > @@ -683,7 +683,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max > default: initial size of receive buffer used by TCP sockets. > This value overrides net.core.rmem_default used by other protocols. > Default: 131072 bytes. > - This value results in initial window of 65535. > + This value results in initial window of 65535. Increasing this value > + won't raise the initial advertised window above 65535. > > max: maximal size of receive buffer allowed for automatically > selected receiver buffers for TCP socket. This value does not override Why not add error check or warning in write to sysctl?
On Mon, Jan 3, 2022 at 4:44 PM Stephen Hemminger <stephen@networkplumber.org> wrote: > > On Mon, 3 Jan 2022 16:37:22 -0800 > Ivan Babrou <ivan@cloudflare.com> wrote: > > > The value of rcv_ssthresh is limited to tcp_rwin, which is limited > > to 64k at the handshake time, since window scaling is not allowed there. > > > > Let's add a note to the docs that increasing tcp_rmem[1] does not have > > any effect on raising the initial value of rcv_ssthresh past 64k. I guess you have to define what is the initial window. There seems to be a confusion between rcv_ssthresh and sk_rcvbuf If you want to document what is rcv_ssthresh and how it relates to sk_rcvbuf, you probably need more than few lines in Documentation/networking/ip-sysctl.rst > > > > Link: https://lkml.org/lkml/2021/12/22/652 > > Signed-off-by: Ivan Babrou <ivan@cloudflare.com> > > --- > > Documentation/networking/ip-sysctl.rst | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst > > index 2572eecc3e86..16528bc92e65 100644 > > --- a/Documentation/networking/ip-sysctl.rst > > +++ b/Documentation/networking/ip-sysctl.rst > > @@ -683,7 +683,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max > > default: initial size of receive buffer used by TCP sockets. > > This value overrides net.core.rmem_default used by other protocols. > > Default: 131072 bytes. > > - This value results in initial window of 65535. > > + This value results in initial window of 65535. Increasing this value > > + won't raise the initial advertised window above 65535. > > > > max: maximal size of receive buffer allowed for automatically > > selected receiver buffers for TCP socket. This value does not override > > Why not add error check or warning in write to sysctl? Please do not. We set this sysctl to 0.5 MB DRS is known to have quantization artifacts.
On Tue, Jan 4, 2022 at 12:33 AM Eric Dumazet <edumazet@google.com> wrote: > I guess you have to define what is the initial window. What I mean here is the first window after scaling is allowed, so the one that appears in the first non-SYN ACK. > There seems to be a confusion between rcv_ssthresh and sk_rcvbuf > > If you want to document what is rcv_ssthresh and how it relates to sk_rcvbuf, > you probably need more than few lines in Documentation/networking/ip-sysctl.rst I can't say I fully understand how buffer sizes grow and how rcv_ssthresh and sk_rcvbuf interact to document this properly. All I want is to document the fact that no matter what you punch into sysctls, you'll end up with an initial scaled window (defined above) that's no higher than 64k. Let me know if this is incorrect and if there's a way we can put this into words without going into too much detail. > Please do not. We set this sysctl to 0.5 MB > DRS is known to have quantization artifacts. Where can I read more about the quantization artifacts you mentioned?
On Wed, Jan 5, 2022 at 8:20 PM Ivan Babrou <ivan@cloudflare.com> wrote: > > On Tue, Jan 4, 2022 at 12:33 AM Eric Dumazet <edumazet@google.com> wrote: > > I guess you have to define what is the initial window. > > What I mean here is the first window after scaling is allowed, so the > one that appears in the first non-SYN ACK. > > > There seems to be a confusion between rcv_ssthresh and sk_rcvbuf > > > > If you want to document what is rcv_ssthresh and how it relates to sk_rcvbuf, > > you probably need more than few lines in Documentation/networking/ip-sysctl.rst > > I can't say I fully understand how buffer sizes grow and how > rcv_ssthresh and sk_rcvbuf interact to document this properly. > > All I want is to document the fact that no matter what you punch into > sysctls, you'll end up with an initial scaled window (defined above) > that's no higher than 64k. Let me know if this is incorrect and if > there's a way we can put this into words without going into too much > detail. Just to clarify, normal TCP 3WHS has a final ACK packet, where window scaling is enabled. You describe a possible issue of passive connections. Most of the time, servers want some kind of control before allowing a remote peer to send MB of payload in the first round trip. However, a typical connection starts with IW10 (rfc 6928), and standard TCP congestion control would implement Slow Start, doubling the payload at every round trip, so this is not an issue. If you want to enable bigger than 65535 RWIN for passive connections, this would violate standards and should be discussed first at IETF. If you want to enable bigger than 65535 RWIN for passive connections in a controlled environment, I suggest using an eBPF program to do so. > > > Please do not. We set this sysctl to 0.5 MB > > DRS is known to have quantization artifacts. > > Where can I read more about the quantization artifacts you mentioned? DRS is implemented in tcp_rcv_space_adjust()/tcp_rcv_rtt_update(), you can look at git history to get plenty of details. https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=c3916ad9320eed8eacd7c0b2cf7f881efceda892
On Thu, Jan 6, 2022 at 12:25 AM Eric Dumazet <edumazet@google.com> wrote: > Just to clarify, normal TCP 3WHS has a final ACK packet, where window > scaling is enabled. Correct, yet this final ACK packet won't signal the initial scaled window above 64k. That's what I'm trying to document, as it seems like a useful thing to keep in mind. If this statement is incorrect, then I'm definitely missing something very basic. Let me know if that's the case. > You describe a possible issue of passive connections. > Most of the time, servers want some kind of control before allowing a > remote peer to send MB of payload in the first round trip. Let's focus purely on the client side of it. The client is willing to receive the large payload (let's say 250K), yet it cannot signal this fact to the server. > However, a typical connection starts with IW10 (rfc 6928), and > standard TCP congestion > control would implement Slow Start, doubling the payload at every round trip, > so this is not an issue. It's not an issue on a low latency link, but when a latency sensitive client is trying to retrieve something across a 300ms RTT link, extra round trips to stretch the window add a lot of latency. > If you want to enable bigger than 65535 RWIN for passive connections, > this would violate standards and should be discussed first at IETF. I understand this and I don't intend to do this. > If you want to enable bigger than 65535 RWIN for passive connections > in a controlled environment, I suggest using an eBPF program to do so. Right, ebpf was your suggestion: https://lkml.org/lkml/2021/12/22/668 The intention of this patch is to say that you can't achieve this even for active connections with the client that is willing to advertise a larger window in the first non-SYN ACK. Currently even with ebpf you cannot do this, but I'm happy to add the support.
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index 2572eecc3e86..16528bc92e65 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -683,7 +683,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max default: initial size of receive buffer used by TCP sockets. This value overrides net.core.rmem_default used by other protocols. Default: 131072 bytes. - This value results in initial window of 65535. + This value results in initial window of 65535. Increasing this value + won't raise the initial advertised window above 65535. max: maximal size of receive buffer allowed for automatically selected receiver buffers for TCP socket. This value does not override
The value of rcv_ssthresh is limited to tcp_rwin, which is limited to 64k at the handshake time, since window scaling is not allowed there. Let's add a note to the docs that increasing tcp_rmem[1] does not have any effect on raising the initial value of rcv_ssthresh past 64k. Link: https://lkml.org/lkml/2021/12/22/652 Signed-off-by: Ivan Babrou <ivan@cloudflare.com> --- Documentation/networking/ip-sysctl.rst | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)