Re: Cwnd grows slowly during slow-start due to LRO of the receiver side.

From: Hans Petter Selasky <hps_at_selasky.org>
Date: Tue, 02 May 2023 09:14:04 UTC
Hi Chen!

The FreeBSD mbufs carry the number of ACKs that have been joined 
together into the following field:

m->m_pkthdr.lro_nsegs

Can this value be of any use to cc_newreno ?

--HPS

On 5/2/23 09:46, Chen Shuo wrote:
> As per newreno_ack_received() in sys/netinet/cc/cc_newreno.c,
> FreeBSD TCP sender strictly follows RFC 5681 with RFC 3465 extension
> That is, during slow-start, when receiving an ACK of 'bytes_acked'
> 
>      cwnd += min(bytes_acked, abc_l_var * SMSS);  // abc_l_var = 2 dflt
> 
> As discussed in sec3.2 of RFC 3465, L=2*SMSS bytes exactly balances
> the negative impact of the delayed ACK algorithm.  RFC 5681 also
> requires that a receiver SHOULD generate an ACK for at least every
> second full-sized segment, so bytes_acked per ACK is at most 2 * SMSS.
> If both sender and receiver follow it. cwnd should grow exponentially
> during slow-slow:
> 
>      cwnd *= 2    (per RTT)
> 
> However, LRO and TSO are widely used today, so receiver may generate
> much less ACKs than it used to do.  As I observed, Both FreeBSD and
> Linux generates at most one ACK per segment assembled by LRO/GRO.
> The worst case is one ACK per 45 MSS, as 45 * 1448 = 65160 < 65535.
> 
> Sending 1MB over a link of 100ms delay from FreeBSD 13.2:
> 
>   0.000 IP sender > sink: Flags [S], seq 205083268, win 65535, options
> [mss 1460,nop,wscale 10,sackOK,TS val 495212525 ecr 0], length 0
>   0.100 IP sink > sender: Flags [S.], seq 708257395, ack 205083269, win
> 65160, options [mss 1460,sackOK,TS val 563185696 ecr
> 495212525,nop,wscale 7], length 0
>   0.100 IP sender > sink: Flags [.], ack 1, win 65, options [nop,nop,TS
> val 495212626 ecr 563185696], length 0
>   // TSopt omitted below for brevity.
> 
>   // cwnd = 10 * MSS, sent 10 * MSS
>   0.101 IP sender > sink: Flags [.], seq 1:14481, ack 1, win 65, length 14480
> 
>   // got one ACK for 10 * MSS, cwnd += 2 * MSS, sent 12 * MSS
>   0.201 IP sink > sender: Flags [.], ack 14481, win 427, length 0
>   0.201 IP sender > sink: Flags [.], seq 14481:31857, ack 1, win 65, length 17376
> 
>   // got ACK of 12*MSS above, cwnd += 2 * MSS, sent 14 * MSS
>   0.301 IP sink > sender: Flags [.], ack 31857, win 411, length 0
>   0.301 IP sender > sink: Flags [.], seq 31857:52129, ack 1, win 65, length 20272
> 
>   // got ACK of 14*MSS above, cwnd += 2 * MSS, sent 16 * MSS
>   0.402 IP sink > sender: Flags [.], ack 52129, win 395, length 0
>   0.402 IP sender > sink: Flags [P.], seq 52129:73629, ack 1, win 65,
> length 21500
>   0.402 IP sender > sink: Flags [.], seq 73629:75077, ack 1, win 65, length 1448
> 
> As a consequence, instead of growing exponentially, cwnd grows
> more-or-less quadratically during slow-start, unless abc_l_var is
> set to a sufficiently large value.
> 
> NewReno took more than 20 seconds to ramp up throughput to 100Mbps
> over an emulated 100ms delay link.  While Linux took ~2 seconds.
> I can provide the pcap file if anyone is interested.
> 
> Switching to CUBIC won't help, because it uses the logic in NewReno
> ack_received() for slow start.
> 
> Is this a well-known issue and abc_l_var is the only cure for it?
> https://calomel.org/freebsd_network_tuning.html
> 
> Thank you!
> 
> Best,
> Shuo Chen
>