On Tue, Sep 16, 2025 at 1:25 AM chia-yu.chang@nokia-bell-labs.com wrote:
From: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com
As SACK blocks tend to eat all option space when there are many holes, it is useful to compromise on sending many SACK blocks in every ACK and attempt to fit the AccECN option there by reducing the number of SACK blocks. However, it will never go below two SACK blocks because of the AccECN option.
As the AccECN option is often not put to every ACK, the space hijack is usually only temporary. Depending on the reuqired AccECN fields (can be either 3, 2, 1, or 0, cf. Table 5 in AccECN spec) and the NOPs used for alignment of other TCP options, up to two SACK blocks will be reduced. Please find below tables for more details:
+====================+=========================================+ | Number of | Required | Remaining | Number of | Final | | SACK | AccECN | option | reduced | number of | | blocks | fields | spaces | SACK blocks | SACK blocks | +===========+==========+===========+=============+=============+ | x (<=2) | 0 to 3 | any | 0 | x | +-----------+----------+-----------+-------------+-------------+ | 3 | 0 | any | 0 | 3 | | 3 | 1 | <4 | 1 | 2 | | 3 | 1 | >=4 | 0 | 3 | | 3 | 2 | <8 | 1 | 2 | | 3 | 2 | >=8 | 0 | 3 | | 3 | 3 | <12 | 1 | 2 | | 3 | 3 | >=12 | 0 | 3 | +-----------+----------+-----------+-------------+-------------+ | y (>=4) | 0 | any | 0 | y | | y (>=4) | 1 | <4 | 1 | y-1 | | y (>=4) | 1 | >=4 | 0 | y | | y (>=4) | 2 | <8 | 1 | y-1 | | y (>=4) | 2 | >=8 | 0 | y | | y (>=4) | 3 | <4 | 2 | y-2 | | y (>=4) | 3 | <12 | 1 | y-1 | | y (>=4) | 3 | >=12 | 0 | y | +===========+==========+===========+=============+=============+
Signed-off-by: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com Co-developed-by: Ilpo Järvinen ij@kernel.org Signed-off-by: Ilpo Järvinen ij@kernel.org
Reviewed-by: Eric Dumazet edumazet@google.com