[tcpm] tcp window size

[Marc <[e-mail]>]

Hello experts,

I've been trying to find definitive information about tcp window size
negotiation, and if it exists at all.

Take a browser that downloads a file (receiver) and a webserver (sender). If
- both sender and receiver support window scaling
- the receiver advertises a TCP winsize of 256KB
- the sender advertises a window size of 32KB
- the sender has enough space in its TCP send buffer
(/proc/sys/net/ipv4/tcp_wmem on linux)

How much 'outstanding unacked data' is the sender allowed to have,
32KB or 256KB ?

I can't see any logic for it being limited to the window size of the
sender (32KB), I think it depends on both the receiver winsize and the
sender TCP Send Buffer Size, but I need to be sure.

Reading https://tools.ietf.org/html/rfc793 also suggests there is no
such thing like window size 'negotiation' and each end of a TCP
connection just advertises its own to indicate how much data it can
receive, independently.

Is that correct ?

Thanks!