[e2e] Question about managing window(rfc1122, 4.2.2.16)

[cys]

Hi.

While I'm implementing ELFN in ns simulator, I got some questions.

I searched some RFC and "tcp/ip illustrated" but I do not have a clue yet.


***TCP/IP vol.2, pp.857***
cancel any rtx timer
snd_nxt<-snd_una
enter the persist state.
probing
when receiver opens its window, start retransmitting form the left of
the window.
******

>RFC1122
>However, a sending TCP MUST be robust against window shrinking, which
may >cause the "useable window" (see Section 4.2.3.4) to become
negative.


>If this happens, the sender SHOULD NOT send new data,
What is the "new data"?? data beyond SND.UNA+SND.WND?
When does sender start transmit new data?

>but SHOULD retransmit normally the old unacknowledged data
>between SND.UNA and SND.UNA+SND.WND.
>The sender MAY also retransmit old data beyond SND.UNA+SND.WND, but
>SHOULD NOT time out the connection if data beyond the right window
edge
>is not acknowledged. If the window shrinks to zero, the TCP
>MUST probe it in the standard way (see next Section).

Does above cluase mean that TCP can transmit data "before" probing or
going to persist mode??

If it is(transmit some pkts before probing), how it can be possible?
SND.WND is already 0, isn't it?

If it It isn't, what is the old unacknowledged data?
TCP already set SND.NXT to SND.UNA. And I think TCP can't distinguish it.

--
cys