TCP FLAGS

Jeg kender ikke så meget til dem, men ved hvad de første to er til.

SYN kan løst oversættes til Synkronisering. Bruges til at etablere en TCP session.
Modparten vil så svare tilbage med SYN + ACK (Acknowledge) og til sidst vil førstepar svare med ACK. Herefter er sessionen etableret.

URG:  Urgent Pointer field significant
ACK:  Acknowledgment field significant
PSH:  Push Function
RST:  Reset the connection
SYN:  Synchronize sequence numbers
FIN:  No more data from sender

Kig i:
ftp://ftp.rfc-editor.org/in-notes/rfc793.txt
ftp://ftp.rfc-editor.org/in-notes/rfc791.txt

http://www.linuxgazette.com/issue42/tag/3.html

Mere konkret:

URG - urgent bit
ACK - acknowledgement bit
PSH - push bit
RST - Reset bit
SYN - Synchronization bit
FIN - Final bit

Altså bit der sættes i en TCP PDU (pakke).

Jeg har problmer med at forstå disse to flags endnu URG og PSH.

Man kan f.eks sige at SYN bruges til at oprette en ny TCP session, men hvad kan man så sige om URG og PSH ?

Jeg har selv fundet ud af hvad URG IG PSH buges til.

URG er bare hvis det er en vigtig besked, der er ikke så meget der.

PSH er hvis man vil sende en pakke hurtigt og sende den vidre til applicationen.

Ved afsendelse af pakke:
        If the PUSH flag is set, the data must be transmitted promptly
        to the receiver, and the PUSH bit will be set in the last TCP
        segment created from the buffer.  If the PUSH flag is not set,
        the data may be combined with data from subsequent SENDs for
        transmission efficiency.

        If the URGENT flag is set, segments sent to the destination TCP
        will have the urgent pointer set.  The receiving TCP will signal
        the urgent condition to the receiving process if the urgent
        pointer indicates that data preceding the urgent pointer has not
        been consumed by the receiving process.  The purpose of urgent
        is to stimulate the receiver to process the urgent data and to
        indicate to the receiver when all the currently known urgent
        data has been received.  The number of times the sending user's
        TCP signals urgent will not necessarily be equal to the number
        of times the receiving user will be notified of the presence of
        urgent data.

Ved modtagelse af pakke:
        If enough data arrive to fill the buffer before a PUSH is seen,
        the PUSH flag will not be set in the response to the RECEIVE.
        The buffer will be filled with as much data as it can hold.  If
        a PUSH is seen before the buffer is filled the buffer will be
        returned partially filled and PUSH indicated.

        If there is urgent data the user will have been informed as soon
        as it arrived via a TCP-to-user signal.  The receiving user
        should thus be in "urgent mode".  If the URGENT flag is on,
        additional urgent data remains.  If the URGENT flag is off, this
        call to RECEIVE has returned all the urgent data, and the user
        may now leave "urgent mode".  Note that data following the
        urgent pointer (non-urgent data) cannot be delivered to the user
        in the same buffer with preceeding urgent data unless the
        boundary is clearly marked for the user.