Subversion Repositories Kolibri OS

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;                                                                 ;;

;; Copyright (C) KolibriOS team 2004-2012. All rights reserved.    ;;

;; Distributed under terms of the GNU General Public License       ;;

;;                                                                 ;;

;;  Part of the tcp/ip network stack for KolibriOS                 ;;

;;                                                                 ;;

;;   Written by hidnplayr@kolibrios.org                            ;;

;;                                                                 ;;

;;    Based on the code of 4.4BSD                                  ;;

;;                                                                 ;;

;;          GNU GENERAL PUBLIC LICENSE                             ;;

;;             Version 2, June 1991                                ;;

;;                                                                 ;;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

$Revision: 2612 $

;-----------------------------------------------------------------

;

; TCP_input:

;

;  IN:  [esp] = ptr to buffer

;       [esp+4] = buffer size

;       ebx = ptr to device struct

;       ecx = segment size

;       esi = ptr to TCP segment

;       edi = ptr to ipv4 source address, followed by ipv4 dest address

;

;  OUT: /

;

;-----------------------------------------------------------------

align 4

TCP_input:

        DEBUGF  1,"TCP_input size=%u\n", ecx

; First, re-calculate the checksum

        push    ecx esi

        pushw   [esi + TCP_header.Checksum]

        mov     [esi + TCP_header.Checksum], 0

        TCP_checksum (edi), (edi+4)

        pop     cx              ; previous checksum

        cmp     cx, dx

        pop     edx ecx

        jne     .drop_no_socket

        DEBUGF  1,"Checksum ok\n"

        and     [edx + TCP_header.DataOffset], 0xf0                     ; Calculate TCP segment header size (throwing away unused reserved bits in TCP header)

        shr     [edx + TCP_header.DataOffset], 2

        cmp     [edx + TCP_header.DataOffset], sizeof.TCP_header        ; Now see if it's at least the size of a standard TCP header

        jb      .drop_no_socket                                         ; If not, drop the packet

        movzx   eax, [edx + TCP_header.DataOffset]

        sub     ecx, eax                                                ; substract TCP header size from total segment size

        jb      .drop_no_socket                                         ; If total segment size is less then the advertised header size, drop packet

        DEBUGF  1,"we got %u bytes of data\n", ecx

;-----------------------------------------------------------------------------------------

; Check if this packet has a timestamp option (We do it here so we can process it quickly)

        cmp     eax, sizeof.TCP_header + 12                             ; Timestamp option is 12 bytes

        jb      .no_timestamp

        je      .is_ok

        cmp     byte [edx + sizeof.TCP_header + 12], TCP_OPT_EOL                ; end of option list

        jne     .no_timestamp

  .is_ok:

        test    [edx + TCP_header.Flags], TH_SYN                                ; SYN flag must not be set

        jnz     .no_timestamp

        cmp     dword [edx + sizeof.TCP_header], 0x0101080a                     ; Timestamp header

        jne     .no_timestamp

        DEBUGF  1,"timestamp ok\n"

        ; TODO: Parse the option

        ; TODO: Set a Bit in the TCP to tell all options are parsed

  .no_timestamp:

;-------------------------------------------

; Convert Big-endian values to little endian

        ntohd   [edx + TCP_header.SequenceNumber]

        ntohd   [edx + TCP_header.AckNumber]

        ntohw   [edx + TCP_header.Window]

        ntohw   [edx + TCP_header.UrgentPointer]

        ntohw   [edx + TCP_header.SourcePort]

        ntohw   [edx + TCP_header.DestinationPort]

;------------------------------------------------------------

; Next thing to do is find the TCPS (thus, the socket pointer)

; IP Packet TCP Destination Port = local Port

; (IP Packet SenderAddress = Remote IP)  OR  (Remote IP = 0)

; (IP Packet TCP Source Port = remote Port) OR (remote Port = 0)

        mov     ebx, net_sockets

        mov     si, [edx + TCP_header.DestinationPort]

  .socket_loop:

        mov     ebx, [ebx + SOCKET.NextPtr]

        or      ebx, ebx

        jz      .drop_with_reset_no_socket

        cmp     [ebx + SOCKET.Domain], AF_INET4

        jne     .socket_loop

        cmp     [ebx + SOCKET.Protocol], IP_PROTO_TCP

        jne     .socket_loop

        cmp     [ebx + TCP_SOCKET.LocalPort], si

        jne     .socket_loop

        mov     eax, [ebx + IP_SOCKET.RemoteIP]

        cmp     eax, [edi]                              ; Ipv4 source addres

        je      @f

        test    eax, eax

        jnz     .socket_loop

       @@:

        mov     ax, [ebx + TCP_SOCKET.RemotePort]

        cmp     [edx + TCP_header.SourcePort], ax

        je      .found_socket

        test    ax, ax

        jnz     .socket_loop

  .found_socket:                                        ; ebx now contains the socketpointer

        DEBUGF  1,"Socket ptr: %x\n", ebx

;----------------------------

; Check if socket isnt closed

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_CLOSED

        je      .drop_not_locked

;----------------

; Lock the socket

        pusha

        lea     ecx, [ebx + SOCKET.mutex]

        call    mutex_lock

        popa

        DEBUGF  1,"Socket locked\n"

;---------------------------------------

; unscale the window into a 32 bit value

        movzx   eax, [edx + TCP_header.Window]

        push    ecx

        mov     cl, [ebx + TCP_SOCKET.SND_SCALE]

        shl     eax, cl

        mov     dword [edx + TCP_header.Window], eax    ; word after window is checksum, we dont need checksum anymore

        pop     ecx

;-----------------------------------

; Is this socket a listening socket?

        test    [ebx + SOCKET.options], SO_ACCEPTCON

        jz      .no_listening_socket

        DEBUGF  1,"Accepting new connection\n"

        pusha

        lea     ecx, [ebx + SOCKET.mutex]

        call    mutex_unlock

        popa

        push    ecx edx esi edi         ;;;

        call    SOCKET_fork

        pop     edi esi edx ecx

        test    eax, eax

        jz      .drop

        push    dword [edi + 4]                         ; Ipv4 destination addres

        pop     [eax + IP_SOCKET.LocalIP]

        push    [edx + TCP_header.DestinationPort]

        pop     [eax + TCP_SOCKET.LocalPort]

        mov     [eax + TCP_SOCKET.t_state], TCPS_LISTEN

        mov     ebx, eax

        jmp     .LISTEN

  .no_listening_socket:

;-------------------------------------

; Reset idle timer and keepalive timer

        mov     [ebx + TCP_SOCKET.t_idle], 0

        mov     [ebx + TCP_SOCKET.timer_keepalive], TCP_time_keep_interval

;--------------------

; Process TCP options

        movzx   eax, [edx + TCP_header.DataOffset]

        cmp     eax, sizeof.TCP_header                  ; Does header contain any options?

        je      .no_options

        DEBUGF  1,"Segment has options\n"

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_LISTEN ; no options when in listen state

        jz      .not_uni_xfer                           ; also no header prediction

        add     eax, edx

        lea     esi, [edx + sizeof.TCP_header]

  .opt_loop:

        cmp     esi, eax                        ; are we scanning outside of header?

        jae     .no_options

        cmp     byte [esi], TCP_OPT_EOL         ; end of option list?

        jz      .no_options

        cmp     byte [esi], TCP_OPT_NOP         ; nop ?

        jz      .opt_nop

        cmp     byte [esi], TCP_OPT_MAXSEG

        je      .opt_maxseg

        cmp     byte [esi], TCP_OPT_WINDOW

        je      .opt_window

        cmp     byte [esi], TCP_OPT_TIMESTAMP

        je      .opt_timestamp

        jmp     .no_options     ; If we reach here, some unknown options were received, skip them all!

  .opt_nop:

        inc     edi

        jmp     .opt_loop

  .opt_maxseg:

        cmp     byte [esi+1], 4

        jne     .no_options             ; error occured, ignore all options!

        test    [edx + TCP_header.Flags], TH_SYN

        jz      @f

        movzx   eax, word[esi+2]

        rol     ax, 8

        DEBUGF  1,"Maxseg: %u\n", ax

        mov     [ebx + TCP_SOCKET.t_maxseg], eax

       @@:

        add     edi, 4

        jmp     .opt_loop

  .opt_window:

        cmp     byte [esi+1], 3

        jne     .no_options

        test    [edx + TCP_header.Flags], TH_SYN

        jz      @f

        DEBUGF  1,"Got window option\n"

        ;;;;;

       @@:

        add     edi, 3

        jmp     .opt_loop

  .opt_timestamp:

        cmp     byte [esi+1], 10

        jne     .no_options

        DEBUGF  1,"Got timestamp option\n"

        ;;;;;

        add     esi, 10

        jmp     .opt_loop

  .no_options:

;-----------------------------------------------------------------------

; Time to do some header prediction (Original Principle by Van Jacobson)

; There are two common cases for an uni-directional data transfer.

;

; General rule: the packets has no control flags, is in-sequence,

;   window width didnt change and we're not retransmitting.

;

; Second rules:

;  -  If the length is 0 and the ACK moved forward, we're the sender side of the transfer.

;      In this case we'll free the ACK'ed data and notify higher levels that we have free space in buffer

;

;  -  If the length is not 0 and the ACK didn't move, we're the receiver side of the transfer.

;      If the packets are in order (data queue is empty), add the data to the socket buffer and request a delayed ACK

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_ESTABLISHED

        jnz     .not_uni_xfer

        test    [edx + TCP_header.Flags], TH_SYN + TH_FIN + TH_RST + TH_URG

        jnz     .not_uni_xfer

        test    [edx + TCP_header.Flags], TH_ACK

        jz      .not_uni_xfer

        mov     eax, [edx + TCP_header.SequenceNumber]

        cmp     eax, [ebx + TCP_SOCKET.RCV_NXT]

        jne     .not_uni_xfer

        mov     eax, dword [edx + TCP_header.Window]

        cmp     eax, [ebx + TCP_SOCKET.SND_WND]

        jne     .not_uni_xfer

        mov     eax, [ebx + TCP_SOCKET.SND_NXT]

        cmp     eax, [ebx + TCP_SOCKET.SND_MAX]

        jne     .not_uni_xfer

;---------------------------------------

; check if we are sender in the uni-xfer

; If the following 4 conditions are all true, this segment is a pure ACK.

;

; - The segment contains no data.

        test    ecx, ecx

        jnz     .not_sender

; - The congestion window is greater than or equal to the current send window.

;     This test is true only if the window is fully open, that is, the connection is not in the middle of slow start or congestion avoidance.

        mov     eax, [ebx + TCP_SOCKET.SND_CWND]

        cmp     eax, [ebx + TCP_SOCKET.SND_WND]

        jb      .not_uni_xfer

; - The acknowledgment field in the segment is less than or equal to the maximum sequence number sent.

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.SND_MAX]

        ja      .not_uni_xfer

; - The acknowledgment field in the segment is greater than the largest unacknowledged sequence number.

        sub     eax, [ebx + TCP_SOCKET.SND_UNA]

        jbe     .not_uni_xfer

        DEBUGF  1,"Header prediction: we are sender\n"

;---------------------------------

; Packet is a pure ACK, process it

; Update RTT estimators

;;; TODO

; Delete acknowledged bytes from send buffer

        pusha

        mov     ecx, eax

        lea     eax, [ebx + STREAM_SOCKET.snd]

        call    SOCKET_ring_free

        popa

; update window pointers

        mov     eax, [edx + TCP_header.AckNumber]

        mov     [ebx + TCP_SOCKET.SND_UNA], eax

; Stop retransmit timer

        mov     [ebx + TCP_SOCKET.timer_retransmission], 0

; Awaken waiting processes

        pusha

        lea     ecx, [ebx + SOCKET.mutex]

        call    mutex_unlock

        popa

        mov     eax, ebx

        call    SOCKET_notify_owner

; Generate more output

        call    TCP_output

        jmp     .drop_not_locked

;-------------------------------------------------

; maybe we are the receiver in the uni-xfer then..

  .not_sender:

; - The amount of data in the segment is greater than 0 (data count is in ecx)

; - The acknowledgment field equals the largest unacknowledged sequence number. This means no data is acknowledged by this segment.

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.SND_UNA]

        jne     .not_uni_xfer

; - The reassembly list of out-of-order segments for the connection is empty (seg_next equals tp).

;;; TODO

        jnz     .not_uni_xfer

; Complete processing of received data

        DEBUGF  1,"header prediction: we are receiver\nreceiving %u bytes of data\n", ecx

        add     [ebx + TCP_SOCKET.RCV_NXT], ecx         ; Update sequence number with number of bytes we have copied

        movzx   esi, [edx + TCP_header.DataOffset]

        add     esi, edx

        lea     eax, [ebx + STREAM_SOCKET.rcv]

        call    SOCKET_ring_write                       ; Add the data to the socket buffer

        mov     eax, ebx

        call    SOCKET_notify_owner

        or      [ebx + TCP_SOCKET.t_flags], TF_DELACK   ; Set delayed ack flag

        jmp     .drop

;--------------------------------------------------

; Header prediction failed, do it the slow way

  .not_uni_xfer:

        DEBUGF  1,"Header prediction failed\n"

; Calculate receive window size

;        mov     eax, [ebx + STREAM_SOCKET.rcv.size]

;        neg     eax

;        add     eax, SOCKETBUFFSIZE

;        mov     edx, [ebx + TCP_SOCKET.RCV_ADV]

;        sub     edx, [ebx + TCP_SOCKET.RCV_NXT]

;        cmp     eax, edx

;        jae     @f

;        mov     eax, edx

;       @@:

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_LISTEN

        je      .LISTEN

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_SYN_SENT

        je      .SYN_SENT

        jmp     .NOT_LISTEN_OR_SYN_SENT

;-------------

; Passive Open

align 4

  .LISTEN:

        DEBUGF  1,"TCP state: listen\n"

        test    [edx + TCP_header.Flags], TH_RST        ;;; TODO: kill new socket on error

        jnz     .drop

        test    [edx + TCP_header.Flags], TH_ACK

        jnz     .drop_with_reset

        test    [edx + TCP_header.Flags], TH_SYN

        jz      .drop

;;; TODO: check if it's a broadcast or multicast, and drop if so

        push    dword [edi]                             ; Ipv4 source addres

        pop     [ebx + IP_SOCKET.RemoteIP]

        push    [edx + TCP_header.SourcePort]

        pop     [ebx + TCP_SOCKET.RemotePort]

        push    [edx + TCP_header.SequenceNumber]

        pop     [ebx + TCP_SOCKET.IRS]

        push    [TCP_sequence_num]                      ;;;;;

        add     [TCP_sequence_num], 64000 / 2

        pop     [ebx + TCP_SOCKET.ISS]

        push    [ebx + TCP_SOCKET.ISS]

        pop     [ebx + TCP_SOCKET.SND_NXT]

        TCP_sendseqinit ebx

        TCP_rcvseqinit ebx

        mov     [ebx + TCP_SOCKET.t_state], TCPS_SYN_RECEIVED

        mov     [ebx + TCP_SOCKET.t_flags], TF_ACKNOW

        mov     [ebx + TCP_SOCKET.timer_keepalive], TCP_time_keep_interval  ;;;; macro

        lea     eax, [ebx + STREAM_SOCKET.snd]

        call    SOCKET_ring_create

        lea     eax, [ebx + STREAM_SOCKET.rcv]

        call    SOCKET_ring_create

;;;        call    SOCKET_notify_owner

        jmp     .trim_then_step6

;------------

; Active Open

align 4

.SYN_SENT:

        DEBUGF  1,"TCP state: syn_sent\n"

        test    [edx + TCP_header.Flags], TH_ACK

        jz      @f

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.ISS]

        jbe     .drop_with_reset

        cmp     eax, [ebx + TCP_SOCKET.SND_MAX]

        ja      .drop_with_reset

       @@:

        test    [edx + TCP_header.Flags], TH_RST

        jz      @f

        test    [edx + TCP_header.Flags], TH_ACK

        jz      .drop

        mov     eax, ebx

        mov     ebx, ECONNREFUSED

        call    TCP_drop

        jmp     .drop

       @@:

        test    [edx + TCP_header.Flags], TH_SYN

        jz      .drop

; at this point, segment seems to be valid

        test    [edx + TCP_header.Flags], TH_ACK

        jz      .no_syn_ack

; now, process received SYN in response to an active open

        mov     eax, [edx + TCP_header.AckNumber]

        mov     [ebx + TCP_SOCKET.SND_UNA], eax

        cmp     eax, [ebx + TCP_SOCKET.SND_NXT]

        jbe     @f

        mov     [ebx + TCP_SOCKET.SND_NXT], eax

       @@:

  .no_syn_ack:

        mov     [ebx + TCP_SOCKET.timer_retransmission], 0      ; disable retransmission

        push    [edx + TCP_header.SequenceNumber]

        pop     [ebx + TCP_SOCKET.IRS]

        TCP_rcvseqinit ebx

        or      [ebx + TCP_SOCKET.t_flags], TF_ACKNOW

        mov     eax, [ebx + TCP_SOCKET.SND_UNA]

        cmp     eax, [ebx + TCP_SOCKET.ISS]

        jbe     .simultaneous_open

        test    [edx + TCP_header.Flags], TH_ACK

        jz      .simultaneous_open

        DEBUGF  1,"TCP: active open\n"

;;; TODO: update stats

; set socket state to connected

        mov     [ebx + SOCKET.state],1  ;;;; FIXME

        mov     [ebx + TCP_SOCKET.t_state], TCPS_ESTABLISHED

;;; TODO: check if we should scale the connection (567-572)

        mov     [ebx + TCP_SOCKET.SND_SCALE], 0

;;; TODO: update RTT estimators

        jmp     .trim_then_step6

  .simultaneous_open:

        DEBUGF  1,"TCP: simultaneous open\n"

; We have received a syn but no ACK, so we are having a simultaneous open..

        mov     [ebx + TCP_SOCKET.t_state], TCPS_SYN_RECEIVED

;-------------------------------------

; Common processing for receipt of SYN

  .trim_then_step6:

        inc     [edx + TCP_header.SequenceNumber]

;;; TODO: Drop any received data that follows receive window (590)

        mov     eax, [edx + TCP_header.SequenceNumber]

        mov     [ebx + TCP_SOCKET.RCV_UP], eax

        dec     eax

        mov     [ebx + TCP_SOCKET.SND_WL1], eax

        jmp     .ack_processed

  .NOT_LISTEN_OR_SYN_SENT:

        DEBUGF  1,"Slow TCP input: not listen or syn_sent state\n"

;--------------------------------------------

; Protection Against Wrapped Sequence Numbers

; First, check if timestamp is present

;;;; TODO 602

; Then, check if at least some bytes of data are within window

;;;; TODO

;----------------------------

; trim any data not in window

; check for duplicate data at beginning of segment

        mov     eax, [ebx + TCP_SOCKET.RCV_NXT]

        sub     eax, [edx + TCP_header.SequenceNumber]

        jbe     .no_duplicate

        DEBUGF  1,"Uh oh.. %u bytes of duplicate data!\n", eax

        test    [edx + TCP_header.Flags], TH_SYN

        jz      .no_dup_syn

; remove duplicate syn

        and     [edx + TCP_header.Flags], not (TH_SYN)

        inc     [edx + TCP_header.SequenceNumber]

        cmp     [edx + TCP_header.UrgentPointer], 1

        jbe     @f

        dec     [edx + TCP_header.UrgentPointer]

        jmp     .dup_syn

       @@:

        and     [edx + TCP_header.Flags], not (TH_URG)

  .dup_syn:

        dec     eax

  .no_dup_syn:

; eax holds number of bytes to drop

; Check for entire duplicate packet

        cmp     eax, ecx

        jae     .duplicate

        DEBUGF  1,"Going to drop %u out of %u bytes\n", eax, ecx

;;; TODO: apply figure 28.30

; Check for duplicate FIN

        test    [edx + TCP_header.Flags], TH_FIN

        jz      @f

        inc     ecx

        cmp     eax, ecx

        dec     ecx

        jne     @f

        mov     eax, ecx

        and     [edx + TCP_header.Flags], not TH_FIN

        or      [ebx + TCP_SOCKET.t_flags], TF_ACKNOW

        jmp     .no_duplicate

       @@:

; Handle the case when a bound socket connects to itself

; Allow packets with a SYN and an ACKto continue with the processing

;-------------------------------------

; Generate duplicate ACK if nescessary

; This code also handles simultaneous half-open or self-connects

        test    eax, eax

        jnz     .drop_after_ack

        cmp     [edx + TCP_header.Flags], TH_ACK

        jz      .drop_after_ack

  .duplicate:

        DEBUGF  1,"Duplicate received\n"

;----------------------------------------

; Update statistics for duplicate packets

;;; TODO

        jmp     .drop_after_ack

  .no_duplicate:

;-----------------------------------------------

; Remove duplicate data and update urgent offset

        add     [edx + TCP_header.SequenceNumber], eax

;;; TODO

        sub     [edx + TCP_header.UrgentPointer], ax

        ja      @f

        and     [edx + TCP_header.Flags], not (TH_URG)

        mov     [edx + TCP_header.UrgentPointer], 0

       @@:

;--------------------------------------------------

; Handle data that arrives after process terminates

        cmp     [ebx + SOCKET.PID], 0

        ja      @f

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_CLOSE_WAIT

        jbe     @f

        test    ecx, ecx

        jz      @f

;;; Close the socket

;;; update stats

        jmp     .drop_with_reset

       @@:

;----------------------------------------

; Remove data beyond right edge of window

        mov     eax, [edx + TCP_header.SequenceNumber]

        add     eax, ecx

        sub     eax, [ebx + TCP_SOCKET.RCV_NXT]

        sub     ax, [ebx + TCP_SOCKET.RCV_WND]

; eax now holds the number of bytes to drop

        jbe     .no_excess_data

;;; TODO: update stats

        cmp     eax, ecx

        jb      .dont_drop_all

;;; TODO 700-736

  .dont_drop_all:

  .no_excess_data:

;-----------------

; Record timestamp

;;; TODO 737-746

;------------------

; Process RST flags

        test    [edx + TCP_header.Flags], TH_RST

        jz      .rst_skip

        DEBUGF  1,"Got an RST flag"

        mov     eax, [ebx + TCP_SOCKET.t_state]

        shl     eax, 2

        jmp     dword [eax + .rst_sw_list]

  .rst_sw_list:

        dd      .rst_skip       ;TCPS_CLOSED

        dd      .rst_skip       ;TCPS_LISTEN

        dd      .rst_skip       ;TCPS_SYN_SENT

        dd      .econnrefused   ;TCPS_SYN_RECEIVED

        dd      .econnreset     ;TCPS_ESTABLISHED

        dd      .econnreset     ;TCPS_CLOSE_WAIT

        dd      .econnreset     ;TCPS_FIN_WAIT_1

        dd      .rst_close      ;TCPS_CLOSING

        dd      .rst_close      ;TCPS_LAST_ACK

        dd      .econnreset     ;TCPS_FIN_WAIT_2

        dd      .rst_close      ;TCPS_TIMED_WAIT

  .econnrefused:

        DEBUGF  1,"Connection refused"

        mov     [ebx + SOCKET.errorcode], ECONNREFUSED

        jmp     .close

  .econnreset:

        DEBUGF  1,"Connection reset"

        mov     [ebx + SOCKET.errorcode], ECONNRESET

    .close:

        DEBUGF  1,"Closing connection"

        mov     [ebx + TCP_SOCKET.t_state], TCPS_CLOSED

        ;;; TODO: update stats

        mov     eax, ebx

        call    TCP_close

        jmp     .drop

  .rst_close:

        DEBUGF  1,"Closing with reset\n"

        mov     eax, ebx

        call    TCP_close

        jmp     .drop

  .rst_skip:

;--------------------------------------

; handle SYN-full and ACK-less segments

        test    [edx + TCP_header.Flags], TH_SYN

        jz      @f

        mov     eax, ebx

        mov     ebx, ECONNRESET

        call    TCP_drop

        jmp     .drop_with_reset

        test    [edx + TCP_header.Flags], TH_ACK

        jz      .drop

      @@:

;---------------

; ACK processing

        cmp     [ebx + TCP_SOCKET.t_state], TCPS_SYN_RECEIVED

        jnz     .no_syn_rcv

        DEBUGF  1,"TCP state = syn received\n"

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     [ebx + TCP_SOCKET.SND_UNA], eax

        ja      .drop_with_reset

        cmp     eax, [ebx + TCP_SOCKET.SND_MAX]

        ja      .drop_with_reset

;;; TODO: update stats

        mov     eax, ebx

        call    SOCKET_is_connected

        mov     [ebx + TCP_SOCKET.t_state], TCPS_ESTABLISHED

; Do window scaling?

        test    [ebx + TCP_SOCKET.t_flags], TF_RCVD_SCALE

        jz      @f

        test    [ebx + TCP_SOCKET.t_flags], TF_REQ_SCALE

        jz      @f

        push    word [ebx + TCP_SOCKET.requested_s_scale]       ; Set send and receive scale factors to the received values

        pop     word [ebx + TCP_SOCKET.SND_SCALE]

       @@:

;;; TODO: copy the data (if any) into the socket

        mov     eax, [edx + TCP_header.SequenceNumber]

        dec     eax

        mov     [ebx + TCP_SOCKET.SND_WL1], eax

        jmp     .not_dup_ack

  .no_syn_rcv:

; check for duplicate ACK

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.SND_UNA]

        ja      .not_dup_ack

        test    ecx, ecx

        jnz     .reset_dupacks

        mov     eax, dword [edx + TCP_header.Window]

        cmp     eax, [ebx + TCP_SOCKET.SND_WND]

        jne     .reset_dupacks

        DEBUGF  1,"Processing a duplicate ACK..\n"

        cmp     [ebx + TCP_SOCKET.timer_retransmission], 10000 ;;;;  FIXME

        ja      @f

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.SND_UNA]

        je      .dup_ack

       @@:

        mov     [ebx + TCP_SOCKET.t_dupacks], 0

        jmp     .not_dup_ack

  .dup_ack:

        inc     [ebx + TCP_SOCKET.t_dupacks]

        cmp     [ebx + TCP_SOCKET.t_dupacks], TCP_re_xmit_thresh

        jne     .no_re_xmit

        push    [ebx + TCP_SOCKET.SND_NXT]              ; >>>>

        mov     eax, [ebx + TCP_SOCKET.SND_WND]

        cmp     eax, [ebx + TCP_SOCKET.SND_CWND]

        cmovg   eax, [ebx + TCP_SOCKET.SND_CWND]

        shr     eax, 1

        push    edx

        xor     edx, edx

        div     [ebx + TCP_SOCKET.t_maxseg]

        cmp     eax, 2

        jae     @f

        mov     ax, 2

       @@:

        mul     [ebx + TCP_SOCKET.t_maxseg]

        pop     edx

        mov     [ebx + TCP_SOCKET.SND_SSTHRESH], eax

        mov     [ebx + TCP_SOCKET.timer_retransmission], 0      ; turn off retransmission timer

        mov     [ebx + TCP_SOCKET.t_rtt], 0

        mov     eax, [edx + TCP_header.AckNumber]

        mov     [ebx + TCP_SOCKET.SND_NXT], eax

        mov     eax, [ebx + TCP_SOCKET.t_maxseg]

        mov     [ebx + TCP_SOCKET.SND_CWND], eax

        mov     eax, ebx

        call    TCP_output                                      ; retransmit missing segment

        push    edx

        xor     edx, edx

        mov     eax, [ebx + TCP_SOCKET.t_maxseg]

        mul     [ebx + TCP_SOCKET.t_dupacks]

        pop     edx

        add     eax, [ebx + TCP_SOCKET.SND_SSTHRESH]

        mov     [ebx + TCP_SOCKET.SND_CWND], eax

        pop     eax                                     ; <<<<

        cmp     eax, [ebx + TCP_SOCKET.SND_NXT]

        jb      @f

        mov     [ebx + TCP_SOCKET.SND_NXT], eax

       @@:

        jmp     .drop

  .no_re_xmit:

        jbe     .not_dup_ack

        DEBUGF  1,"Increasing congestion window\n"

        mov     eax, [ebx + TCP_SOCKET.t_maxseg]

        add     [ebx + TCP_SOCKET.SND_CWND], eax

        mov     eax, ebx

        call    TCP_output

        jmp     .drop

  .not_dup_ack:

;-------------------------------------------------

; If the congestion window was inflated to account

; for the other side's cached packets, retract it

        mov     eax, [ebx + TCP_SOCKET.SND_SSTHRESH]

        cmp     eax, [ebx + TCP_SOCKET.SND_CWND]

        ja      @f

        cmp     [ebx + TCP_SOCKET.t_dupacks], TCP_re_xmit_thresh

        jbe     @f

        mov     [ebx + TCP_SOCKET.SND_CWND], eax

       @@:

        mov     [ebx + TCP_SOCKET.t_dupacks], 0

        mov     eax, [edx + TCP_header.AckNumber]

        cmp     eax, [ebx + TCP_SOCKET.SND_MAX]

        jbe     @f

        ;;; TODO: update stats

        jmp     .drop_after_ack

       @@:

        mov     edi, [edx + TCP_header.AckNumber]

        sub     edi, [ebx + TCP_SOCKET.SND_UNA]         ; now we got the number of acked bytes in edi

        ;;; TODO: update stats

        DEBUGF  1,"We have an acceptable ACK of %x bytes\n", esi