# ip65 technical reference

# File : ip65/tcp.s

    TCP (transmission control protocol) functions
    NB to use these functions, you must pass "-DTCP" to ca65 when assembling "ip.s"
    otherwise inbound tcp packets won't get passed in to tcp_process
    currently only a single outbound (client) connection is supported
    to use, first call "tcp_connect" to create a connection. to send data on that connection, call "tcp_send".
    whenever data arrives, a call will be made to the routine pointed at by tcp_callback.

## functions

function | description
--- | ---
tcp_close |
tcp_connect |
    make outbound tcp connection
    inputs:
     tcp_connect_ip:  destination ip address (4 bytes)
     AX: destination port (2 bytes)
     tcp_callback: vector to call when data arrives on this connection
    outputs:
       carry flag is set if an error occured, clear otherwise
tcp_init |
     initialize tcp
    called automatically by ip_init if "ip.s" was compiled with -DTCP
     inputs: none
     outputs: none
tcp_listen |
    listen for an inbound tcp connection
    this is a 'blocking' call, i.e. it will not return until a connection has been made
    inputs:
     AX: destination port (2 bytes)
     tcp_callback: vector to call when data arrives on this connection
    outputs:
       carry flag is set if an error occured, clear otherwise
tcp_process |
    process incoming tcp packet
    called automatically by ip_process if "ip.s" was compiled with -DTCP
    inputs:
     eth_inp: should contain an ethernet frame encapsulating an inbound tcp packet
    outputs:
     none but if connection was found, an outbound message may be created, overwriting eth_outp
     also tcp_state and other tcp variables may be modified
tcp_send |
    send tcp data
    inputs:
       tcp connection should already be opened
       tcp_send_data_len: length of data to send (exclusive of any headers)
       AX: pointer to buffer containing data to be sent
    outputs:
       carry flag is set if an error occured, clear otherwise
tcp_send_keep_alive |
    send an empty ACK packet on the current connection
    inputs:
       none
    outputs:
       carry flag is set if an error occured, clear otherwise
tcp_send_string |
    send a string over the current tcp connection
    inputs:
       tcp connection should already be opened
       AX: pointer to buffer - data up to (but not including)
     the first nul byte will be sent. max of 255 bytes will be sent.
    outputs:
       carry flag is set if an error occured, clear otherwise

## variables

variable | description | size (bytes)
--- | --- | ---
tcp_callback | vector to routine to be called when data is received over tcp connection | 2
tcp_connect_ip | ip address of remote server to connect to | 4
tcp_connect_remote_port |  | 2
tcp_inbound_data_length | length of data just received over tcp connection | 2
tcp_inbound_data_ptr | pointer to data just recieved over tcp connection | 2
tcp_send_data_len | length (in bytes) of data to be sent over tcp connection | 2
tcp_state |  | 1

## constants

constants | description | value
--- | --- | ---
tcp_remote_ip |  |

## implementation

    ;TCP (transmission control protocol) functions
    ;NB to use these functions, you must pass "-DTCP" to ca65 when assembling "ip.s"
    ;otherwise inbound tcp packets won't get passed in to tcp_process
    ;currently only a single outbound (client) connection is supported
    ;to use, first call "tcp_connect" to create a connection. to send data on that connection, call "tcp_send".
    ;whenever data arrives, a call will be made to the routine pointed at by tcp_callback.

    MAX_TCP_PACKETS_SENT=8     ;timeout after sending 8 messages will be about 7 seconds (1+2+3+4+5+6+7+8)/4

    .include "../inc/common.i"
    .ifndef KPR_API_VERSION_NUMBER
      .define EQU     =
      .include "../inc/kipper_constants.i"
    .endif

    .import ip65_error

    .export tcp_init
    .export tcp_process
    .export tcp_connect
    .export tcp_callback
    .export tcp_connect_ip
    .export tcp_send_data_len
    .export tcp_send
    .export tcp_send_string
    .export tcp_close
    .export tcp_listen
    .export tcp_send_keep_alive
    .export tcp_connect_remote_port
    .export tcp_remote_ip
    .export tcp_state
    .export tcp_inbound_data_ptr
    .export tcp_inbound_data_length

    .import ip_calc_cksum
    .import ip_send
    .import ip_create_packet
    .import ip_inp
    .import ip_outp
    .import ip65_process

    .import check_for_abort_key
    .import timer_read
    .import ip65_random_word

    .importzp acc32
    .importzp op32
    .importzp acc16

    .import add_32_32
    .import add_16_32
    .import cmp_32_32
    .import cmp_16_16
    .import sub_16_16

    .importzp ip_cksum_ptr
    .importzp ip_header_cksum
    .importzp ip_src
    .importzp ip_dest
    .importzp ip_data
    .importzp ip_proto
    .importzp ip_proto_tcp
    .importzp ip_id
    .importzp ip_len

    .import copymem
    .importzp copy_src
    .importzp copy_dest

    .import cfg_ip

    tcp_cxn_state_closed      = 0
    tcp_cxn_state_listening   = 1  ;(waiting for an inbound SYN)
    tcp_cxn_state_syn_sent    = 2  ;(waiting for an inbound SYN/ACK)
    tcp_cxn_state_established = 3  ;

    ; tcp packet offsets
    tcp_inp    = ip_inp + ip_data  ;pointer to tcp packet inside inbound ethernet frame
    tcp_outp  = ip_outp + ip_data ;pointer to tcp packet inside outbound ethernet frame
    tcp_src_port  = 0 ;offset of source port field in tcp packet
    tcp_dest_port  = 2 ;offset of destination port field in tcp packet
    tcp_seq    = 4 ;offset of sequence number field in tcp packet
    tcp_ack  = 8 ;offset of acknowledgement field in tcp packet
    tcp_header_length  = 12 ;offset of header length field in tcp packet
    tcp_flags_field  = 13 ;offset of flags field in tcp packet
    tcp_window_size = 14 ; offset of window size field in tcp packet
    tcp_checksum = 16 ; offset of checksum field in tcp packet
    tcp_urgent_pointer = 18 ; offset of urgent pointer field in tcp packet
    tcp_data=20   ;offset of data in tcp packet

    ; virtual header
    tcp_vh    = tcp_outp - 12
    tcp_vh_src  = 0
    tcp_vh_dest  = 4
    tcp_vh_zero  = 8
    tcp_vh_proto  = 9
    tcp_vh_len  = 10

    ;
    tcp_flag_FIN  =1
    tcp_flag_SYN  =2
    tcp_flag_RST  =4
    tcp_flag_PSH  =8
    tcp_flag_ACK  =16
    tcp_flag_URG  =32

    .segment "TCP_VARS"
    tcp_state:  .res 1
    tcp_local_port: .res 2
    tcp_remote_port: .res 2
    tcp_remote_ip: .res 4
    tcp_sequence_number: .res 4
    tcp_ack_number: .res 4
    tcp_data_ptr: .res 2
    tcp_data_len: .res 2
    tcp_send_data_ptr: .res 2
    tcp_send_data_len: .res 2 ;length (in bytes) of data to be sent over tcp connection
    tcp_callback: .res 2 ;vector to routine to be called when data is received over tcp connection
    tcp_flags: .res 1
    tcp_fin_sent: .res 1

    tcp_listen_port: .res 2

    tcp_inbound_data_ptr: .res 2 ;pointer to data just recieved over tcp connection
    tcp_inbound_data_length: .res 2 ;length of data just received over tcp connection
    ;(if this is $ffff, that means "end of file", i.e. remote end has closed connection)
    tcp_connect_sequence_number: .res 4   ;the seq number we will next send out
    tcp_connect_expected_ack_number: .res 4 ;what we expect to see in the next inbound ack
    tcp_connect_ack_number: .res 4 ;what we will next ack
    tcp_connect_last_received_seq_number: .res 4 ;the seq field in the last inbound packet for this connection
    tcp_connect_last_ack: .res 4 ;ack field in the last inbound packet for this connection
    tcp_connect_local_port: .res 2 ;
    tcp_connect_remote_port: .res 2
    tcp_connect_ip: .res 4 ;ip address of remote server to connect to

    tcp_timer:  .res 1
    tcp_loop_count: .res 1
    tcp_packet_sent_count: .res 1

    .code

    ; initialize tcp
    ;called automatically by ip_init if "ip.s" was compiled with -DTCP
    ; inputs: none
    ; outputs: none
    tcp_init:

      rts

    jmp_to_callback:
      jmp (tcp_callback)

    ;listen for an inbound tcp connection
    ;this is a 'blocking' call, i.e. it will not return until a connection has been made
    ;inputs:
    ; AX: destination port (2 bytes)
    ; tcp_callback: vector to call when data arrives on this connection
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_listen:
      stax  tcp_listen_port
      lda #tcp_cxn_state_listening
      sta tcp_state
      lda #0  ;reset the "packet sent" counter
      sta tcp_packet_sent_count
      sta tcp_fin_sent

      ;set the low word of seq number to $0000, high word to something random
      sta tcp_connect_sequence_number
      sta tcp_connect_sequence_number+1
      jsr ip65_random_word
      stax  tcp_connect_sequence_number+2
      jsr set_expected_ack;       ;due to various ugly hacks, the 'expected ack' value is now what is put into the 'SEQ' field in outbound packets
    @listen_loop:
      jsr ip65_process
      jsr check_for_abort_key
      bcc @no_abort
      lda #KPR_ERROR_ABORTED_BY_USER
      sta ip65_error
      rts
    @no_abort:
      lda #tcp_cxn_state_listening
      cmp tcp_state
      beq @listen_loop

      jmp tcp_connection_established
      rts

    ;make outbound tcp connection
    ;inputs:
    ; tcp_connect_ip:  destination ip address (4 bytes)
    ; AX: destination port (2 bytes)
    ; tcp_callback: vector to call when data arrives on this connection
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_connect:
      stax  tcp_connect_remote_port
      jsr ip65_random_word
      stax  tcp_connect_local_port
      lda #tcp_cxn_state_syn_sent
      sta tcp_state
      lda #0  ;reset the "packet sent" counter
      sta tcp_packet_sent_count
      sta tcp_fin_sent

      ;set the low word of seq number to $0000, high word to something random
      sta tcp_connect_sequence_number
      sta tcp_connect_sequence_number+1
      jsr ip65_random_word
      stax  tcp_connect_sequence_number+2

    @tcp_polling_loop:

      ;create a SYN packet
      lda #tcp_flag_SYN
      sta tcp_flags
      lda  #0
      sta  tcp_data_len
      sta  tcp_data_len+1

      ldx #3        ;
    :  lda tcp_connect_ip,x
      sta tcp_remote_ip,x
      lda tcp_connect_sequence_number,x
      sta tcp_sequence_number,x
      dex
      bpl :-
      ldax  tcp_connect_local_port
      stax  tcp_local_port
      ldax  tcp_connect_remote_port
      stax  tcp_remote_port

      jsr tcp_send_packet
      lda tcp_packet_sent_count
      adc #1
      sta tcp_loop_count       ;we wait a bit longer between each resend
    @outer_delay_loop:
      jsr timer_read
      stx tcp_timer            ;we only care about the high byte
    @inner_delay_loop:
      jsr ip65_process
      jsr check_for_abort_key
      bcc @no_abort
      lda #KPR_ERROR_ABORTED_BY_USER
      sta ip65_error
      rts
    @no_abort:
      lda tcp_state
      cmp #tcp_cxn_state_syn_sent
      bne @got_a_response

      jsr timer_read
      cpx tcp_timer            ;this will tick over after about 1/4 of a second
      beq @inner_delay_loop

      dec tcp_loop_count
      bne @outer_delay_loop

      inc tcp_packet_sent_count
      lda tcp_packet_sent_count
      cmp #MAX_TCP_PACKETS_SENT-1
      bpl @too_many_messages_sent
      jmp @tcp_polling_loop

    @too_many_messages_sent:
    @failed:
      lda #tcp_cxn_state_closed
      sta tcp_state
      lda #KPR_ERROR_TIMEOUT_ON_RECEIVE
      sta ip65_error
      sec             ;signal an error
      rts
    @got_a_response:
      lda tcp_state
      cmp #tcp_cxn_state_closed
      bne @was_accepted
      sec     ;if we got here, then the other side sent a RST or FIN, so signal an error to the caller
      rts
    @was_accepted:
    tcp_connection_established:
    ;inc the sequence number to cover the SYN we have sent
      ldax  #tcp_connect_sequence_number
      stax  acc32
      ldax  #$01
      jsr add_16_32

    set_expected_ack:
    ;set the expected ack number with current seq number
      ldx #3        ;
    :  lda tcp_connect_sequence_number,x
      sta tcp_connect_expected_ack_number,x
      dex
      bpl :-

      clc
      rts

    tcp_close:
    ;close the current connection
    ;inputs:
    ;   none
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise

      lda tcp_state
      cmp #tcp_cxn_state_established
      beq :+
    @connection_closed:
      lda #tcp_cxn_state_closed
      sta tcp_state
      clc
      rts
    :
      ;increment the expected sequence number for the SYN we are about to send
      ldax #tcp_connect_expected_ack_number
      stax acc32
      ldax #1
      sta tcp_fin_sent
      jsr add_16_32

    @send_fin_loop:
      lda #tcp_flag_FIN+tcp_flag_ACK
      sta tcp_flags
      ldax  #0
      stax  tcp_data_len
      ldx #3        ;
    :  lda tcp_connect_ip,x
      sta tcp_remote_ip,x
      lda tcp_connect_ack_number,x
      sta tcp_ack_number,x
      lda tcp_connect_sequence_number,x
      sta tcp_sequence_number,x
      dex
      bpl :-
      ldax  tcp_connect_local_port
      stax  tcp_local_port
      ldax  tcp_connect_remote_port
      stax  tcp_remote_port

      jsr tcp_send_packet

      lda tcp_packet_sent_count
      adc #1
      sta tcp_loop_count       ;we wait a bit longer between each resend
    @outer_delay_loop:
      jsr timer_read
      stx tcp_timer            ;we only care about the high byte
    @inner_delay_loop:
      jsr ip65_process
      lda tcp_state
      cmp #tcp_cxn_state_established
      bne @connection_closed

      jsr timer_read
      cpx tcp_timer            ;this will tick over after about 1/4 of a second
      beq @inner_delay_loop

      dec tcp_loop_count
      bne @outer_delay_loop

      inc tcp_packet_sent_count
      lda tcp_packet_sent_count
      cmp #MAX_TCP_PACKETS_SENT-1
      bpl @too_many_messages_sent
      jmp @send_fin_loop
    @too_many_messages_sent:
    @failed:
      lda #tcp_cxn_state_closed
      sta tcp_state
      lda #KPR_ERROR_TIMEOUT_ON_RECEIVE
      sta ip65_error
      sec             ;signal an error
      rts

    ;send a string over the current tcp connection
    ;inputs:
    ;   tcp connection should already be opened
    ;   AX: pointer to buffer - data up to (but not including)
    ; the first nul byte will be sent. max of 255 bytes will be sent.
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_send_string:
      stax tcp_send_data_ptr
      stax copy_src
      lda #0
      tay
      sta tcp_send_data_len
      sta tcp_send_data_len+1
      lda (copy_src),y
      bne @find_end_of_string
      rts ; if the string is empty, don't send anything!
    @find_end_of_string:
      lda (copy_src),y
      beq @done
      inc tcp_send_data_len
      iny
      bne @find_end_of_string
    @done:
      ldax tcp_send_data_ptr
      ;now we can fall through into tcp_send

    ;send tcp data
    ;inputs:
    ;   tcp connection should already be opened
    ;   tcp_send_data_len: length of data to send (exclusive of any headers)
    ;   AX: pointer to buffer containing data to be sent
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_send:

      stax tcp_send_data_ptr

      lda tcp_state
      cmp #tcp_cxn_state_established
      beq @connection_established
      lda #KPR_ERROR_CONNECTION_CLOSED
      sta ip65_error
      sec
      rts
      lda #0  ;reset the "packet sent" counter
      sta tcp_packet_sent_count

    @connection_established:
      ;increment the expected sequence number
      ldax #tcp_connect_expected_ack_number
      stax acc32
      ldax tcp_send_data_len
      jsr add_16_32

    @tcp_polling_loop:

      ;create a data packet
      lda #tcp_flag_ACK+tcp_flag_PSH
      sta tcp_flags
      ldax tcp_send_data_len
      stax tcp_data_len

      ldax tcp_send_data_ptr
      stax tcp_data_ptr

      ldx #3        ;
    :  lda tcp_connect_ip,x
      sta tcp_remote_ip,x
      lda tcp_connect_sequence_number,x
      sta tcp_sequence_number,x

      dex
      bpl :-
      ldax  tcp_connect_local_port
      stax  tcp_local_port
      ldax  tcp_connect_remote_port
      stax  tcp_remote_port

      jsr tcp_send_packet
      lda tcp_packet_sent_count
      adc #1
      sta tcp_loop_count       ;we wait a bit longer between each resend
    @outer_delay_loop:
      jsr timer_read
      stx tcp_timer            ;we only care about the high byte
    @inner_delay_loop:
      jsr ip65_process
      jsr check_for_abort_key
      bcc @no_abort
      lda #KPR_ERROR_ABORTED_BY_USER
      sta ip65_error
      lda #tcp_cxn_state_closed
      sta tcp_state

      rts
    @no_abort:
      ldax #tcp_connect_last_ack
      stax acc32
      ldax #tcp_connect_expected_ack_number
      stax op32
      jsr cmp_32_32
      beq @got_ack

      jsr timer_read
      cpx tcp_timer            ;this will tick over after about 1/4 of a second
      beq @inner_delay_loop

      dec tcp_loop_count
      bne @outer_delay_loop

      inc tcp_packet_sent_count
      lda tcp_packet_sent_count
      cmp #MAX_TCP_PACKETS_SENT-1
      bpl @too_many_messages_sent
      jmp @tcp_polling_loop

    @too_many_messages_sent:
    @failed:

      lda #tcp_cxn_state_closed
      sta tcp_state
      lda #KPR_ERROR_TIMEOUT_ON_RECEIVE
      sta ip65_error
      sec             ;signal an error
      rts
    @got_ack:
      ;finished - now we need to advance the sequence number for the data we just sent
      ldax #tcp_connect_sequence_number
      stax acc32
      ldax tcp_send_data_len
      jsr add_16_32

      clc
      rts

    ;send a single tcp packet
    ;inputs:
    ; tcp_remote_ip: IP address of destination server
    ; tcp_remote_port: destination tcp port
    ; tcp_local_port: source tcp port
    ; tcp_flags: 6 bit flags
    ; tcp_data_ptr: pointer to data to include in this packet
    ; tcp_data_len: length of data pointed at by tcp_data_ptr
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_send_packet:
      ldax  tcp_data_ptr
      stax copy_src      ; copy data to output buffer
      ldax #tcp_outp + tcp_data
      stax copy_dest
      ldax tcp_data_len
      jsr copymem

      ldx #3        ; copy virtual header addresses
    :  lda tcp_remote_ip,x
      sta tcp_vh + tcp_vh_dest,x  ; set virtual header destination
      lda cfg_ip,x
      sta tcp_vh + tcp_vh_src,x  ; set virtual header source
      dex
      bpl :-

      lda tcp_local_port    ; copy source port
      sta tcp_outp + tcp_src_port + 1
      lda tcp_local_port + 1
      sta tcp_outp + tcp_src_port

      lda tcp_remote_port    ; copy destination port
      sta tcp_outp + tcp_dest_port + 1
      lda tcp_remote_port + 1
      sta tcp_outp + tcp_dest_port

      ldx #3        ; copy sequence and ack (if ACK flag set) numbers (in reverse order)
      ldy #0
    :  lda tcp_sequence_number,x
      sta tcp_outp + tcp_seq,y
      lda #tcp_flag_ACK
      bit tcp_flags
      bne @ack_set
      lda #0
      beq @sta_ack
      @ack_set:
      lda tcp_ack_number,x
      @sta_ack:
      sta tcp_outp + tcp_ack,y
      iny
      dex
      bpl :-

      lda #$50    ;4 bit header length in 32bit words + 4 bits of zero
      sta tcp_outp+tcp_header_length
      lda tcp_flags
      sta tcp_outp+tcp_flags_field

      lda #ip_proto_tcp
      sta tcp_vh + tcp_vh_proto

      ldax  #$0010  ;$1000 in network byte order
      stax  tcp_outp+tcp_window_size

      lda #0        ; clear checksum
      sta tcp_outp + tcp_checksum
      sta tcp_outp + tcp_checksum + 1
      sta tcp_vh + tcp_vh_zero  ; clear virtual header zero byte

      ldax #tcp_vh      ; checksum pointer to virtual header
      stax ip_cksum_ptr

      lda tcp_data_len    ; copy length + 20
      clc
      adc #20
      sta tcp_vh + tcp_vh_len + 1  ; lsb for virtual header
      tay
      lda tcp_data_len + 1
      adc #0
      sta tcp_vh + tcp_vh_len    ; msb for virtual header

      tax        ; length to A/X
      tya

      clc        ; add 12 bytes for virtual header
      adc #12
      bcc :+
      inx
    :
      jsr ip_calc_cksum    ; calculate checksum
      stax tcp_outp + tcp_checksum

      ldx #3        ; copy addresses
    :  lda tcp_remote_ip,x
      sta ip_outp + ip_dest,x    ; set ip destination address
      dex
      bpl :-

      jsr ip_create_packet    ; create ip packet template

      lda tcp_data_len   ; ip len = tcp data length +20 byte ip header + 20 byte tcp header
      ldx tcp_data_len +1
      clc
      adc #40
      bcc :+
      inx
    :  sta ip_outp + ip_len + 1  ; set length
      stx ip_outp + ip_len

      ldax #$1234          ; set ID
      stax ip_outp + ip_id

      lda #ip_proto_tcp    ; set protocol
      sta ip_outp + ip_proto

      jmp ip_send      ; send packet, sec on error

    check_current_connection:
    ;see if the ip packet we just got is for a valid (non-closed) tcp connection
    ;inputs:
    ; eth_inp: should contain an ethernet frame encapsulating an inbound tcp packet
    ;outputs:
    ; carry flag clear if inbound tcp packet part of existing connection

      lda tcp_state
      cmp #tcp_cxn_state_closed
      bne @connection_not_closed
      sec
      rts
    @connection_not_closed:
      ldax  #ip_inp+ip_src
      stax  acc32
      ldax  #tcp_connect_ip
      stax  op32
      jsr   cmp_32_32
      beq @remote_ip_matches

      sec
      rts
    @remote_ip_matches:
      ldax  tcp_inp+tcp_src_port
      stax  acc16
      lda   tcp_connect_remote_port+1 ;this value in reverse byte order to how it is presented in the TCP header
      ldx   tcp_connect_remote_port
      jsr   cmp_16_16
      beq @remote_port_matches
      sec
      rts
    @remote_port_matches:
      ldax  tcp_inp+tcp_dest_port
      stax  acc16
      lda   tcp_connect_local_port+1 ;this value in reverse byte order to how it is presented in the TCP header
      ldx   tcp_connect_local_port
      jsr   cmp_16_16
      beq   @local_port_matches
      sec
      rts
    @local_port_matches:
      clc
      rts

    ;process incoming tcp packet
    ;called automatically by ip_process if "ip.s" was compiled with -DTCP
    ;inputs:
    ; eth_inp: should contain an ethernet frame encapsulating an inbound tcp packet
    ;outputs:
    ; none but if connection was found, an outbound message may be created, overwriting eth_outp
    ; also tcp_state and other tcp variables may be modified
    tcp_process:

      lda #tcp_flag_RST
      bit tcp_inp+tcp_flags_field
      beq @not_reset
      jsr check_current_connection
      bcs @not_current_connection_on_rst
      ;for some reason, search.twitter.com is sending RSTs with ID=$1234 (i.e. echoing the inbound ID)
      ;but then keeps the connection open and ends up sending the file.
      ;so lets ignore a reset with ID=$1234
      lda ip_inp+ip_id
      cmp #$34
      bne @not_invalid_reset
      lda ip_inp+ip_id+1
      cmp #$12
      bne @not_invalid_reset
      jmp @send_ack
    @not_invalid_reset:
      ;connection has been reset so mark it as closed
      lda #tcp_cxn_state_closed
      sta tcp_state
      lda #KPR_ERROR_CONNECTION_RESET_BY_PEER
      sta ip65_error

      lda #$ff
      sta tcp_inbound_data_length
      sta tcp_inbound_data_length+1
      jsr jmp_to_callback   ;let the caller see the connection has closed

    @not_current_connection_on_rst:
      ;if we get a reset for a closed or nonexistent connection, then ignore it
      rts
    @not_reset:
      lda tcp_inp+tcp_flags_field
      cmp #tcp_flag_SYN+tcp_flag_ACK
      bne @not_syn_ack

      ;it's a SYN/ACK
      jsr check_current_connection
      bcc @current_connection_on_syn_ack
      ;if we get a SYN/ACK for something that aint the connection we're expecting,
      ;terminate with extreme prejudice
      jmp @send_rst
    @current_connection_on_syn_ack:
      lda tcp_state
      cmp #tcp_cxn_state_syn_sent
      bne @not_expecting_syn_ack
      ;this IS the syn/ack we are waiting for :-)
      ldx #3        ; copy sequence number to ack (in reverse order)
      ldy #0
    :  lda tcp_inp + tcp_seq,y
      sta tcp_connect_ack_number,x
      iny
      dex
      bpl :-

      ldax #tcp_connect_ack_number
      stax acc32
      ldax  #$0001  ;
      jsr add_16_32 ;increment the ACK counter by 1, for the SYN we just received

      lda #tcp_cxn_state_established
      sta tcp_state

    @not_expecting_syn_ack:
    ;we get a SYN/ACK for the current connection,
    ;but we're not expecting it, it's probably
    ;a retransmist - just ACK it
      jmp @send_ack

    @not_syn_ack:

    ;is it an ACK - alone or with PSH/URGENT but not a SYN/ACK?
      lda #tcp_flag_ACK
      bit tcp_inp+tcp_flags_field
      bne @ack
      jmp @not_ack
    @ack:
      ;is this the current connection?
      jsr check_current_connection
      bcc @current_connection_on_ack
      ;if we get an ACK for something that is not the current connection
      ;we should send a RST
      jmp @send_rst
    @current_connection_on_ack:
      ;if it's an ACK, then record the last ACK (reorder the bytes in the process)
      ldx #3        ; copy seq & ack fields (in reverse order)
      ldy #0
    :  lda tcp_inp + tcp_ack,y
      sta tcp_connect_last_ack,x
      lda tcp_inp + tcp_seq,y
      sta tcp_connect_last_received_seq_number,x
      iny
      dex
      bpl :-

      ;was this the next sequence number we're waiting for?
      ldax  #tcp_connect_ack_number
      stax  acc32
      ldax  #tcp_connect_last_received_seq_number
      stax  op32
      jsr   cmp_32_32

      bne   @not_expected_seq_number

      ;what is the size of data in this packet?
      lda ip_inp+ip_len+1 ;payload length (lo byte)
      sta acc16
      lda ip_inp+ip_len ;payload length (hi byte)
      sta acc16+1
      lda tcp_inp+tcp_header_length   ;high 4 bits is header length in 32 bit words
      lsr ; A=A/2
      lsr ; A=A/2
      clc ; A now equal to tcp header length in bytes
      adc #20 ;add 20 bytes for IP header. this gives length of IP +TCP headers
      ldx #0
      sta tcp_header_length
      jsr sub_16_16

      ;acc16 now contains the length of data in this TCP packet

      lda acc16
      sta tcp_inbound_data_length
      lda acc16+1
      sta tcp_inbound_data_length+1
      bne @not_empty_packet
      lda acc16
      bne @not_empty_packet
      jmp @empty_packet
    @not_empty_packet:

      ;calculate ptr to tcp data
      clc
      lda tcp_header_length
      adc #ip_inp
      adc #0
      sta tcp_inbound_data_ptr+1

      ;  do a callback
      jsr jmp_to_callback

      ; move ack ptr along
      ldax #tcp_connect_ack_number
      stax acc32
      ldax tcp_inbound_data_length
      jsr add_16_32

    @not_expected_seq_number: ;send an ACK with the sequence number we expect

      ;send the ACK for any data in this packet, then return to check for FIN flag
      jsr @send_ack

    @not_ack:
    @empty_packet:

    ;is it a FIN?
      lda #tcp_flag_FIN
      bit tcp_inp+tcp_flags_field
      bne @fin
      jmp @not_fin
    @fin:
      ;is this the current connection?
      jsr check_current_connection
      bcc :+
      jmp @send_rst ;reset if not current connection
    :
      ldx #3        ; copy seq field (in reverse order)
      ldy #0
    :  lda tcp_inp + tcp_seq,y
      sta tcp_connect_last_received_seq_number,x
      iny
      dex
      bpl :-

      ;was this the next sequence number we're waiting for?
      ldax  #tcp_connect_ack_number
      stax  acc32
      ldax  #tcp_connect_last_received_seq_number
      stax  op32
      jsr   cmp_32_32

      beq :+
      rts ;bail if not expected sequence number
    :

      ;set the length to $ffff
      lda #$ff
      sta tcp_inbound_data_length
      sta tcp_inbound_data_length+1
      jsr jmp_to_callback   ;let the caller see the connection has closed

      lda #tcp_cxn_state_closed
      sta tcp_state

      ;send a FIN/ACK
      ; move ack ptr along for the inbound FIN
      ldax #tcp_connect_ack_number
      stax acc32
      ldax #$01
      sta  tcp_fin_sent
      jsr add_16_32

      ;if we've already sent a FIN then just send back an ACK
      lda tcp_fin_sent
      beq @send_fin_ack
    ;if we get here, we've sent a FIN, and just received an inbound FIN.
    ;when we sent the fin, we didn't update the sequence number, since
    ;we want to use the old sequence on every resend of that FIN
    ;now that our fin has been ACKed, we need to inc the sequence number
    ;and then send another ACK.

      ldax #tcp_connect_sequence_number
      stax acc32
      ldax  #$0001  ;
      jsr add_16_32 ;increment the SEQ counter by 1, for the FIN we have been sending

      lda #tcp_flag_ACK
      jmp @send_packet

    @send_fin_ack:

      lda #tcp_flag_FIN+tcp_flag_ACK

      jmp @send_packet

    @not_fin:

      lda tcp_inp+tcp_flags_field
      cmp #tcp_flag_SYN
      beq @syn
      jmp @not_syn
    @syn:

      ;is this the port we are listening on?
      lda tcp_inp+tcp_dest_port+1
      cmp tcp_listen_port
      bne @decline_syn_with_reset
      lda tcp_inp+tcp_dest_port
      cmp tcp_listen_port+1
      bne @decline_syn_with_reset

      ;it's the right port - are we actually waiting for a connecting?
      lda #tcp_cxn_state_listening
      cmp tcp_state
      beq @this_is_connection_we_are_waiting_for
      ;is this the current connection? that would mean our ACK got lost, so resend
      jsr check_current_connection
      bcc @this_is_connection_we_are_waiting_for

      rts ;if we've currently got a connection open, then ignore any new requests
          ;the sender will timeout and resend the SYN, by which time we may be
          ;ready to accept it again.

    @this_is_connection_we_are_waiting_for:

      ; copy sequence number to ack (in reverse order) and remote IP
      ldx #3
      ldy #0
    :  lda tcp_inp + tcp_seq,y
      sta tcp_connect_ack_number,x
      lda ip_inp+ip_src,x
      sta tcp_connect_ip,x
      iny
      dex
      bpl :-

      ;copy ports
      ldax tcp_listen_port
      stax tcp_connect_local_port

      lda tcp_inp+tcp_src_port+1
      sta tcp_connect_remote_port
      lda tcp_inp+tcp_src_port
      sta tcp_connect_remote_port+1

      lda #tcp_cxn_state_established
      sta tcp_state

      ldax #tcp_connect_ack_number
      stax acc32
      ldax  #$0001  ;
      jsr add_16_32 ;increment the ACK counter by 1, for the SYN we just received
      lda #tcp_flag_SYN+tcp_flag_ACK
      jmp @send_packet

    @decline_syn_with_reset:
    ;create a RST packet
      ldx #3        ; copy sequence number to ack (in reverse order)
      ldy #0
    :  lda tcp_inp + tcp_seq,y
      sta tcp_ack_number,x
      iny
      dex
      bpl :-

      ldax #tcp_ack_number
      stax acc32
      ldax  #$0001  ;
      jsr add_16_32 ;increment the ACK counter by 1, for the SYN we just received

    @send_rst:

      lda #tcp_flag_RST+tcp_flag_ACK
      sta tcp_flags
      ldax  #0
      stax  tcp_data_len
      ldx #3        ;
    :  lda ip_inp+ip_src,x
      sta tcp_remote_ip,x
      dex
      bpl :-

      ;copy src/dest ports in inverted byte order
      lda tcp_inp+tcp_src_port
      sta tcp_remote_port+1
      lda tcp_inp+tcp_src_port+1
      sta tcp_remote_port

      lda tcp_inp+tcp_dest_port
      sta tcp_local_port+1
      lda tcp_inp+tcp_dest_port+1
      sta tcp_local_port

      jsr tcp_send_packet
      rts

    @not_syn:
      rts

    @send_ack:

    ;create an ACK packet
      lda #tcp_flag_ACK

    @send_packet:
      sta tcp_flags
      ldax  #0
      stax  tcp_data_len
      ldx #3        ;
    :  lda tcp_connect_ip,x
      sta tcp_remote_ip,x
      lda tcp_connect_ack_number,x
      sta tcp_ack_number,x
    ;if we have just sent a packet out, we may not yet have updated tcp_connect_sequence_number yet
    ;so use current value of tcp_connect_expected_ack_number as outbound sequence number instead
      lda tcp_connect_expected_ack_number,x
      sta tcp_sequence_number,x
      dex
      bpl :-
      ldax  tcp_connect_local_port
      stax  tcp_local_port
      ldax  tcp_connect_remote_port
      stax  tcp_remote_port

      jmp tcp_send_packet

    ;send an empty ACK packet on the current connection
    ;inputs:
    ;   none
    ;outputs:
    ;   carry flag is set if an error occured, clear otherwise
    tcp_send_keep_alive=@send_ack

    ;-- LICENSE FOR tcp.s --
    ; The contents of this file are subject to the Mozilla Public License
    ; Version 1.1 (the "License"); you may not use this file except in
    ; compliance with the License. You may obtain a copy of the License at
    ; http://www.mozilla.org/MPL/
    ;
    ; Software distributed under the License is distributed on an "AS IS"
    ; basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
    ; License for the specific language governing rights and limitations
    ; under the License.
    ;
    ; The Original Code is ip65.
    ;
    ; The Initial Developer of the Original Code is Jonno Downes,
    ; jonno@jamtronix.com.
    ; Portions created by the Initial Developer are Copyright (C) 2009
    ; Jonno Downes. All Rights Reserved.
    ; -- LICENSE END --