TCPCHECKSUM - Revision history

Admin: Created page with "= Action = Return a TCP/IP checksum, also called Internet Checksum, or IP Checksum.
2013-02-12T17:55:07Z

Created page with "= <span class="f_Header">Action</span> = Return a TCP/IP checksum, also called Internet Checksum, or IP Checksum. <span style="font-family: Arial;"> </span> <span styl..."

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= <span class="f_Header">Action</span> =

Return a TCP/IP checksum, also called Internet Checksum, or IP Checksum.

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= <span class="f_Header">Syntax</span> =

res= <span class="f_Syntax">TCPCHECKSUM</span>(buffer , bytes [,w1] [,w2])

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= <span class="f_Header">Remarks</span> =
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Res

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A word variable that is assigned with the TCP/IP checksum of the buffer

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Buffer

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A variable or array to get the checksum of.

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Bytes

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The number of bytes that must be examined.

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w1,w2

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Optional words that will be included in the checksum.

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Checksum's are used a lot in communication protocols.  A checksum is a way to verify that received data is the same as it was sent.  In the many Internet Protocols (TCP, UDP, IP, ICMP …) a special Internet checksum is used.  Normally the data to calculate the checksum on is stored in an array of bytes, but in some cases like TCP, and UDP, a pseudo header is added.  The optional words (w1, w2) can be used for these cases.  Most often w1 and w2 will be used for the Protocol number, and the UDP or TCP packet length.

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This checksum is calculated by grouping the bytes in the array into 2-byte words.  If the number of Bytes is an odd number, then an extra byte of zero is used to make the last 2-byte word.  All of the words are added together, keeping the total in a 4-byte Long variable.  If the optional words w1, w2, are included, they are also added to the total.  Next, the 4-byte Long total is split into two, 2-byte words, and these words are added together to make a new 2-byte Word total.  Finally the total is inverted.  This is the value returned as Res.

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This function using w1, w2, are very useful when working directly with Ethernet chips like the RTL8019AS or with protocols not directly supported by the WIZnet chips.

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See the samples directory for more examples of use (IP_Checksum.bas).

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You can use it for the PING sample below.

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= <span class="f_Header">See also</span> =

[[CRC8]] , [[CRC16]], [[CRC32]] , [[CHECKSUM]]

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= <span class="f_Header">ASM</span> =

NONE

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= <span class="f_Header">Example</span> =

<br/><source lang="bascomavr">
'-----------------------------------------------------------------------------------------
'name : PING_TWI.bas http://www.faqs.org/rfcs/rfc792.html
'copyright : (c) 1995-2005, MCS Electronics
'purpose : Simple PING program
'micro : Mega88
'suited for demo : yes
'commercial addon needed : no
'-----------------------------------------------------------------------------------------
$regfile = "m32def.dat" ' specify the used micro

$crystal = 8000000 ' used crystal frequency
$baud = 19200 ' use baud rate
$hwstack = 80 ' default use 32 for the hardware stack
$swstack = 128 ' default use 10 for the SW stack
$framesize = 80 ' default use 40 for the frame space

Const Debug = 1

'we do the usual
Print "Init TCP" ' display a message
Enable Interrupts ' before we use config tcpip , we need to enable the interrupts
Config Tcpip = Int0 , Mac = 12.128.12.34.56.78 , Ip = 192.168.0.8 , Submask = 255.255.255.0 , Gateway = 192.168.0.1 , Localport = 1000 , Tx = $55 , Rx = $55 , Twi = &H80 , Clock = 400000
Print "Init done"

Dim Peersize As Integer , Peeraddress As Long , Peerport As Word
Dim Idx As Byte , Result As Word , J As Byte , Res As Byte
Dim Ip As Long
Dim Dta(12) As Byte , Rec(12) As Byte

Dta(1) = 8 'type is echo
Dta(2) = 0 'code

Dta(3) = 0 ' for checksum initialization
Dta(4) = 0 ' checksum
Dta(5) = 0 ' a signature can be any number
Dta(6) = 1 ' signature
Dta(7) = 0 ' sequence number - any number
Dta(8) = 1
Dta(9) = 65

Dim W As Word At Dta + 2 Overlay 'same as dta(3) and dta(4)
W = Tcpchecksum(dta(1) , 9) ' calculate checksum and store in dta(3) and dta(4)

#if Debug
For J = 1 To 9
Print Dta(j)
Next
#endif

Ip = Maketcp(192.168.0.16) 'try to check this server

Print "Socket " ; Idx ; " " ; Idx
Setipprotocol Idx , 1 'set protocol to 1
'the protocol value must be set BEFORE the socket is openend

Idx = Getsocket(idx , 3 , 5000 , 0)

Do
Result = Udpwrite(ip , 7 , Idx , Dta(1) , 9) 'write ping data '
Print Result
Waitms 100
Result = Socketstat(idx , Sel_recv) 'check for data
Print Result
If Result >= 11 Then
Print "Ok"
Res = Tcpread(idx , Rec(1) , Result) 'get data with TCPREAD !!!
#if Debug
Print "DATA RETURNED :" ; Res '
For J = 1 To Result
Print Rec(j) ; " " ;
Next
Print
#endif
Else 'there might be a problem
Print "Network not available"
End If
Waitms 1000
Loop
</source><br/>{{Languages}}

[[Category:BASCOM Language Reference]]