Web51 TCP sockets

Processing packets  software.html  ARP
Due to lack of memory, Web51 does not use anything close to the usual TCP sockets. It uses its own specific interface.

The system is mainly event-driven. It is not possible to write a procedure that would process incoming characters in a loop and have a background process deliver them. Rather, Web51 behaves more like the background process that calls a character-processing procedure whenever a character is received. Characters are usually processed in several state machines with state transitions dependent on the received character. For example, the states of the http state machine expect the characters 'G' 'E' 'T' ' ' in this order, transiting to the filesystem index calculation mode afterwards.

Behavior of the TCP stack is controlled by several configuration constants.

Example 3.1

; "telnet"
	.global	STACK1MODE
	.equ	STACK1MODE, RESENDMODE
	.global	ETHRETRY1
	.equ	ETHRETRY1,  5*50	; timeout = 50 s
	.global PORT1			; local port
	.equ	PORT1, 23
	.global PORT3			; remote port
	.equ	PORT3, 23
; "http"
	.global	STACK2MODE
	.equ	STACK2MODE, NORESENDMODE
	.global	ETHRETRY2
	.equ	ETHRETRY2,  5*2		; timeout = 2 s
	.global PORT2			; local port
	.equ	PORT2, 80

First, there are the STACKxMODE constants with valid values RESENDMODE and NORESENDMODE. These affect the TCP stack operation in terms of acknowledging outgoing data. RESENDMODE is more correct with respect to TCP definition; data are removed from the buffer when the other side acknowledges them. Unacknowledged data remain in the buffer and the transmission can be repeated. However, the transmit buffer size is limited; for example, the sendchar procedure limits the buffer size to about 1280 bytes. If the limit is exceeded, the buffer contents are transmitted and the associated pointers are reset. This mode is for instance suitable for transferring data from the serial line, with the data being copied to the TCP packet from the serial line buffer only if there is space.
If it is impossible to regulate the size of transmitted data, the NORESENDMODE shoud be used. This mode supports the "transmit and forget" function; contents of the buffer are transmitted, automatically acknowledged, and split into several TCP segments if necessary. The last TCP packet has the FIN flag set. This mode is suitable for example for serving WWW pages - if the communication is corrupted, data can be thrown away and the entire communication starts over from the beginning.

The ETHRETRYx constants specify the timeout for individual TCP stacks.

Stack configuration constants :

Stack 2 is not capable of active connection establishing.


Example 3.2

During operation, the TCP stack calls three callback procedures depending on the TCP connection state. Procedure open_stack is called when the TCP connection is open. It can be used for instance to initialize the procedure for processing of incoming data. Procedure process_stack is called whenever there are incoming data or acknowledgements. This procedure processes the incoming data and also passes data to the TCP stack. Procedure close_stack is called when the connection is terminated.

	.text

	.global open_stack1
	.global process_stack1
	.global close_stack1

open_stack1:	ret
process_stack1:	ljmp	telnet
close_stack1:	ljmp	close_telnet

	.global open_stack2
	.global process_stack2
	.global close_stack2

open_stack2:	ljmp	open_http
process_stack2:	ljmp	http
close_stack2:	ret

All of the above code examples are extracted from www8051.asm from the "serial" example. The above sample demonstrates assignment of the callbacks to actual procedures that transfer data between TCP and the serial line and control the http protocol.

Let's use the example of transferring data over TCP to demonstrate the function of the callback procedure named telnet. Two parameters are passed to the process_stack procedure:

Before process_stack is called, the variables unwrited, tcpWritePointer and flaghide are also set. They control writing data to the TCP stack. However, they are usually masked from the user by functions for writing to the TCP stack, such as sendchar, sendDecb, sendHexb, sNibl, send_string.

Unlike writing to the TCP stack, Web51 does not have character-oriented function for reading from the TCP stack. Processing of received data is managed by the user, using the variables data_addr and data_len.

Example 3.3

First, check if there are data to be processed in the received packet and save the length of the data.

telnet:
;;send max. data_len bytes to serial line
	mov	a,data_len+1	;LSB
	mov	workreg+3,a	;LSB
	mov	workreg+2,data_len	;MSB
	orl	a,data_len	;MSB
	jz	sendTelnetDta

Example 3.4

Copy part of the received packet from RTL8019AS buffer to the work buffer in the microcontroller for processing.

tgetbuf:
	mov	Timeout1,#ethtiming	;restart retry timer
;;  Retry1   = ETHRETRY1;
	mov	Retry1,#ETHRETRY1
	setb	flag1retry		;; NEW data in packet, send delayed Ack
	LCALL   pcode
;;  copy_r2s (buf, data_addr, sizeof(buf));
      	.pcode pr2s BYTE buf, @data_addr, #BYTE sizeofbuf	;copy telnet data
;;  data_addr += sizeof(buf);
      	.pcode paddwi BYTE data_addr, #BYTE sizeofbuf
	.byte 0
	mov	R0,#buf-1

Example 3.5

Process the copied data. In case of the telnet procedure, processing is simple - just copy the data byte by byte into the serial line buffer. If the serial line buffer is full, processing terminates prematurely.

tnxtin:inc	R0
	mov	a,#(buf + sizeofbuf)
	xrl	a,R0
	jz	tgetbuf
	mov	a,data_len
	jnz	tdata
	mov	a,data_len+1
	jnz	tdata
	sjmp	tallsend
tdata:
	mov	a,@r0
	lcall	xputchar
	jz	ttimeout
;;  data_len--;
	mov	a,data_len+1	; LSB
	dec	a
	mov	data_len+1,a
	cjne	A,#0xFF,sndx
	dec	data_len	; MSB
sndx:	sjmp	tnxtin

Example 3.6

Acknowledge processed data from the passed TCP packet with a call to AckBytes.

ttimeout:
;; workreg -= data_len;
	mov	r0,#workreg+3
	clr	C
	mov	a,@r0
	subb	a,data_len+1
	mov	@r0,a
	dec	r0
	mov	a,@r0
	subb	a,data_len
	mov	@r0,a
tallsend:

; Acknowledge only processed data
;;  tx_eth_pkt.pkt.ip.ipdata.tcp.tcpheader.Ack =
;;    rcvSeq + sendbytes;
	mov	R7,workreg+3	;LSB
	mov	R6,workreg+2	;MSB
      	lcall	AckBytes

Example 3.7

TCP connection is full duplex; therefore, a reply acknowledging received data may contain other outgoing data. If there is space in the transmitted packet (keep in mind the RESENDMODE used), we can fill it with data from the serial line buffer. The entire packet is stored in the RTL8019AS for retransmission in case of errors. Unlike in older TCP implementations in Web51, data processed by getchar/sendchar are removed from the serial line buffer. Removal does not wait for acknowledgement from the other side.[5]

sendTelnetDta:
	MOV	A,tcpWritePointer	; MSB
	anl	A,#0xFC			; send data only 
					; if tcpWritePointer %lt; 0x0400
	jnz	tnodata
;; if data in buffer, transmit
	jnb	rxint,tnodata
	lcall	getchar
	lcall	sendchar
	sjmp	sendTelnetDta
tnodata:
	ret

Upon return from the process_stack procedure, sendchar buffers, if any, are copied to the packet, TCP flags and checksums are added, ..., and the packet is transmitted.

[5] In the NORESENDMODE operation, length of the transmitted packet is not important. If necessary, the outgoing data are transmitted in several packets. Upon return from the process_stack procedure, the last packet sent has the FIN flag set and the connection is terminated.




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