TCP/IP研究(1)-Timer

 

 

       ARM and TOE Rx/Tx module will initialize, reset and clean the timer list. The timer list is some timer descriptor arrays stored in the SRAM and maintained by Timer Manager. All the timeout process are handled by ARM. When several TCP connections are timeout , the Timer Manager will generate an interrupt to ARM and indicate timeout happend. ARM will

read all the connections’ timeout events from the registers of Timer Manager and process in the ISR of firmware.

 

The main functions of Timer Manager:

l  Maintain descriptor array

 

l  Generate interrupt when timeout happened

 

l  Tell the ARM which connections are timeout

 

 

2.Timer Descriptor and Array

 

 

 

The hardware maintains six sequential arrays of timer descriptors which are stored in the SRAM. The array size is the maximum TCP connections supported. The entry of the array is short , middle and long timer descriptors:

 

 

 

l  The short timer descriptors is used by four general timer: SYNACK timer, FIN_WAIT2 time, Delayed ACK timer, TIME_WAIT Kill timer.

 

l  The middle timer descriptors is used by one general timer: Write timer(inherit from Linux, re-used by Retransmit timer and Zero Window Probe timer).

 

l  The long timer descriptors is used by one general timer: Keepalive timer.

 

Linux General TCP Timer	TOE Timer		Timeout Value	Reset Frequence	Accuracy	Descriptor Type
Keepalive timer	SYNACK timer		3 seconds	Very low	0.1s	Short
	FIN_WAIT2 timer		(FIN time – 60) second.	Very low	0.5s	Short
	Keepalive timer		75 seconds or 2 hours	Very low	5s	Long
Write timer	Write timer	Retransmit timer	200ms ~ 2min	High in WAN Low in LAN	50ms	Middle
		Zero Window Probe timer	(RTT*2 ^backoff) , Maximum 120 seconds	Low
Delayed ACK timer	Delayed ACK time		40ms ~ 200ms	Extreme High (reset whenever receive a packet)	10ms	Short
TIME_WAIT Kill timer	TIME_WAIT Kill timer		60 seconds	Very low	1s	Short

 

The operation of timer is described by the following, by SYNACK timer as instance:

l  Initialize/ Clear:

Software initialize the SYNACK timer of TCP Connection ID X. Software accesses the timer descriptor in the Keepalive_timer1_Array[ X ] and set the USED to 0( USED is 0, indicates the descriptor is not used)

 

l  Reset:

Software initialize the SYNACK timer of TCP Connection ID N.Software/Hardware accesses the timer descriptor in the Keepalive_timer1_Array[X ] and set the USED to 1. And set VALUE:

Equation[1]:

VALUE = timeout_second / accuracy_second.

If timeout_second is 3 second, accuracy_second is 0.5 second, VALUE will be set to 6.

 

l  Maintain:

Timer manager will access each entry of  Keepalive_timer1_Array,and subtract  1 from the VALUE every accuracy_second.

Equation[2]:

VALUE = VALUE – 1

If the Keepalive_timer1_Array X].VALUE become to zero, it is indicate the TCP Connection ID X is timeout. Then, Timer manager should interrupt the ARM to process the timeout process.

 

The design doesn’t follow the Linux. In the Linux, 4 general timers are re-used as 7 individual timers. For example , the Keepalive timer is re-used as 3 timers (SYNACK timer, FIN_WAIT2 timer and Keepalive timer). But those three times have different accuracy. It has to use 0.1 second as the minium accuracy and the use 3 bytes timer descriptors. The hardware need the whole 3 bytes * Max_Connection_ID array every 0.1 second. The Read/Write memory overhead is too big.

 

3. Overhead Estimation

On the assumption that the TOE support 1K connections. The hardware maintains six sequential array of timer descriptors: SYNACK timer, FIN_WAIT2 time, Delayed ACK timer, TIME_WAIT Kill timer, Write timer and Keepalive timer. Now, we will estimation the overhead of Read/Write memory and store quantity.

 

3.1 Read/Write memory

 

Equation[3]:

Read/Write_Memory byte/second = Length_of_Descriptor * Number_of_Connections / Accuracy_second

 

Overhead of SYNACK timer:

Read/Write Memory = 1byte * 1K /0.1 = 10 Kbyte/second

 

Overhead of FIN_WAIT2 timer:

Read/Write Memory = 1byte * 1K /0.5 = 2 Kbyte/second

 

Overhead of Delayed ACK timer:

Read/Write Memory = 1byte * 1K /0.01 = 100 Kbyte/second

 

Overhead of TIME_WAIT Kill timer:

Read/Write Memory = 1byte * 1K /1 = 1 Kbyte/second

 

Overhead of Write time:

Read/Write Memory = 1.5byte * 1K /0.05 = 30 Kbyte/second

 

Overhead of Keepalive e time:

Read/Write Memory = 2byte * 1K / 5 = 0.4 Kbyte/second

 

Read/Write memory  Summary = 10 + 2 +100 +1 + 30+ 0.4 = 143.4 Kbyte/second

 

3.2 Store quantity

Support 1K connections, need SRAM:

(1 + 1 + 1 +1 +1.5 + 2) * 1K = 7.5Kbyte

 

4. Interrupt

Timer Manage generates interrupt when timeout happened( VALUE became to 0 in the descriptor.)

…….

 

5. Tell the ARM which connections are timeout

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