Hi all,
This is a question regarding WWBump example code in 'Network System Design, using network processors', from Prof. Douglas Comer.
Trace through the code in WWBump.uc, i have some questions on the section regarding extracting destination port from SDRAM transfer register. I might have misunderstood the overall concept behind these codes, and hope if someone could point out.
Thanks in advance.
....
/* 'dpoff' contains the offset for destination port in TCP packet,*/
/* that is 14 bytes Ethernet header + n bytes IP header + 2 */
/* bytes source port, where n has been calculated earlier. */
/* Use lower three bits of the byte offset to determine which */
/* byte the destination port will be in. If value >= 4, dest. */
/* port is in the 2nd longword; otherwise it's in the first. */
alu[ dpoff, dpoff, AND, 0x7 ] ; Get lowest three bits
alu[ --, dpoff, -, 4] ; Test and conditional
br>=0[SecondWord#] ; branch if value >=4
FirstWord#: /* Load upper two bytes of register $$hdr0 */
ld_field_w_clr[dport, 0011, $$hdr0, >>16] ; Shift before mask
br[GotDstPort#] ; Check port number
SecondWord#: /* Load lower two bytes of register $$hdr1 */
ld_field_w_clr[dport, 0011, $$hdr1, >>16] ; Shift before mask
.....
May i know why the author uses lower 3 bits of the byte offset to determine byte the destination port will be in?
Isnt' dpoff has pointed to the offset of destination port, extracting 8 bytes from the offset onwards will always start from destination port until the first 2 bytes of ACK number, (means it will always be in $$hdr0) ?
Then why must the code check for condition as it will never branch to SecondWord#?
If it will branch to SecondWord#, under what condition?
I have done some calculation, please kindly indicate if i have misunderstood the concept.
Case I: (minimum IP header length)
Assume IP HLEN = 20, dpoff = 14 + 20 + 2 = 36 bytes
That is destination port offset will be at 37 & 38 bytes of the packet.
36 bytes -> 288 bits -> 100100000
100100000 & 000000111 = 000000000 /* dpoff AND 0x7 */
result: branch to FirstWord#
Case II: (40 bytes IP header length)
Assume IP HLEN = 40, dpoff = 14 + 40 + 2 = 56 bytes
That is destination port offset will be at 57 & 58 bytes of the packet.
56 bytes -> 448 bits -> 111000000
111000000 & 000000111 = 000000000 /* dpoff AND 0x7 */
result: branch to FirstWord#
Case III: (60 bytes IP header length)
Assume IP HLEN = 60, dpoff = 14 + 60 + 2 = 76 bytes
That is destination port offset will be at 77 & 78 bytes of the packet.
76 bytes -> 608 bits -> 1001100000
1001100000 & 000000111 = 000000000 /* dpoff AND 0x7 */
result: branch to FirstWord#
Best regards,
Lim Boon Ping
---------------------------------------
Post-Graduate Student
Multimedia University
Cyberjaya, Malaysia
---------------------------------
Do you Yahoo!?
The New Yahoo! Shopping - with improved product search
