[ixp1200] WWBump Code in 'Network System Design'

[Shyamal Pandya]

When SDRAM is accessed from microengines the address must be quadword 
(8-byte) aligned.
Consider the following two cases:

Case I: (minimum IP header length)

Assume IP HLEN = 20, dpoff = 14 + 20 + 2 = 36 bytes
That is destination port offset will be at byte ofset 36 & 37.

The packet data will have to be accessed from byte offset 32 because of the 
8-byte alignment requirement. So the first longword contains bytes 32, 33, 
34 and 35. The second longword begins with the destination port.
also 36 AND 0x7 == 4

Case II: (IP header length 24 bytes)

Assume IP HLEN = 24, dpoff = 14 + 24 + 2 = 40 bytes
That is destination port offset will be at byte offset 40 & 41.

Here the packet data can be accessed from byte 40, so that the first 
longword contains the destination port.

Also 40 AND 0x7 == 0

Hope this clears the doubt.

--
Shyamal Pandya

Arizona State University

Addr:
920 S Terrace Rd #102
Tempe AZ 85281
Phone:
1-480-966-1982





>From: Lim Boon Ping <syseeker at yahoo.com>
>Reply-To: ixp1200 at CS.Princeton.EDU
>To: ixp1200 at CS.Princeton.EDU
>Subject: [ixp1200] WWBump Code in 'Network System Design'
>Date: Tue, 30 Sep 2003 23:37:34 -0700 (PDT)
>
>
>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
>
>---------------------------------
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