[lwip-devel] Question on comparations of sequence numbers

[narke]

Hi,

To check if a sequence number a within a window [b, c], a macro was used:

  #define TCP_SEQ_BETWEEN(a,b,c) (TCP_SEQ_GEQ(a,b) && TCP_SEQ_LEQ(a,c))

And, because, TCP_SEQ_LEQ and TCP_SEQ_GEQ are defined as,

  #define TCP_SEQ_LEQ(a,b)    ((s32_t)((a)-(b)) <= 0)
  #define TCP_SEQ_GEQ(a,b)    ((s32_t)((a)-(b)) >= 0)

so it means, a is within [b, c] iif

 b <= a <= c

However, because sequence number cycles (module 2**32), so I cannot
understand why a - b >=  0 mean  a >= b or a - c <= 0 means a <= c.
For example, in a 2 bit window, and a window could be [3, 1] and 0
falls in the window.  But here 3 <= 0 <= 1 does not hold.

How to understand that?

Thanks in advance.

-- 
Life is the only flaw in an otherwise perfect nonexistence
    -- Schopenhauer

narke
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