h = vlib_buffer_get_current (b) + first_buffer_offset;
sum = ip_incremental_checksum (sum, h, n);
if (PREDICT_FALSE (b->flags & VLIB_BUFFER_NEXT_PRESENT))
h = vlib_buffer_get_current (b) + first_buffer_offset;
sum = ip_incremental_checksum (sum, h, n);
if (PREDICT_FALSE (b->flags & VLIB_BUFFER_NEXT_PRESENT))